Chapter 12: Patterns of Heredity and Human Genetics€¦ · Teacher’s Corner 314A 314B Patterns...

Activities/FeaturesObjectivesSection

Chapter 12 OrganizerChapter 12 Organizer

Mendelian Inheritanceof Human TraitsNational Science EducationStandards UCP.2, UCP.3;A.1, A.2; C.2; F.1; G.1, G.2(1 session, 1/2 block)

When HeredityFollows DifferentRulesNational Science EducationStandards UCP.2, UCP.3;A.1, A.2; C.2; F.4; G.1-3(3 sessions, 21/2 blocks)

Complex Inheritanceof Human TraitsNational Science EducationStandards UCP.2, UCP.3,UCP.5; A.1, A.2; C.2; F.1;G.1-3 (2 sessions, 11/2 blocks)

1. Interpret a pedigree.2. Determine human genetic disorders

that are caused by inheritance of reces-sive alleles.

3. Predict how a human disorder can bedetermined by a simple dominant allele.

4. Distinguish between incompletely dominant and codominant alleles.

5. Compare multiple allelic and polygenicinheritance.

6. Analyze the pattern of sex-linked inheri-tance.

7. Summarize how internal and externalenvironments affect gene expression.

8. Compare codominance, multiple allelic,sex-linked, and polygenic patterns ofinheritance in humans.

9. Distinguish among conditions in whichextra autosomal or sex chromosomesexist.

MiniLab 12-1: Illustrating a Pedigree, p. 316Problem-Solving Lab 12-1, p. 317

Problem-Solving Lab 12-2, p. 324Design Your Own BioLab: What is the pattern of cytoplasmic inheritance? p. 336

Inside Story: The ABO Blood Group, p. 331Problem-Solving Lab 12-3, p. 332MiniLab 12-2: Detecting Colors and Patternsin Eyes, p. 333Social Studies Connection: Queen Victoriaand Royal Hemophilia, p. 338

MATERIALS LIST

BioLabp. 336 Brassica rapa seeds (normal andvariegated), potting soil, potting trays,paintbrushes, forceps, single-edgerazor blade, light source, labels

MiniLabsp. 316 pencil, paperp. 333 hand lens, colored pencils,paper

Alternative Labp. 322 petri dish, label, paper towels,scissors, tobacco seedsp. 332 microscope, prepared slides ofmale and female human cheek cells

Quick Demosp. 318 nonep. 323 nonep. 334 none

Need Materials? Contact Carolina Biological Supply Company at 1-800-334-5551or at http://www.carolina.com

Index to National Geographic MagazineThe following articles may be used forresearch relating to this chapter:“The Family Line: The Human-CatConnection,” by Stephen J. O’Brien, June1997.

Teacher’s Corner

314B314A

Patterns of Heredity and Human Genetics


TransparenciesReproducible MastersSection

MendelianInheritance ofHuman Traits

When HeredityFollows DifferentRules

ComplexInheritance ofHuman Traits

Section 12.1

Section 12.2

Section 12.3

Teacher Classroom Resources

Reinforcement and Study Guide, p. 51Critical Thinking/Problem Solving, p. 12BioLab and MiniLab Worksheets, p. 57Tech Prep Applications, pp. 19-20Content Mastery, pp. 57-58, 60

Reinforcement and Study Guide, pp. 52-53Concept Mapping, p. 12Laboratory Manual, pp. 83-86Content Mastery, pp. 57, 59-60

Reinforcement and Study Guide, p. 54BioLab and MiniLab Worksheets, pp. 58-60Laboratory Manual, pp. 87-90Content Mastery, pp. 57, 60 L1

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Section Focus Transparency 29Reteaching Skills Transparency 20

Section Focus Transparency 30Reteaching Skills Transparency 21

Section Focus Transparency 31

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Assessment Resources Additional ResourcesSpanish ResourcesEnglish/Spanish AudiocassettesCooperative Learning in the Science ClassroomLesson Plans/Block Scheduling

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Chapter Assessment, pp. 67-72MindJogger VideoquizzesPerformance Assessment in the Biology ClassroomAlternate Assessment in the Science ClassroomComputer Test BankBDOL Interactive CD-ROM, Chapter 12 quiz

Section 12.2

Section 12.1

Section 12.3

Refer to pages 4T-5T of the Teacher Guide for an explanation of the National Science Education Standards correlations.

Key to Teaching StrategiesKey to Teaching Strategies

Level 1 activities should be appropriatefor students with learning difficulties.Level 2 activities should be within theability range of all students.Level 3 activities are designed for above-average students.ELL activities should be within the abilityrange of English Language Learners.

Cooperative Learning activitiesare designed for small group work.These strategies represent student prod-ucts that can be placed into a best-workportfolio.These strategies are useful in a blockscheduling format.

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The following multimedia resources are available from Glencoe.

Biology: The Dynamics of LifeCD-ROM

Exploration: Trait InheritanceVideo: Fruit Fly GeneticsAnimation: Sex-linked TraitsExploration: Blood Types

Videodisc ProgramSex-linked Traits

The Infinite VoyageA Taste of HealthThe Geometry of Life

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http://www.carolina.com

Section

Making a PedigreeAt some point, you have probably

seen a family tree, either for yourfamily or for someone else’s. A familytree traces a family name and variousfamily members through successivegenerations. Through a family tree,you can trace your cousins, aunts,uncles, grandparents, and great-grandparents.

Pedigrees illustrate inheritanceGeneticists often need to map the

inheritance of genetic traits fromgeneration to generation. A pedigreeis a graphic representation of geneticinheritance. At a glance, it looks verysimilar to any family tree.

A pedigree is made up of a set ofsymbols that identify males and

females, individuals affected by thetrait being studied, and family rela-tionships. Some commonly usedsymbols are shown in Figure 12.1. Acircle represents a female; a square

12.1 MENDELIAN INHERITANCE OF HUMAN TRAITS 315

s you learn about traits, you willsee that some, such as tonguerolling or a widow’s peak hair-

line, are relatively harmless. Other traitsproduce devastating disorders and evendeath. All of these traits demonstrate howgenes are inherited, and this iswhat you need to learn. Thedisorders caused by genetictransmission of traits arethe motivation that drivesscientists to do research todiscover treatments andcures.

SECTION PREVIEW

ObjectivesInterpret a pedigree.Determine humangenetic disorders thatare caused by inheri-tance of recessive alleles.Predict how a humantrait can be determinedby a simple dominantallele.

Vocabularypedigreecarrierfetus

12.1 Mendelian Inheritance of Human Traits

Tongue rolling (above) andwidow’s peak hairline (inset)

Figure 12.1 Geneticists use thesesymbols to make andanalyze a pedigree.

Male Parents

Siblings

Knownheterozygotesfor recessiveallele

DeathMating

Female

Affectedmale

Affectedfemale

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Section 12.1

BIOLOGY: The Dynamics of Life SECTION FOCUS TRANSPARENCIES

Use with Chapter 12,Section 12.1

Which of these traits do you have?

For each trait, how would knowing your parents’phenotypes help you determine your genotype?

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Transparency Simple DominantHuman Traits29

Hanging earlobe,freckles

Rolled tongue,dimpled chin

Cheek dimples,widow’s peak

PrepareKey ConceptsThe section begins with a discus-sion of pedigrees and their inter-pretation. Then the inheritanceof autosomal recessive disorderssuch as cystic fibrosis, phenylke-tonuria, and Tay-Sachs disease isdescribed. The section closeswith a discussion of autosomaltraits such as tongue rolling,widow’s peak, and Huntington’sdisease.

Planning■ Obtain a long piece of wire for

the Getting Started Demo.

1 FocusBellringer Before presenting the lesson, display Section Focus Trans-parency 29 on the overhead pro-jector and have students answerthe accompanying questions.

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Assessment PlannerAssessment PlannerPortfolio Assessment

Portfolio, TWE, pp. 316, 321, 325, 330Problem-Solving Lab, TWE, p. 324Assessment, TWE, p. 330

Performance AssessmentProblem-Solving Lab, TWE, p. 317Assessment, TWE, pp. 318, 323, 328, 334Alternative Lab, TWE, p. 332-333MiniLab, SE, pp. 316, 333BioLab, SE, pp. 336-337

Alternative Lab, pp. 322-323, 332-333Knowledge Assessment

MiniLab, TWE, pp. 316, 333Problem-Solving Lab, TWE, p. 332Section Assessment, SE, pp. 320, 328, 335Chapter Assessment, SE, pp. 339-341

Skill AssessmentAssessment, TWE, pp. 320, 335Alternative Lab, TWE, pp. 322-323BioLab, TWE, pp. 336-337


What You’ll Learn■ You will compare the inheri-

tance of recessive and dominant traits in humans.

■ You will analyze the inheri-tance of incompletely domi-nant and codominant traits.

■ You will determine the inher-itance of sex-linked traits.

Why It’s ImportantThe transmission of traits fromgeneration to generation affectsyour appearance, your behavior,and your health. Understandinghow these traits are inherited isimportant in understanding thetraits you may pass on to afuture generation.

All in a Family Examine the family photo onthis page. Notice the physicaltraits of each family member,how they are similar, and howthey are different. How dogenes make these people looklike they do?

To find outmore about

human genetics, visit theGlencoe Science Web Site.www.glencoe.com/sec/science

12

GETTING STARTEDGETTING STARTED

ChapterChapter

It is difficult to imagine howthe information for such var-ied traits as eye or hair colorand athletic talent could becontained in the nucleic acidscomposing this chromosome.

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Magnification: 4500�

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Theme DevelopmentThe main theme of the chapter ishomeostasis, which is normallymaintained during the transmis-sion of genetic material but isdisrupted by the inheritance ofparticular alleles that result ingenetic disorders. The nature ofscience is illustrated by the con-cepts developed by Morgan as heworked with and interpreted datafrom sex-linked traits.

Chapter 12Chapter 12

MultipleLearningStyles

Look for the following logos for strategies that emphasize different learning modalities.

Kinesthetic Meeting IndividualNeeds, pp. 324, 326; Project,

p. 327Visual-Spatial Quick Demo, pp. 318, 334; Reteach, pp. 320,

335; Portfolio, p. 325; Meeting Indivi-dual Needs, p. 326; MicroscopeActivity, p. 330

Intrapersonal Meeting Individ-ual Needs, p. 317; Extension,

p. 327Linguistic Portfolio, pp. 316,330; Biology Journal, pp. 319,

331; Check for Understanding, p. 335Logical-Mathematical Activity,p. 328

GETTING STARTED DEMOGETTING STARTED DEMO

Many genes are needed tocode for all the traits shownby the family in the photo.Demonstrate how this largeamount of DNA can fit intothe small volume of a chromo-some by coiling a long piece of wire. First, coil the wire sothat it looks like a telephonecord, then coil the coiled wireto make it even more com-pact.

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If time does not permit teach-ing the entire chapter, use theBioDigest at the end of theunit as an overview.

http://www.glencoe.com/sec/science

would find, according to Mendeliansegregation, that the ratio ofhomozygous dominant to heterozy-gous to homozygous recessive geno-types among their children would be1: 2: 1. Of those genotypes possiblefor the members of generation II,only the homozygous recessive geno-type will express the trait, which isthe case for II-3.

You can’t tell the genotypes of II-4and II-5, but they have a normal phe-notype. If you look at the Punnettsquare you made, you can see thatthe probability of II-4 and of II-5being a carrier is each two out ofthree because they can have only twopossible genotypes—homozygousnormal and heterozygous. Thehomozygous recessive genotype isnot a possibility in these individualsbecause neither of them shows theaffected phenotype.

Because none of the children ingeneration III are affected andbecause the recessive allele is rare, itis reasonably safe to assume that II-1is not a carrier. You know that indi-vidual II-2 must be a carrier like herparents because she has passed on therecessive allele to subsequent genera-tion IV. Because individual III-1 hasone parent who is heterozygous andthe other parent who is assumed tobe homozygous normal, III-1 mostlikely has a one-in-two chance ofbeing a carrier. If her parent II-1 hadbeen heterozygous instead ofhomozygous normal, III-1’s chancesof being a carrier are increased totwo in three.

Simple RecessiveHeredity

Most genetic disorders are causedby recessive alleles. Many of thesealleles are relatively rare, but a few are common in certain ethnic


What are the chances?Using a Punnett squareallows you to calculatethe chance that off-spring will be born with certain traits. Inorder to do this, how-ever, you must firstknow the genotype ofthe parents and wheth-er the trait that is beingdescribed is dominant orrecessive.

AnalysisThe following traits and their alleles are to be used in

solving problems.

Thinking Critically1. What are the chances that a child will be born with six fin-

gers ifa. both parents are heterozygous?b. one parent has five fingers, the other is heterozygous?c. both parents have five fingers?

2. How many children in a family of four will be able to rolltheir tongues ifa. one parent is a nonroller and the other is a homozy-

gous roller?b. both parents are rollers and both are homozygous?c. both parents are rollers and each of them has a parent

who cannot roll his or her tongue?3. A child is born with cystic fibrosis but both parents are

normal.a. What are the genotypes and phenotypes for the

parents?b. What is the genotype and phenotype for the child?

Problem-Solving Lab 12-1Problem-Solving Lab 12-1

groups. You can practice calculatingthe chance that offspring will be bornwith some of these genetic traits in the Problem-Solving Lab above.

Trait Recessive alleleDominant allele

Number of fingers

Tongue rolling

Cystic fibrosis

d = five

t = cannot roll

c = disorder

D = six

T = can roll

C = normal

Table 12.1 Human traits

Polydactyly—having six fingers

Applying Concepts

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Purpose Students will use Punnett squaresto determine the chance of off-spring receiving certain traits.

Process Skillsapply concepts, draw a conclu-sion, predict, think critically

Teaching Strategies■ Review the technique for set-ting up, using, and completingPunnett squares.■ Review the terminology usedin describing traits: homozygous,heterozygous, dominant, reces-sive, genotype, phenotype.■ Review the meaning of thephrase “chance that” when refer-ring to the outcome of a Punnettsquare.

Thinking Critically1. a. three out of four

b. two out of fourc. no chance of child having

six fingers2. a. all will roll their tongues

b. all children will be rollersc. three out of four will be

rollers3. a. Both parents are Cc and

have normal phenotypes.b.The child is cc and has cys-

tic fibrosis.

Performance Take a classsurvey for tongue rolling. Ask stu-dents to predict their genotypes.Only those who cannot roll theirtongues can predict correctly. All oth-ers are either homozygous dominantor heterozygous. Use the Perform-ance Task Assessment List forConducting a Survey and Graph-ing the Results in PASC, p. 35.L1

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Problem-Solving Lab 12-1Problem-Solving Lab 12-1represents a male. Shaded circles and squares represent individualsshowing the trait being studied.Unshaded circles and squares desig-nate individuals that do not show thetrait. A half-shaded circle or squarerepresents a carrier, a heterozygousindividual. A horizontal line connect-ing a circle and a square indicatesthat the individuals are parents, and avertical line connects a set of parentswith their offspring. Each horizontalrow of circles and squares in a pedi-gree designates a generation, withthe most recent generation shown atthe bottom. The generations areidentified in sequence by Romannumerals, and each individual isgiven an Arabic number. You canpractice using these symbols to makea pedigree in the MiniLab on thispage.

Analyzing a pedigreeAn example of a pedigree for a fic-

titious rare, recessive disorder inhumans is shown in Figure 12.2.This disorder could be any of severalrecessive disorders in which the dis-order shows up only if the affectedperson carries two recessive allelesfor the trait. Follow the pedigree asyou read how to analyze it.

Suppose individual III-1 in thepedigree wants to know the likeli-hood of passing on this allele to herchildren. By studying the pedigree,the individual will be able to deter-mine the likelihood that she carriesthe allele. Notice that informationcan also be gained about other mem-bers of the family by studying thepedigree. For example, you knowthat I-1 and I-2 are both carriers ofthe recessive allele for the traitbecause they have produced II-3,who shows the recessive phenotype.If you drew a Punnett square for themating of individuals I-1 and I-2, you

316 PATTERNS OF HEREDITY AND HUMAN GENETICS

Illustrating a Pedigree The pedigree method of studying a trait in a family uses records of phenotypes extending for two or more generations. Studies of pedigrees can be used to yield a great deal of genetic information about a related group.

Procedure! Working with a partner, choose one human trait, such as

attached and free-hanging earlobes or tongue rolling,that interests both of you.

@ Using either your or your partner’s family, collect informa-tion about your chosen trait. Include whether each indi-vidual is male or female, does or does not have the trait,and the relationship of the individual to others in thefamily.

# Use your information to draw a pedigree for the trait.$ Try to determine how your trait is inherited.

Analysis1. What trait did you study? Can you determine from your

pedigree what the apparent inheritance pattern of thetrait is?

2. How is the study of inheritance patterns limited by pedi-gree analysis?

MiniLab 12-1MiniLab 12-1 Analyzing Information

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Figure 12.2 This pedigreeshows how arare, recessiveallele is trans-mitted fromgeneration togeneration.

Sample pedigree

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2 Teach

Purpose Students will observe a specifichuman trait and prepare a pedi-gree.

Process Skillsobserve and infer, interpret scien-tific illustrations

Teaching Strategies■ If any of your students areadopted, be sure he or she ispaired with a student who is notadopted and can contribute fam-ily information to the pair’s pedi-gree.■ Provide students with infor-mation about various humantraits that are inherited in a sim-ple Mendelian pattern.

Expected ResultsStudents will construct pedigreesof a human trait.

Analysis1. Answers may include earlobe

shape, widow’s peak, tonguerolling, or ability to tastePTC paper.

2. The number of individualsmay be too small to deter-mine the inheritance pattern.

Knowledge Provide apedigree that consists of only afew individuals and has enoughinformation so that students candetermine genotypes for thegiven phenotypes. Use thePerformance Task AssessmentList for Analyzing the Data inPASC, p. 27. L1

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MiniLab 12-1MiniLab 12-1

PortfolioPortfolio

Blue PeopleLinguistic Have students read “TheBlue People of Troublesome Creek,”

by Cathy Trost, Science 82, Nov. 1982, pp.34-39, and construct a pedigree from thearticle. These people have an autosomalrecessive gene that causes their skin toappear dark blue. Have students explainthis disorder.

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Human Genetic DisordersHave students make a notebook collectionof newspaper or magazine articles onhuman genetic disorders. Ask them towrite a paragraph or two about the sensi-tivity of these articles.

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MEETING INDIVIDUAL NEEDS MEETING INDIVIDUAL NEEDS

GiftedIntrapersonal Have interested stu-dents research a genetic disorder

commonly found in Amish populations,polydactyly, or other disorders not men-tioned in the chapter (achondroplasia,Marfan’s syndrome, albinism, galac-tosemia, or thalassemia are examples).P

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CD-ROMBiology: The Dynamics of LifeExploration: Trait Inheritance

Disc 2

Resource ManagerResource Manager

BioLab and MiniLab Work-sheets, p. 57

Section Focus Transparency 29and Master

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noticed PKU warnings on cans ofdiet soft drinks. Because most dietdrinks are sweetened with an artifi-cial sweetener that contains phenyl-alanine, a pregnant woman who ishomozygous recessive must limit herintake of diet foods.

Simple DominantHeredity

Unlike the inheritance of recessivetraits in which a recessive allele mustbe inherited from both parents for aperson to show the recessive pheno-type, many traits are inherited just asthe rule of dominance predicts.Remember that in Mendelian inheri-tance, a single dominant allele inher-ited from one parent is all that isneeded for a person to show thedominant trait.

Simple dominant traitsTongue rolling is one example of a

simple dominant trait. If you can rollyour tongue, you’ve inherited thedominant allele from at least one of your parents. A Hapsburg lip is

shown in Figure 12.4along with earlobetypes, another dominanttrait that is determinedby simple Mendelianinheritance. Having ear-lobes that are attached to thehead is a recessive trait (ff), whereasheterozygous (Ff) and homozygousdominant (FF) individuals have ear-lobes that hang freely.

There are many other human traitsthat are inherited by simple dominantinheritance. Figure 12.5 shows oneof these traits—hitchhiker’s thumb,the ability to bend your thumb tip


Figure 12.5Hitchhiker’s thumb isa dominant trait.

Figure 12.4 The allele F for freelyhanging earlobes (a) is dominant to theallele f for attachedearlobes (b). TheHapsburg lip, a pro-truding lower lip thatresults in a half-openmouth, has been tracedback to the fourteenthcentury through por-traits of members ofthe Hapsburg Dynastyof Europe (c).

a b

c

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Chalkboard ExamplePlace Punnett squares on thechalkboard to demonstrate possi-ble inheritance patterns of eachgenetic disorder described in thetext.

DisplayCollect articles and pamphlets onvarious genetic disorders andpost them on the bulletin board.The March of Dimes organiza-tion is a good source of materials.

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Cystic fibrosis Cystic fibrosis (CF) is the most

common genetic disorder amongwhite Americans. Approximately onein 20 white Americans carries therecessive allele, and one in 2000 chil-dren born to white Americans inher-its the disorder. Due to a defectiveprotein in the plasma membrane,cystic fibrosis results in the formationand accumulation of thick mucus inthe lungs and digestive tract. Physicaltherapy, special diets, and new drugtherapies have continued to raise theaverage life expectancy of CFpatients.

Tay-Sachs diseaseTay-Sachs (tay saks) disease is a

recessive disorder of the central ner-vous system. In this disorder, a reces-sive allele results in the absence of anenzyme that normally breaks down alipid produced and stored in tissuesof the central nervous system.Therefore, this lipid fails to break

down properly and accumulates inthe cells. The allele for Tay-Sachs isespecially common in the UnitedStates among the Amish people andamong Ashkenazic Jews, whoseancestors came from eastern Europe.Figure 12.3 shows a typical pedigreefor Tay-Sachs disease.

Phenylketonuria Phenylketonuria (fen ul keet un

YOOR ee uh), also called PKU, is arecessive disorder that results fromthe absence of an enzyme that con-verts one amino acid, phenylalanine,to a different amino acid, tyrosine.Because phenylalanine cannot bebroken down, it and its by-productsaccumulate in the body and result insevere damage to the central nervoussystem. The PKU allele is most com-mon among people whose ancestorscame from Norway or Sweden.

A homozygous PKU newbornappears healthy at first because itsmother’s normal enzyme level pre-vented phenylalanine accumulationduring development. However, oncethe infant begins drinking milk,which is rich in phenylalanine, theamino acid accumulates and mentalretardation occurs. Today, a PKUtest is normally performed on allinfants a few days after birth. Infantsaffected by PKU are given a diet thatis low in phenylalanine until theirbrains are fully developed. With thisspecial diet, the toxic effects of thedisorder can be avoided.

Ironically, the success of treatingphenylketonuria infants has resultedin a new problem. If a female who ishomozygous recessive for PKUbecomes pregnant, the high phenyl-alanine levels in her blood can dam-age her fetus—the developing baby.This problem occurs even if the fetusis heterozygous and would be pheno-typically normal. You may have


Figure 12.3 A study of familieswho have childrenwith Tay-Sachs dis-ease shows typicalpedigrees for traitsinherited as simplerecessives. Note thatthe trait appears toskip generations, acharacteristic of arecessive trait.

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Performance Assessmentin the Biology Classroom, p.15, Inheritance of Human Traits.Have students carry out thisactivity after they have learnedabout Mendelian inheritance.

Visual LearningFigure 12.3 Ask students whythis pedigree is a characteristic ofa recessive trait. An individualmust have two recessive alleles toshow a trait. Because these traits arerare, an affected individual mostlikely will mate with an individualwith two dominant alleles. The off-spring will be heterozygous and willnot show the trait. The trait mayreappear when two heterozygotesmate.

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Quick DemoQuick Demo

Visual-Spatial Draw aPunnett square on the

board showing the matingbetween two heterozygotesfor a trait such as Tay-Sachs dis-ease. Then draw a pedigree forthe trait. Explain to studentsthat a Punnett square predictsthe probability of inheriting atrait in one mating, whereas apedigree can follow a trait forgenerations.

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Note Internet addressesthat you find useful in

the space below for quick reference.

BIOLOGY JOURNAL BIOLOGY JOURNAL

Huntington’s DiseaseLinguistic Have students write aparagraph in which they discuss the

pros and cons of genetic testing for thisdisorder. Would they want to knowwhether they carry the allele if either par-ent had the disorder?

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VIDEODISCThe Secret of Life Pedigree/Family—PKU

!7;@VI"

VIDEODISCThe Infinite Voyage A Taste of Health

Genetic Links to Cholesterol(Ch. 6), 4 min.

The Infinite Voyage The Geometry of LifeHuntington’s Disease andInheritance of the Deadly Gene(Ch. 6), 6 min. 30 sec.

VIDEODISCVIDEOTAPEThe Secret of Life

Tinkering with our Genes:Genetic Medicine

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!7P((É~I"Resource ManagerResource Manager

Reteaching Skills Trans-parency 20 and Master

Concept Mapping, p. 12

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Tech Prep Applications, pp. 17-18

Reinforcement and StudyGuide, p. 51

Content Mastery, p. 58 L1

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Section AssessmentSection Assessment

Understanding Main Ideas1. In your own words, define the following

symbols used in a pedigree: a square, a circle, anunshaded circle, a shaded square, a horizontalline, and a vertical line.

2. Describe one genetic disorder that is inherited asa recessive trait.

3. How are the cause and onset of symptoms ofHuntington’s disease different from those of PKUand Tay-Sachs disease?

4. Describe one trait that is inherited as a dominantallele. If you carried that trait, would you neces-sarily pass it on to your children?

Thinking Critically5. Suppose that a child with free-hanging earlobes

has a mother with attached earlobes. Can a manwith attached earlobes be the child’s father?

6. Interpreting Scientific Illustrations Make a pedigree for three generations of a family that shows at least one member of each gen-eration who demonstrates a particular trait.Would this trait be dominant or recessive? Formore help, refer to Thinking Critically in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Huntington’s disease Huntington’s disease is a lethal

genetic disorder caused by a rare dom-inant allele. It results in a breakdownof certain areas of the brain. Noeffective treatment exists.

Ordinarily, a dominant allele withsuch severe effects would result indeath before the affected individualcould have children and pass the alleleon to the next generation. But becausethe onset of Huntington’s disease usu-ally occurs between the ages of 30 and50, an individual may have childrenbefore knowing whether he or shecarries the allele. A genetic test hasbeen developed that allows individu-als to check their DNA. Although thistest allows carriers to decide whetherthey want to have children and riskpassing the trait on to future genera-tions, it also places a tremendousburden on them in knowing they willdevelop the disease. For this reason,some people may choose not to betested. The pedigree in Figure 12.6shows a typical pattern of occurrenceof Huntington’s disease in a family.

Notice that every child of anaffected individual has a 50 percentchance of being affected and then a 50percent chance of passing the defec-tive allele to his or her own child.


Figure 12.6 A typical pedigree for a simple dominant trait such asHuntington’s disease shows the trait in each generationand equally distributed among males and females.

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backward more than 30 degrees. Astraight thumb is recessive. Otherdominant traits in humans includealmond-shaped eyes (round eyes arerecessive), thick lips (thin lips arerecessive), and the presence of hairon the middle section of your fingers.

3 AssessCheck for UnderstandingAsk students to summarize whythe study of genetics is importantto couples considering havingchildren.

ReteachVisual-Spatial Have studentsmake a table of the genetic

disorders described in this sec-tion, including the type of inheri-tance and the effects of thedisorder on an affected individ-ual.

ExtensionAsk student groups to contact thelocal March of Dimes organiza-tion to gather information on thehelp it gives individuals withgenetic disorders. Students cangive a report on their findings.

Skill Ask students to de-sign a handout that tells about agenetic disorder. Students coulddraw Punnett squares and illus-trate the chances of offspringinheriting the disorder from par-ents who are carriers.

4 CloseDiscussionDiscuss with students whethergenetic testing should be re-quired before obtaining a mar-riage license.

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Section AssessmentSection AssessmentSection Assessment1. A square represents a male, a circle

a female. An unshaded circle is an unaffected female. A shaded square isan affected male. A horizontal lineindicates two parents. A vertical lineindicates offspring (children).

2. Students could describe cystic fibrosis,Tay-Sachs, or phenylketonuria.

3. Huntington’s disease is an autosomaldominant disorder with onset betweenthe ages of 30 and 50, whereas PKUand Tay-Sachs disease are autosomalrecessive disorders with onset at birth.

4. Huntington’s disease, tongue rolling,widow’s peak, a Hapsburg lip are allexamples of dominant traits. If yourchildren inherit even one dominant

allele from you, they will express adominant trait.

5. The man cannot be the father becausethe child had to receive an allele forfree-hanging earlobes from one par-ent; the father would have to have atleast one dominant allele for this trait.

6. This trait would be dominant. SeeFigure 12.6 for a sample pedigree.320

Section

Complex Patterns of Inheritance

Patterns of inheritance that areexplained by Mendel’s experimentsare often referred to as simpleMendelian inheritance—the inheri-tance controlled by dominant andrecessive paired alleles. However,many inheritance patterns are morecomplex than those studied byMendel. As you will learn, most traitsare not simply dominant or recessive.The BioLab at the end of this chapterinvestigates a type of inheritance thatdoesn’t even involve chromosomes.

Incomplete dominance:Appearance of a third phenotype

When inheritance follows a pat-tern of dominance, heterozygous andhomozygous dominant individualsboth have the same phenotype.When traits are inherited in anincomplete dominance pattern,however, the phenotype of the het-erozygote is intermediate betweenthose of the two homozygotes. Forexample, if a homozygous red-flow-ered snapdragon plant (RR) is crossedwith a homozygous white-floweredsnapdragon plant (R'R'), all of the F1 offspring will have pink

12.2 WHEN HEREDITY FOLLOWS DIFFERENT RULES 321

ariations in the pattern ofinheritance explained by

Mendel became known soonafter his work was discovered. Whatdo geneticists do when observed pat-terns of inheritance, such as kernelcolor in this ear of corn, do not appearto follow Mendel’s laws? They oftenuse a strategy of piecing together bitsof a puzzle until the basis for theunfamiliar inheritance pattern isunderstood.

SECTION PREVIEW

ObjectivesDistinguish betweenincompletely dominantand codominant alleles.Compare multipleallelic and polygenicinheritance.Analyze the pattern ofsex-linked inheritance.Summarize how internal and externalenvironments affectgene expression.

Vocabularyincomplete dominancecodominant allelesmultiple allelesautosomesex chromosomesex-linked traitpolygenic inheritance

12.2 When Heredity FollowsDifferent Rules

The genetics of Indian corn (above)is often like a puzzle (inset).

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Section 12.2

PrepareKey ConceptsStudents are shown the differ-ence between codominance andincomplete dominance and aregiven examples of multiple-allelictraits, sex-linked traits, and poly-genic inheritance. The sectionends with a brief description ofhow internal and external envi-ronmental factors can affect theappearance of certain traits.

Planning■ Gather small pots, soil, and

lights for the BioLab.■ Make cardboard X and Y

chromosomes for MeetingIndividual Needs.

■ Buy mustard seeds for theProject.


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Use with Chapter 12,Section 12.2

What is the dominant flower color in the Mendelian cross?

How does the snapdragon cross differ from the Mendelian cross?

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Transparency Complex InheritancePatterns30

Purple

Purple Purple Purple Pink Pink Pink

Snapdragon CrossMendel’s Pea Plants

White Red White


CodominanceProvide students with practice workingwith codominance by having them deter-mine the phenotypes of offspring result-ing from the following crosses. (a) acheckered rooster mated to a checkeredhen; (b) a checkered rooster mated to awhite hen; (c) a checkered rooster matedto a black hen.

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Comparing Inheritance PatternsHave students show through a series ofPunnett squares how the genotypic andphenotypic ratios of the offspring woulddiffer if the trait for chicken feather color were inherited through Mendeliandominance and incomplete dominancecompared with the actual pattern ofcodominance.

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incompletely dominant. Notice, how-ever, that the heterozygote is neitherblack nor gray. Instead, all of the off-spring are checkered; some feathersare black and other feathers are white.In such situations, the inheritance pat-tern is said to be codominant.Codominant alleles cause the pheno-types of both homozygotes to be pro-duced in heterozygous individuals. Incodominance, both alleles areexpressed equally.

Multiple phenotypes from multiple alleles

Although each trait has only twoalleles in the patterns of heredity youhave studied thus far, it is commonfor more than two alleles to control atrait in a population. This is under-standable when you recall that a newallele can be formed any time a muta-tion occurs in a nitrogen base some-where within a gene. Although onlytwo alleles of a gene can exist withina diploid cell, multiple alleles for a

single gene can be studied in a popu-lation of organisms.

Traits controlled by more than twoalleles have multiple alleles. Thepigeons pictured in Figure 12.9show the effects of multiple allelesfor feather color. Three alleles of asingle gene govern their feathercolor, although each pigeon can haveonly two of these alleles. The num-ber of alleles for any particular trait isnot limited to three, and there areinstances in which more than 100


Figure 12.8When a certain vari-ety of black chicken iscrossed with a whitechicken, all of the off-spring are checkered.Both feather colorsare produced bycodominant alleles.

Figure 12.9 In pigeons, a single gene that con-trols feather color has three alleles.An enzyme that activates the pro-duction of a pigment is controlledby the B allele. This enzyme is lack-ing in bb pigeons.

The dominant BA

allele producesash-red coloredfeathers.

AA

The B allele produces wild-type blue feathers. B is domi-nant to b but recessive to BA.

BB

The allele b pro-duces a chocolate-colored feather andis recessive to bothother alleles.

CC

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data show? Is this ratio close to theexpected ratio of 1: 2: 1 with incom-plete dominance? ratio of phenotypeswill vary; not very close to the expectedratio

2. Why might your data not be close to a1: 2: 1 ratio? small sample size

3. What ratio of phenotypes do you getwhen using class totals? Why is thisratio closer to what was expected?close to 1: 2: 1; large sample size

Skill Have students assign lettersto represent the genotypes of the parentsand offspring. Then have them draw aPunnett square to show the expectedresults of a cross between plants withgreen leaves and plants with yellowleaves. Use the Performance TaskAssessment List for Scientific Drawingin PASC, p. 55.

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flowers, as shown in Figure 12.7.The intermediate pink form of thetrait occurs because neither allele of the pair is completely dominant.Note that the letters R and R', ratherthan R and r , are used to show that these alleles are incompletelydominant.

The new phenotype occurs becausethe flowers contain a colored pig-ment. The R allele codes for anenzyme that produces a red pigment.The R' allele codes for a defectiveenzyme that makes no pigment.Because the heterozygote has onlyone copy of the R allele, its flowersproduce only half the amount of redpigment that the flowers of the redhomozygote produce, and theyappear pink. The R'R' homozygotehas no normal enzyme, produces nored pigment, and appears white.

Note that the segregation of allelesis the same as in simple Mendelianinheritance. However, because nei-ther allele is dominant, the plants of

the F1 generation all have pink flow-ers. When pink-flowered F1 plantsare crossed with each other, the off-spring in the F2 generation appear ina 1: 2: 1 phenotypic ratio of red topink to white flowers. This resultsupports Mendel’s law of independentassortment, which states that the al-leles are inherited independently.

Codominance: Expression of both alleles

In chickens, black-feathered andwhite-feathered birds are homozy-gotes for the B and W alleles, respec-tively. Two different uppercase lettersare used to represent the alleles incodominant inheritance.

One of the resulting heterozygousoffspring in a breeding experimentbetween a black rooster and a whitehen is shown in Figure 12.8. Youmight expect that heterozygous chick-ens, BW, would be black if the patternof inheritance followed Mendel’s lawof dominance, or gray if the trait were


Figure 12.7 A Punnett square of snapdragon colorshows that the red snapdragon ishomozygous for the allele R, and thewhite snapdragon is homozygous for theallele R'. All of the pink snapdragons areheterozygous, or RR'.

RR'

R'

R'

R R

RR'

RR' RR'

All pink flowers

White(R'R')

Red(RR)

RR

R

R'

R R'

RR'

RR' R'R'

1 red: 2 pink: 1 white

Allpink

Red White

Pink(RR')

Pink(RR')

2 TeachDiscussionExhibit photos of a red shorthornbull, a white shorthorn cow, and aroan shorthorn cow. Tell studentsthat the roan cow, offspring ofthe red bull and the white cow,has both red and white hairs. Askstudents whether Mendel’s law ofdominance applies to the inheri-tance of coat color in cattle. Whyor why not? No, because a thirdphenotype appears when the red andwhite cattle are crossed, and traitsthat Mendel studied had only twophenotypes. Ask students what typeof inheritance pattern explainshow roan cattle express their coatcolor. codominance

Alternative Lab 12.1Incomplete Dominance

Purpose Students will observe the phenotypic ratiothat appears with incomplete dominance.Materials petri dish, paper toweling, tobacco seeds

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ProcedureGive the following directions to students.

1. Label the top of a petri dish with yourname and the date. Place several layersof paper toweling inside the dish.

2. Moisten the toweling with water andplace 20 tobacco seeds on it.

3. Cover the dish and place it where itwill receive light.

4. Check the seeds for the next 10 days.Keep the toweling moist but not

soaked. After 8-10 days, count thenumber of plants with green, yellow-green, and yellow leaves.

5. Design a data table to record yourresults and class totals.

6. Wash your hands after handling seeds.Expected ResultsData will approach a 1: 2: 1 ratio of pheno-types.Analysis

1. What ratio of phenotypes does your322


Copy the Punnett squaresdemonstrating incompletedominance in Figure 12.7 ontothe chalkboard using coloredchalk to demonstrate differentphenotypes. To demonstratecodominance, draw a Punnettsquare for a mating thatwould produce a checkeredchicken like the one shown inFigure 12.8.

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VIDEODISCThe Secret of LifeIncomplete Dominance

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The BioLab at theend of the chaptercan be used at thispoint in the lesson.

YOUR OWNDESIGN

YOUR OWNDESIGN



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probability, the ratio usually is notexactly 1: 1 in a small population.

Sex-linked inheritanceDrosophila (droh SAHF uh luh),

commonly known as fruit flies,inherit sex chromosomes in the sameway as humans do. Traits controlledby genes located on sex chromo-somes are called sex-linked traits.The alleles for sex-linked traits arewritten as superscripts of the X or Ychromosome. Because the X and Ychromosomes are not homologous,the Y chromosome has no corre-sponding allele to one on the X chro-mosome and no superscript is used.Also remember that any allele on theX chromosome of a male will not bemasked by a corresponding allele onthe Y chromosome.

In 1910, Thomas Hunt Morgandiscovered traits linked to sex chro-mosomes. Morgan noticed one daythat one male fly had white eyesrather than the usual red eyes. Hecrossed the white-eyed male with a

homozygous red-eyed female. All ofthe F1 offspring had red eyes, indi-cating that the white-eyed trait isrecessive. Then Morgan allowed theF1 flies to mate among themselves.According to simple Mendelianinheritance, if the trait were reces-sive, the offspring in the F2 genera-tion would show a 3: 1 ratio of red-eyed to white-eyed flies. As you cansee in Figure 12.11, this is whatMorgan observed. However, he alsonoticed that the trait of white eyesappeared only in male flies.

Morgan hypothesized that the red-eye allele was dominant and thewhite-eye allele was recessive. Healso reasoned that the gene for eyecolor was located on the X chromo-some and was not present on the Ychromosome. In heterozygousfemales, the dominant allele for redeyes masks the recessive allele forwhite eyes. In males, however, a sin-gle recessive allele is expressed as awhite-eyed phenotype. When Morgancrossed a heterozygous red-eyed


Figure 12.11 Morgan crossed a white-eyed male fruit flywith a normal homozygous red-eyed female(a). He then allowed the F1 flies to mate (b).The superscripts R and r are the dominant andrecessive alleles for eye color in fruit flies.

a b

XRXr

Xr Y

XR

XR

XRY

XRXr XRY

F1 All red eyed

White-eyedmale (XrY)

Red-eyedfemale (XRXR)

XRXR

XR Y

XR

Xr

XRY

XRXr XrY

F2

Females:all red eyed

Males:1/2 red eyed1/2 white eyed

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Visual LearningFigure 12.10 Ask students whythe X and Y chromosomes arenot homologous? They do not havethe same size, shape, or similargenes. However, they are able to pairduring meiosis because they have asmall homologous region at one endof each of the chromosomes.

Make it clear to students thatthere is a 50% chance that eachoffspring will receive XX chro-mosomes, which is female, and a50% chance that each offspringwill receive XY chromosomes,which is male.

ReinforcementDraw a Punnett square. Place sexchromosomes X and X along theleft side, and X and Y along thetop to represent gametes. Askstudents why each gamete hasonly one sex chromosome. Thesex chromosomes are separated dur-ing meiosis.

alleles are known to exist for a singletrait! You can learn about anotherexample of multiple alleles in theProblem-Solving Lab shown here.

Sex determination Recall that in humans the diploid

number of chromosomes is 46, or 23pairs. There are 22 pairs of matchinghomologous chromosomes calledautosomes. Homologous autosomeslook exactly alike. The 23rd pair ofchromosomes differs in males andfemales. These two chromosomes,which determine the sex of an indi-vidual, are called sex chromosomes.In humans, the chromosomes thatcontrol the inheritance of sex charac-teristics are indicated by the letters Xand Y. If you are a human female,XX, your 23rd pair of chromosomesare homologous and look alike, asshown in Figure 12.10A. However,if you are a male, XY, your 23rd pairof chromosomes look different.Males, which have one X and one Ychromosome, produce two kinds ofgametes, X and Y, by meiosis.Females have two X chromosomes,so they produce only X gametes.Figure 12.10B shows that after fertilization, a 1: 1 ratio of males to females is expected. Because fertil-ization is governed by the laws of


How is coat color in rabbits inherited? Coat color in rab-bits is inherited as a series of multiple alleles. This means thatthere can be more than just two alleles for a single gene. Inthe case of coat color in rabbits, there are four alleles, andeach one is expressed with a different phenotype.

AnalysisExamine Table 12.2. Use this information to answer the

questions. Remember, each rabbit can have only two allelesfor coat color.

Thinking Critically1. List all possible genotypes for a

a. dark gray-coated rabbit (there are 4).b. chinchilla rabbit (there are 3).c. Himalayan rabbit (there are 2).d. white rabbit (there is 1).

2. Predict the phenotype for a rabbit with a chcch and with aCch genotype. Explain your answer.

3. Would it be possible to obtain white rabbits if one parentis white and the other is chinchilla? Explain your answer.

4. Would it be possible to obtain chinchilla rabbits if oneparent is Himalayan and the other is white? Explain.

5. A chinchilla rabbit is mated with a Himalayan. Some off-spring are white. What are the parents’ genotypes?

Problem-Solving Lab 12-2Problem-Solving Lab 12-2 Predicting

Figure 12.10The sex chromo-somes in humans arecalled X and Y.

Phenotype Pattern of inheritanceAllele

Dark gray coat

Chinchilla

Himalayan

White

dominant to all other alleles

dominant to Himilayan and to white

dominant to white

recessive

C

cch

ch

c

Table 12.2 Coat color in rabbits

Half the offspring of any matingbetween humans will have two Xchromosomes, XX, which is female.The other half of the offspring willhave one X and one Y chromosome,XY, which is male.

BB

The sex chromo-somes are namedfor the lettersthey resemble.

AAX X X Y

MaleFemale XXFemale

XXFemale

XXFemale

X

X Y

X

XYMale

XYMale

XYMale

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Purpose Students will work with problemsthat deal with multiple alleles.

Process Skillsacquire information, apply con-cepts, predict, think critically

Teaching Strategies■ Have students sequence thegenotypes in order from domi-nant to recessive.■ Emphasize again that only twoalleles at a time govern coatcolor. However, there are 10 dif-ferent combinations that are pos-sible when dealing with fourdifferent alleles in a multipleallele situation.■ Have students define in theirown words the meaning of multi-ple allele.

Thinking Critically

1. a. CC, Ccch, Cch, Ccb. cchcch, cchch, cchcc. chch, chcd. cc

2. Chinchilla; the cch allele forchinchilla is dominant to thech allele for Himalayan.Dark gray; the C allele isdominant to all other alleles.

3. Yes, if the chinchilla rabbit iscchc.

4. No; Himalayan and whiterabbits have no alleles forchinchilla.

5. The chinchilla parent is cchcand the Himalayan parent ischc.

Portfolio Ask students toplan a breeding program thatwould produce all Himalayanrabbit offspring when startingwith one white parent and onedark gray parent known to beCcch. Punnett squares should beincluded where appropriate. Allwork should be placed in studentportfolios. Use the PerformanceTask Assessment List for De-signing an Experiment in PASC,p. 23. PP

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Problem-Solving Lab 12-2Problem-Solving Lab 12-2

MEETING INDIVIDUAL NEEDS MEETING INDIVIDUAL NEEDS

Learning Disabled/VisuallyImpaired

Kinesthetic Make large cardboard Xand Y chromosomes. Have students

manipulate them to see how each parentdonates one of his/her sex chromosomes toeach gamete and how they come togetherduring fertilization to produce offspring ofeach sex.

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PortfolioPortfolio

Eye ColorVisual-Spatial Provide studentswith a pedigree outline that

involves a sex-linked trait such as whiteeye color in Drosophila. Have studentsrecord above each symbol the sex chro-mosomes and alleles that each fly pos-sesses for the trait.

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VIDEODISCVIDEOTAPEThe Secret of Life

Sex and the Single Gene: Cell Development

!71Z%ÉLB"

will show a broad range of heights. APunnett square of this trihybrid crosswould show that 10-cm-tall plantsare most often expected, and thetallest and shortest plants are seldomexpected. Notice in Figure 12.12that when these results are graphed,the shape of the graph confirms theprediction of the Punnett square.

EnvironmentalInfluences

Even when you understand domi-nance and recessiveness and you havesolved the puzzles of the other pat-terns of heredity, the inheritance pic-ture is not complete. The geneticmakeup of an organism at fertiliza-tion determines only the organism’spotential to develop and function. Asthe organism develops, many factorscan influence how the gene isexpressed, or even whether the geneis expressed at all. Two such influ-ences are the organism’s external andinternal environments.

Influence of external environmentSometimes, individuals known to

have a particular gene fail to expressthe phenotype specified by that gene.Temperature, nutrition, light, chemi-cals, and infectious agents all can

influence gene expression. In certainbacteria, temperature has an effect onthe expression of color, as shown inFigure 12.13. External influencescan also be seen in leaves. Leaves ona tree can have different sizes andshapes depending on the amount oflight they receive.

Influence of internal environmentThe internal environments of

males and females are differentbecause of hormones and structuraldifferences, Figure 12.14. For exam-ple, traits such as horn size in moun-tain sheep, male-pattern baldness inhumans, and feather color in pea-cocks are expressed differently in the

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Figure 12.13 Serratia marcescens isa bacterium that formsbrick-red growth onsolid media at 25oC.When the same bacte-ria are grown at 30oC,the growth is creamcolored.

Figure 12.14 The horns of a ram(male) are muchheavier and morecoiled than those ofa ewe (female)although their geno-types are identical.

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3 AssessCheck for UnderstandingHave students explain how thefollowing differ from oneanother. (a) incomplete domi-nance and codominance; (b) mul-tiple alleles and polygenicinheritance; (c) autosomes andsex chromosomes; (d) sex-linkedtrait and autosomal trait.

ReteachAsk students to provide an exam-ple of each of the following pat-terns or types of inheritance: (a)incomplete dominance; (b)codominance; (c) multiple allelictrait; (d) polygenic trait; (e) sex-linked trait.

ExtensionIntrapersonal Have studentsdescribe and record some of

the changes associated withhuman aging that alter geneexpression. To get studentsstarted, provide them with a hintsuch as a change in hair colorfrom black or blond to gray. L2

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female with a white-eyed male, halfof all the males and half of all thefemales inherited white eyes. Theonly explanation of these results isMorgan’s hypothesis: The allele foreye color is carried on the X chromo-some and the Y chromosome has noallele for eye color.

Traits dependent on genes that follow the inheritance pattern of a sex chromosome are called sex-linked traits. Eye color in fruit flies is an example of an X-linked trait. Y-linked traits are passed only frommale to male.

Polygenic inheritanceSome traits, such as skin color and

height in humans, and cob length incorn, vary over a wide range. Suchranges occur because these traits aregoverned by many different genes.Polygenic inheritance is the inheri-tance pattern of a trait that is controlled by two or more genes.The genes may be on the same chro-mosome or on different chromo-somes, and each gene may have twoor more alleles. For simplicity,uppercase and lowercase letters are

used to represent the alleles, as theyare in Mendelian inheritance. Keepin mind, however, that the allele rep-resented by an uppercase letter is notdominant. All heterozygotes areintermediate in phenotype.

In polygenic inheritance, eachallele represented by an uppercaseletter contributes a small, but equal,portion to the trait being expressed.The result is that the phenotypesusually show a continuous range ofvariability from the minimum valueof the trait to the maximum value.

Suppose, for example, that stemlength in a plant is controlled bythree different genes: A, B, and C.Each gene is on a different chromo-some and has two alleles, which canbe represented by uppercase or low-ercase letters. Thus, each diploidplant has a total of six alleles for stemlength. A plant that is homozygousfor short alleles at all three gene loca-tions (aabbcc) might grow to be only 4cm tall, the base height. A plant thatis homozygous for tall alleles at allthree gene locations (AABBCC)might be 16 cm tall. The differencebetween the tallest possible plant and

the shortest possible plant is12 cm, or 2 cm per each ofthe six tall alleles. You couldsay that each allele repre-sented by an uppercase lettercontributes 2 cm to the totalheight of the plant.

Suppose a 16-cm-tall plantwere crossed with a 4-cm-tallplant. In the F1 generation,all the offspring would beAaBbCc. If each of the threetall genes A, B, and C con-tributed 2 cm of height tothe base height of 4 cm, theplants would be 10 cm tall (4cm + 6 cm)—intermediate inheight. If they are allowed tointerbreed, the F2 offspring

Figure 12.12 In this example ofpolygenic inheritance,three genes each havetwo alleles that con-tribute to the trait.When the distributionof plant heights isgraphed, a bell-shapedcurve is formed.Intermediate heightsoccur most often.

Stem length (cm)

Expe

cted

num

ber o

f F2 p

lant

s

4 6 8 10 12 14 16

5

10

0

15

20

Stem Length Variation in a Plant Polygenic for the Trait

OriginWORDWORD

polygenicFrom the Greekwords polys, mean-ing “many,” andgenos, meaning“kind.” Polygenicinheritance involvesmany genes.


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MisconceptionStudents may believe that all thealleles present on the X and Ychromosomes are related tomaleness or femaleness. This isnot the case, however. Explainthat the alleles for blood clottingand color vision are located onhuman sex chromosomes. Thesetraits have little to do with beingmale or female.

MMEETING EETING IINDIVIDUAL NDIVIDUAL NNEEDS EEDS MEETING INDIVIDUAL NEEDS

Visually ImpairedKinesthetic Provide large cutouts ofleaves of different sizes. Have students

group the leaves according to size, thencount the number of leaves in each pile andmake a bar graph. A partner can record thedata and make a tactile bar graph for thevisually impaired student.

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GiftedVisual-Spatial Have students illustratethe cross between two plants that are

each heterozygous for three genes that control a single trait using a Punnett squarethat consists of 64 squares. Have studentsdetermine the gametes that appear along theside and top of the square, using AaBbCc asthe genotype for each parent.

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P R O J E C TGenes and the Environment

Kinesthetic Have student groups ger-minate mustard seeds (available from

the condiment section of a grocery store) inpetri dishes that have been lined with papertoweling. They should moisten the towelingwith water and add 20 seeds. Cover the dishand place it in the dark. Examine the

seedlings after about 7 days. All seedlingswill be white. Remove the dishes from thedark, place them in bright light for 24 hours,and then note their color. Have studentswrite a report in which they explain how thisproject illustrates the role of the environ-ment in influencing gene expression.

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CD-ROMBiology: The Dynamicsof Life

Video: Fruit Fly Genetics Disc 2


Reteaching Skills Trans-parency 21 and Master

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sexes, as you can see in Figure 12.15.These differences are controlled bydifferent hormones, which are deter-mined by different sets of genes.

An organism’s age also affects genefunction. The nature of such a pat-tern is not well understood, but it isknown that the internal environmentof an organism changes with age.

You can now see that you mustlearn how genes interact with eachother and with the environment toform a more complete picture ofinheritance. Mendel’s idea that hered-ity is a composite of many individualtraits still holds. Later researchershave filled in more details of Mendel’sgreat contributions.



Understanding Main Ideas1. A cross between a purebred animal with red hairs

and a purebred animal with white hairs producesan animal that has both red hairs and white hairs.What type of inheritance pattern is involved?

2. In a cross between individuals of a species oftropical fish, all of the male offspring have long tail fins, and none of the females possessthe trait. Mating of the F1 fish fails to producefemales with the trait. Explain a possible inheri-tance pattern of the trait.

3. A red-flowered sweet pea plant is crossed with awhite-flowered sweet pea plant. All of the off-spring are pink. What is the inheritance patternbeing expressed?

4. The color of wheat grains shows a wide vari-ability between red and white with multiple

phenotypes. What type of inheritance pattern isbeing expressed?

Thinking Critically5. Armadillos always have four offspring that have

identical genetic makeup. Suppose that, within alitter, each young armadillo is found to have adifferent phenotype for a particular trait. Howcould you explain this phenomenon?

6. Forming a Hypothesis An ecologist observesthat a population of plants in a meadow hasflowers that may be red, yellow, white, pink, orpurple. Hypothesize what the inheritance pat-tern might be. For more help, refer to PracticingScientific Methods in the Skill Handbook.


Figure 12.15Some traits areexpressed differentlyin the sexes.

Human male-patternbaldness, prematurebalding that occurs in a characteristic pattern,affects males but notfemales.

CC

The plumage of thefemale peahen is dullby comparison.

BB

The plumage ofthe male peacockis highly decoratedand colored.

AA

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Performance Ask studentsto illustrate through the use ofPunnett squares how sex-linkedinheritance differs from autoso-mal inheritance. They should usethe same allele letters and par-ental genotypes.

4 CloseActivity

Logical-Mathematical Askstudents to prepare geno-

types that illustrate the differencebetween Mendelian and incom-plete dominance in radish rootshape. Radish roots are long,oval, or round. Have students usethe symbols L, l, and L�. ForMendelian dominance, the geno-types are LL and Ll for long andll for short. For incomplete dom-inance, the genotypes are LL forlong, LL� for oval, and L�L� forround.

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Section AssessmentSection AssessmentSection Assessment1. Alleles for both red and white hairs are

expressed, which is typical of a pattern ofcodominance.

2. The inheritance pattern of this trait isthat of a sex-linked gene located on theY chromosome, which is possessed bymales only.

3. Incomplete dominance is being expressedbecause the heterozygous plant is an

intermediate form between red andwhite.

4. polygenic inheritance5. The environment in the mother’s uterus

before birth may have been different foreach of the offspring. The external envi-ronment can affect gene expression.

6. This is probably a case of multiple allelicor polygenic inheritance.


Reinforcement and StudyGuide, pp. 52-53

Content Mastery, p. 59L1

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Section

Codominance in Humans

Remember that in codominance,the phenotypes of both homozygotesare produced in the heterozygote.One example of this type of inheri-tance in humans is the disordersickle-cell anemia.

Sickle-cell anemiaSickle-cell anemia is a major health

problem in the United States and inAfrica. In the United States, it ismost common in black Americanswhose families originated in Africaand in white Americans whose fami-lies originated in the countries

surrounding the Mediterranean Sea.About one in 12 African Americans, amuch larger proportion than in mostpopulations, is heterozygous for thedisorder.

In an individual who is homozy-gous for the sickle-cell allele, theoxygen-carrying protein hemoglobindiffers by one amino acid from nor-mal hemoglobin. This defectivehemoglobin forms crystal-like struc-tures that change the shape of the redblood cells. The abnormal red bloodcells are shaped like a sickle, or half-moon. The change in shape occurs inthe body’s narrow capillaries after thehemoglobin releases oxygen to thecells. Abnormally shaped blood cells,

12.3 COMPLEX INHERITANCE OF HUMAN TRAITS 329

For decades, movies and lit-erature have portrayedtimes when people could

genetically program their futureoffspring to have specific charac-teristics. You have probablythought about how you wouldchange certain of your featuresif given the opportunity. As sci-entists study the inheritance oftraits such as height andeye color, they are dis-covering that thepassing of thesetraits can be verycomplex.

SECTION PREVIEW

ObjectivesCompare codominance,multiple allelic, sex-linked, and polygenic patterns of inheritance in humans.Distinguish among conditions in which extra autosomal or sexchromosomes exist.

Vocabularykaryotype

12.3 Complex Inheritance of Human Traits

Eye color and heightare complex traits.

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Section 12.3

PrepareKey ConceptsComplex inheritance patterns inhumans are presented. Sickle-cellanemia is used as an example ofcodominance, blood types as anexample of multiple allelic inheri-tance, color-blindness and hemo-philia as examples of sex-linkedpatterns, and skin color as anexample of polygenic inheritance.The section concludes with a dis-cussion of changes in chromo-some numbers.

Planning■ Obtain magnifying glasses and

colored pencils for MiniLab12-2.

■ Obtain color blindness testingcharts for Meeting IndividualNeeds.


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Cultural DiversityMary Styles HarrisIn your discussion of sickle-cell anemia, pointout contributions of contemporary AfricanAmerican scientists in the treatment and eti-ology of this disorder. One of the moreprominent workers in this field has beengeneticist Mary Styles Harris (born 1949).From 1977 to 1979, Harris was the executive

director of the Sickle Cell Foundation ofGeorgia, and she has published many paperson this subject. In 1980, Harris was honoredas one of Glamour magazine’s OutstandingWomen Scientists. Around this time, she alsowrote, narrated, and produced an educa-tional science series for Georgia TV through agrant from the National Science Foundation.


The ABO Blood Group

The gene for blood type, gene I, codes for a mem-brane protein found on the surface of red blood

cells. Each of the three alleles codes for a different pro-tein. Your immune system recognizes the red blood cellsas belonging to you. If cells with a different protein enteryour body, your immune system will attack them.

Critical Thinking If you inherit ii from your parents,what is your blood type?

Magnification: 21 600�

Red blood cells

INSIDESSTORTORYY

INSIDE

Phenotype A The IA allele is dominant toi, so inheriting eitherthe IAi alleles or the IA IA alleles from your two parentswill give you type Ablood. Surface proteinA is produced.

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Phenotype B The IB allele is also dominantto i. To have type B blood, you must inheritthe IB allele from one par-ent and either another IB

allele or the i allele fromthe other. Surface proteinB is produced.

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Phenotype O The i allele isrecessive and produces no sur-face molecule. Therefore, ifyou are homozygous ii, your

blood cells have nosurface proteins

and you haveblood type O.

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Genotypes Surface Proteins Phenotypes

IAIA or IAi A A

IBIB or IBi B B

IAIB A and B AB

ii none O

Table 12.3 Human blood types

Phenotype AB The IA and IB alleles arecodominant to each other. This means that ifyou inherit the IA allele from one parent andthe IB allele from the other, your red bloodcells will produce both surface proteins andyou will have type AB blood.

33

Surface protein A

I A I A

orIAi

Surface protein B

I B I B

orIBi

Surface protein B

Surface protein A

I A I B

i i

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IINSIDENSIDESSTORTORYY

INSIDE

Figure 12.16, slow blood flow, blocksmall vessels, and result in tissuedamage and pain. Because sickledcells have a shorter life span thannormal red blood cells, the personsuffers from anemia, a condition inwhich there is a low number of redblood cells.

Individuals who are heterozygousfor the allele produce both normaland sickled hemoglobin, an exampleof codominance. They produceenough normal hemoglobin that theydo not have the serious health prob-lems of those homozygous for theallele and can lead relatively normallives. Individuals who are heterozy-gous are said to have the sickle-celltrait because they can show somesigns of sickle-cell anemia if theavailability of oxygen is reduced.

Multiple Alleles in Humans

Traits that are governed by simpleMendelian heredity have only twoalleles. However, you have learnedthat more than two alleles of a geneare possible for certain traits. TheABO blood group is a classic example

of a single gene that has multiplealleles in humans. How many allelesdoes this gene have? Read the InsideStory to answer this question.

Multiple alleles govern blood typeHuman blood types, listed in

Table 12.3, are determined by thepresence or absence of certain mole-cules on the surfaces of red bloodcells. As the determinant of bloodtypes A, B, AB, and O, the gene I hasthree alleles: IA, IB, and i.

The importance of blood typingDetermining blood type is neces-

sary before a person can receive ablood transfusion because the redblood cells of incompatible bloodtypes could clump together, causingdeath. Blood typing can also be help-ful in solving cases of disputedparentage. For example, if a child hastype AB blood and its mother hastype A, a man with type O bloodcould not possibly be the father. Butblood tests cannot prove that a cer-tain man definitely is the father; theyindicate only that he could be. DNAtests are necessary to determineactual parenthood.


Figure 12.16 In sickle-cell anemia,the gene for hemo-globin produces aprotein that is differ-ent by one aminoacid. This hemoglo-bin crystallizes whenoxygen levels arelow, forcing the redblood cells into asickle shape (a). Anormal red blood cellis disc shaped (b).

a bMagnification: 90 000� Magnification: 90 000�

330

2 TeachMicroscope Activity

Visual-Spatial Have studentsview a prepared slide of

sickled blood cells and compare itwith a slide of normal blood cells.

Portfolio Have studentswrite a short paragraph explainingwhy it is so important to typeblood before it is used for a trans-fusion. PP

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PortfolioPortfolio

Alzheimer’s DiseaseLinguistic Have students researchthe connection between chromo-

some 21 and Alzheimer’s disease. Askthem to include a copy of their findings intheir portfolios. PP

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“Improved” ChildrenLinguistic Some day, scientists mayhave the capability to change genes,

making offspring smarter, physicallystronger, and better looking. Ask studentsto write a short essay on whether theywould have their genes changed to“improve” their children if they weregiven the opportunity.

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VIDEOTAPEThe Secret of Life

It’s in the Genes: EvolutionCD-ROMBiology: The Dynamicsof Life

Exploration: Blood Types Disc 5

VIDEODISCThe Secret of LifeBlood Types

Biology: The Dynamics of LifeSex-Linked Traits (Ch. 35)

Disc 1, Side 1, 1 min. 49 sec.

CD-ROMBiology: The Dynamicsof Life

Animation: Sex-Linked Traits Disc 2

Purpose Students will learn the geneticbasis of ABO blood types

Teaching Strategies■ Give students a genotype andask them to identify the pheno-type on red blood cells.

Visual Learning■ Have students create a table

with genotypes as the head-ings. Under each genotype,have them correctly place theresulting phenotype.

■ Ask students to draw red bloodcells with cell surface proteinsin their journals. Have themdraw red blood cells for allfour blood types and writepossible genotypes underneaththe cells.

Critical ThinkingYou will be blood type O.

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Critical Thinking/ProblemSolving, p. 12


Laboratory Manual, pp. 83-86 L2

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colors. Red-green color blindnesswas first described in a boy whocould not be trained to harvest onlythe ripe, red apples from his father’sorchard. Instead, he chose greenapples as often as he chose red.

Other more serious problems canresult from this disorder, such as theinability of color-blind people toidentify red and green traffic lights bycolor. Color blindness is caused bythe inheritance of either of two reces-sive alleles at two gene sites on the Xchromosome that affect red and greenreceptors in the cells of the eyes.

Hemophilia: An X-linked disorderDid you ever wonder about why a

cut stops bleeding so quickly? Thishuman adaptation is essential. If yourblood didn’t have the ability to clot atall, any cut could take a long time tostop bleeding. Of greater concernwould be internal bleeding resultingin a bruise, which a person may notimmediately notice.

Hemophilia A is an X-linked disor-der that causes just such a problemwith blood clotting. About one malein every 10 000 has hemophilia, butonly about one in 100 million femalesinherits the same disorder. Why?Males inherit the allele for hemo-philia on the X chromosome fromtheir carrier mothers. A single reces-sive allele for hemophilia will causethe disorder in males. Females wouldneed two recessive alleles to inherithemophilia. The family of QueenVictoria, pictured in the Social StudiesConnection at the end of this chapter,is the best-known study of hemo-philia A, also called royal hemophilia.

Hemophilia A can be treated withblood transfusions and injections ofFactor VIII, the blood-clottingenzyme that is absent in peopleaffected by the condition. However,both treatments are expensive. New

methods of DNA technology arebeing used to develop a cheapersource of the clotting factor.

Polygenic Inheritance in Humans

Think of all the traits you inher-ited from your parents. Althoughmany of your traits were inheritedthrough simple Mendelian patternsor through multiple alleles, manyother human traits are determined bypolygenic inheritance. These kinds oftraits usually represent a range ofvariation that is measurable. TheMiniLab shown here examines one ofthese traits—the color variations inhuman eyes.


Detecting Colors and Patterns inEyes Human eye color, like skincolor, is determined by polygenicinheritance. You can detect severalshades of eye color, especially if youlook closely at the iris with a magni-fying glass. Often, the pigment isdeposited so that light reflects fromthe eye, causing the iris to appearblue, green, gray, or hazel (brown-green). In actuality, the pigment maybe yellowish or brown, but not blue.

Procedure! Use a magnifying glass to observe the patterns and colors

of pigment in the eyes of five classmates.@ Use colored pencils to make drawings of the five irises.# Describe your observations in your journal.

Analysis1. How many different pigments were you able to detect in

each eye?2. From your data, do you suspect that eye color might not

be inherited by simple Mendelian rules? Explain.3. Suppose that two people have brown eyes. They have two

children with brown eyes, one with blue eyes, and onewith green eyes. What pattern might this suggest?

MiniLab 12-2MiniLab 12-2

Hazel eye color

Observing and InferringPurpose Students will observe the colorsand patterns in the eyes of severalclassmates in order to hypothe-size how eye color is inherited.

Process Skillsobserve and infer, recognizecause and effect

Teaching Strategies■ Provide students with fivecards, each with a circle and aline under the circle. Studentsshould write the name of the per-son on the line and then draw theiris within the circle. Provide col-ored pencils.

Expected ResultsStudents will detect many differ-ent pigments, suggesting poly-genic inheritance.

Analysis1. Students may detect brown,

blue, yellow, gray, green, andblack pigments in each eye.

2. Eye color is probably notinherited by simple Men-delian rules because there areso many phenotypes.

3. The pattern suggests poly-genic inheritance.

Knowledge Ask studentsto write a paragraph telling howthey could determine the numberof genes involved in the inheri-tance of eye color. Use thePerformance Task AssessmentList for Writing in Science inPASC, p. 87. L2

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the microscope.2. Examine cells under high power. A Barr

body will be seen in each cell as adarkly stained mass just inside thenuclear membrane.

3. Examine the slide of male cheek cellsfor the presence of Barr bodies.

4. Draw a cell with a Barr body and onewithout a Barr body.

5. Wash your hands when you have fin-ished your observations.

Expected ResultsStudents will see Barr bodies in cells fromfemales but not in those from males.Analysis

1. Do all your body cells have Barr bod-ies? yes for females; no for males

2. What percentage of a female’s cellshave visible Barr bodies? 100%

3. How many Barr bodies would you findin a cell of an XXX female? 2

Performance Students shouldinclude a summary of the lab, theirdrawings, and the answers to theAnalysis questions in their journals. Usethe Performance Task Assessment Listfor Lab Report in PASC, p. 47. L3

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333

Sex-Linked Traits in Humans

Several human traits are deter-mined by genes that are carried on thesex chromosomes; most of these genesare located on the X chromosome.The pattern of sex-linked inheritanceis explained by the fact that males,who are XY, pass an X chromosometo each of their daughters and a Ychromosome to each son. Females,who are XX, pass one of their X chro-mosomes to each child, Figure 12.17.If a son receives an X chromosomewith a recessive allele from hismother, he will express the recessivephenotype because he has no chanceof inheriting from his father a domi-nant allele that would mask theexpression of the recessive allele.

Two traits that are governed by X-linked inheritance in humans arecertain forms of color blindness andhemophilia. Determine whetherDuchenne’s muscular dystrophy issex-linked by reading the Problem-Solving Lab on this page.

Red-green color blindnessPeople who have red-green color

blindness can’t differentiate these two


How is Duchenne’smuscular dystrophyinherited? Muscular dystrophy is oftenthought of as a single disorder, but it is actuallya group of genetic disor-ders that produce muscu-lar weakness, progressivedeterioration of musculartissue, and loss of coordi-nation. Different forms ofmuscular dystrophy canbe inherited as a domi-nant autosomal, a reces-sive autosomal, or a sex-linked disorder. These three patterns of inheritanceappear different from one another when a pedigree is made.One rare form of muscular dystrophy, called Duchenne’s mus-cular dystrophy, affects three in 10 000 American males.

AnalysisThe pedigree shown here represents the typical inheri-

tance pattern for Duchenne’s muscular dystrophy. Refer to Figure 12.1 if you need help interpreting the symbols.Analyze the pedigree to determine the pattern of inheri-tance. Is this an autosomal or a sex-linked disorder?

Thinking CriticallyIf individual IV-1 had a daughter and a son, what would

be the probability that the daughter is a carrier? That the soninherited the disorder?

Problem-Solving Lab 12-3Problem-Solving Lab 12-3 Drawing a Conclusion

Figure 12.17 If a trait is X-linked,males pass the X-linked allele to all oftheir daughters butto none of their sons(a). Heterozygousfemales have a 50percent chance ofpassing a recessive X-linked allele to eachchild (b).

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1 2 3 4

2 3 4 5

2

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I

II

III

IV

Typical pedigree

XY XYAA BB

FemaleMale

Sperm Eggs

XY

Male

XY

Male

XX

Female

XXXX

XX Y X X

Female

XXXX

Eggs SpermXX YXXX

Female Male

XYXY

Male

XY

Male

XX

Female

XXXX

Female

XX

XYa b

Alternative Lab 12.2Barr Bodies

PurposeStudents will locate Barr bodies in cells.Materials prepared slides of male and female cheekcells, microscope

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BackgroundFemales have two X chromosomes, butearly in development of a female embryo,one of the X chromosomes in each cellbecomes inactive. The inactive chromosomebecomes condensed and can be seen as aBarr body. Cells of males do not containBarr bodies.ProcedureGive students the following directions.

1. Place the slide of female cheek cell on332

Problem-Solving Lab 12-3Problem-Solving Lab 12-3 MiniLab 12-2MiniLab 12-2

PurposeStudents will determine the pat-tern of inheritance for Duch-enne’s muscular dystrophy.

Process Skillsobserve and infer, recognizecause and effect

BackgroundBecause various forms of muscu-lar dystrophy can be inherited asan autosomal dominant, an auto-somal recessive, or a sex-linkeddisorder, a family pedigree mustbe analyzed to determine theexact pattern of inheritance.From the pedigree, the mode ofinheritance of Duchenne’s mus-cular dystrophy can be inferredto be X-linked.

Teaching Strategies■ Ask students which individualsin the pedigree were keys todetermining the type of inheri-tance involved.

Thinking CriticallyThere is a 100% probability thatthe daughter will be a carrierbecause the only X chromosomeshe can inherit from her fatherwould have the defective gene.The son would not inherit thedisorder because his X chromo-some would come from hismother, who presumably is not acarrier.

Knowledge Ask studentswhat mating would have to occurto produce a female child with Duchenne’s muscular dystrophy.mating a female carrier or anaffected female with an affected male Use the Performance TaskAssessment List for Formulatinga Hypothesis in PASC, p. 21. L2

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according to length and location ofthe centromere, as Figure 12.19shows. This chart of chromosomepairs is called a karyotype, and it isvaluable in pinpointing unusual chro-mosome numbers in cells.

Down syndrome: Trisomy 21Most disorders of chromosome

number that occur in humans causesymptoms so severe that the develop-ing fetus dies, often before thewoman even realizes she is pregnant.Fortunately, these disorders occuronly rarely. Down syndrome is theonly autosomal trisomy in whichaffected individuals survive to adult-hood. It occurs in about one in 700live births.

Down syndrome is a group ofsymptoms that results from trisomyof chromosome 21. Individuals whohave Down syndrome have at leastsome degree of mental retardation.The incidence of Down syndromebirths is higher in older mothers,especially those over 40.

Unusual numbers of sex chromosomes

Many abnormalities in the numberof sex chromosomes are known to

exist. An X chromosome may bemissing (designated as XO) or theremay be an extra one (XXX or XXY).There may also be an extra Y chro-mosome (XYY), as you can see byexamining Figure 12.19. Any indi-vidual with at least one Y chromo-some is a male, and any individualwithout a Y chromosome is a female.Most of these individuals lead normallives, but they cannot have childrenand some have varying degrees ofmental retardation.


Figure 12.19 This karyotypedemonstrates XYYsyndrome, wheretwo Y chromosomesare inherited in addi-tion to an X chromo-some.


Understanding Main Ideas1. Why are sex-linked traits such as red-green color

blindness and hemophilia more commonly found in males than in females? Explain your answer in terms of the X chromosome.

2. In addition to revealing chromosome abnormalities,what other information would a karyotype show?

3. What would the genotypes of parents have to be for them to have a color-blind daughter? Explain.

4. Describe a genetic trait in humans that is inherited as codominance. Describe the phenotypes of the two homozygotes and that of the heterozygote. Why is this trait an example of codominance?

Thinking Critically5. A man is accused of fathering two children, one

with type O blood and another with type Ablood. The mother of the children has type Bblood. The man has type AB blood. Could he bethe father of both children? Explain your answer

6. Making and Using Tables Construct a table ofthe traits discussed in this section. For columnheads, use Trait, Pattern of inheritance, andCharacteristics. For more help, refer toOrganizing Information in the Skill Handbook.


3 AssessCheck for Understanding

Linguistic Ask students tosummarize the patterns

shown by multiple allelic, poly-genic, and sex-linked inheritance.

ReteachVisual-Spatial Have studentsdraw the phases of meiosis

to demonstrate how variouschanges in chromosome numbersoccur.

ExtensionAsk students to research the rare sex-linked disorder severe combined immune deficiency(SCID). The “boy in the bubble”had this disorder. What experi-mental treatments are currentlybeing used to treat or cure thisdisorder?

Skill Provide students withfilled-in Punnett squares showinggenotypes and phenotypes of off-spring. Have students determinethe pattern of inheritance fromthe information given.

4 CloseDiscussionAsk students to explain whyhemophilia is extremely rare infemales. Because the allele is rare inthe general population, there is onlya small likelihood that a male withhemophilia would marry a carrierfemale or a female with hemophilia.

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LSSkin color: A polygenic trait

In the early 1900s, theidea that polygenic inheritance

occurs in humans was first testedusing data collected on skin color.Scientists found that when light-skinned people mate with dark-skinned people, their offspring haveintermediate skin colors. When thesechildren produce the F2 generation,the resulting skin colors range fromthe light-skin color to the dark-skincolor of the grandparents (the P1 gen-eration), with most children having anintermediate skin color. As Figure12.18 shows, the variation in skincolor indicates that between threeand four genes are involved.

Changes inChromosome Numbers

You have been reading about traitsthat are caused by one or severalgenes on chromosomes. What wouldhappen if an entire chromosome orpart of a chromosome were missingfrom the complete set? What if cells

had an extra chromosome? As youhave learned, abnormal numbers ofchromosomes usually, but not always,result from accidents of meiosis.Many abnormal phenotypic effectsresult from such mistakes.

Unusual numbers of autosomesYou know that a human usually has

23 pairs of chromosomes, or 46 chro-mosomes altogether. Of these 23pairs of chromosomes, 22 pairs areautosomes. Humans who have anunusual number of autosomes all aretrisomic—that is, they have three of aparticular autosome instead of justtwo. In other words, they have 47chromosomes. Recall that trisomyusually results from nondisjunction,which occurs when paired homolo-gous chromosomes fail to separateproperly during meiosis.

To identify an abnormal number ofchromosomes, a sample of cells isobtained from an individual or froma fetus. Metaphase chromosomes arephotographed, and the chromosomepictures are then enlarged, cut apart,and arranged in pairs on a chart


b

OriginWORDWORD

autosomeFrom the Greekwords autos, mean-ing “self,” and soma,meaning “body.” Anautosome is a chro-mosome that is nota sex chromosome.

Num

ber o

f ind

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Classes of skin color

4 genes

Observeddistributionof skin color

Expected distribution

3 genes

1 gene

I II III IV V VI VII VIII IX

Genes Involved in Skin Color

a

Figure 12.18This graph (b) shows theexpected distribution ofhuman skin color if con-trolled by one, three, orfour genes. The observeddistribution of skin color(a) closely matches thedistribution shown byfour genes.

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Concept DevelopmentMany different types of color-blindness are possible. Makecolor-blindness testing chartsavailable to students. Color-blindness testing kits are alsoavailable. Have pairs of studentsuse the kits.

Visual LearningFigure 12.18 The graph of whatnumber of genes most closelymatches the observed distribu-tion of skin color? The actual datamatch the 4-gene graph best.

Performance Assessmentin the Biology Classroom,Analyzing Human Pedigrees, p. 21.Have students carry out thisactivity after they have learnedabout inheritance of human traits.L2

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Visual-Spatial Have stu-dents re-examine the sec-

tion opener photograph ofstudents whose heights form abell-shaped curve. Which fig-ure in this chapter does theinheritance pattern of heightmost closely resemble? Thepattern of inheritance exhib-ited by the students’ heightsmost closely resembles that ofFigures 12.12 and 12.18demonstrating polygenicinheritance.

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Section AssessmentSection AssessmentSection Assessment1. Males inherit only one X chromosome.

If it carries an allele for a disorder, themale will show the trait.

2. the sex of the child3. The father must be color-blind (have

recessive allele on his X chromosome).The mother must have at least one Xchromosome with the color-blind allelesince a female receives one X from

each parent.4. In sickle-cell anemia, an individual

homozygous for normal hemoglobinwill produce only normal hemoglobin.An individual homozygous for sickle-cell anemia will produce only abnor-mal hemoglobin. A heterozygote willproduce both types of hemoglobin.

5. The mother could be IBi or IBIB and the

father is IAIB. If a child received IA fromthe father and i from the mother, itwould be type A so the man couldhave fathered the type A child. Thisman could not father a type O childbecause he has no i allele.

6. Material for the table can be foundon pages 329-335 of the text.

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Internet Address Book

Note Internet addressesthat you find useful in

the space below for quick reference.

Resource ManagerResource ManagerBioLab and MiniLab Worksheets,

pp. 58-60Reinforcement and Study Guide,

p. 54Content Mastery, pp. 57, 60Laboratory Manual, pp. 87-90 L2

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PLAN THE EXPERIMENTPLAN THE EXPERIMENT

1. Decide which crosses will beneeded to test your hypothesis.

2. Keep the available materials inmind as you plan your proce-dure. How many seeds will youneed?

3. Record your procedure, andlist the materials and quantitiesyou will need.

4. Assign a task to each member ofthe group. One person shouldwrite data in a journal, anothercan pollinate the flowers, whilea third can set up the planttrays. Determine who will setup and clean up materials.

5. Design and construct a datatable for recording your obser-vations.

Check the PlanDiscuss the following points

with other group members todecide the final procedure for yourexperiment.

1. What data will you collect, andhow will data be recorded?

2. When will you pollinate theflowers? How many flowerswill you pollinate?

3. How will you transfer pollenfrom one flower to another?

4. How and when will you collectthe seeds that result from yourcrosses?

5. What variables will have to becontrolled? What controls willbe used?

6. When will youend the experi-ment?

7. Make sure yourteacher hasapproved yourexperimentalplan before youproceed further.

8. Carry out yourexperiment.

1. Checking Your Hypothesis Didyour data support your hypothe-sis? Why or why not?

2. Interpreting Observations Whatis the inheritance pattern of var-iegated leaves in Brassica?

3. Making Inferences Explain whygenes in the chloroplast areinherited in this pattern.

4. Drawing Conclusions Whichparent is responsible for passingthe variegated trait to its offspring?

5. Making Scientific IllustrationsDraw a diagram tracing the

inheritance of this trait throughcell division.

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE


Going FurtherGoing Further

Project Make crosses between normalBrassica plants and genetically dwarfed,mutant Brassica plants to determine theinheritance pattern of the dwarf mutation.

To find out more aboutinheritance of traits, visit

the Glencoe Science Web Site.www.glencoe.com/sec/science

YOUR OWNDESIGN

YOUR OWNDESIGN

1. Student answers will varydepending on their hypothe-ses.

2. Variegation is inherited as acytoplasmic trait from thefemale parent.

3. Sperm cells contain little tono cytoplasm. Cytoplasmcontaining chloroplasts iscontributed only by the egg.

4. female5. Diagrams should show the

trait being transmitted in anegg cell of the female but notin pollen of the male.

Error AnalysisSufficient light must be suppliedto the plants to maximize thecontrast between green and whiteleaves.

Skill Have students drawPunnett squares to explain theinheritance pattern in offspring ifthis trait were inherited as a sim-ple dominant allele. Have themshow how cytoplasmic inheri-tance deviates from this pattern.Use the Performance TaskAssessment List for ScientificDrawing in PASC, p. 55. L2

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ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

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should be planted and grown. After 13-15days, the plants will flower. Students willthen perform the following cross-pollina-tions depending on their hypotheses: var-iegated female with nonvariegated male(transfer of pollen from nonvariegatedmale anther via brush to pistil of varie-gated female) and nonvariegated femalewith variegated male. Pods of seeds willmature between days 28 and 30. These

seeds will then be planted, and new off-spring will be observed for the presenceof the variegated trait.

Data and ObservationsVariegated F1 plants will appear in crosseswhere the female parent was variegated.

Going FurtherGoing Further

Have students carry out crossesbetween nonvariegated maleand female parents as well asvariegated male and femaleparents.

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What is the pattern of cytoplasmic inheritance?

T he mitochondria of all eukaryotes and the chloroplasts of plants andalgae contain DNA. This DNA is not coiled into chromosomes, but

it still carries genes that control genetic traits. Many of the mitochondrialgenes control steps in the respiration process.

The DNA in chloroplasts controls traits such as chlorophyll produc-tion. Lack of chlorophyll in some cells causes the appearance of whitepatches in a leaf. This trait is known as variegated leaf. In this BioLab,you will carry out an experiment to determine the pattern of cytoplasmicinheritance of the variegated leaf trait in Brassica rapa.

YOUR OWNDESIGN

YOUR OWNDESIGN

ProblemWhat inheritance pattern does the

variegated leaf trait in Brassica show?

HypothesesConsider the possible evidence you

could collect that would answer theproblem question. Among the peoplein your group, form a hypothesis thatyou can test to answer the question,and write the hypothesis in yourjournal.

ObjectivesIn this BioLab, you will:■ Determine which crosses of Brassica

plants will reveal the pattern ofcytoplasmic inheritance.

■ Analyze data from Brassica crosses.

Possible MaterialsBrassica rapa seeds, normal

and variegatedpotting soil and trayspaintbrushesforcepssingle-edge razor bladelight sourcelabels

Safety PrecautionsAlways wear goggles in the lab.

Handle the razor blade with extremecaution. Always cut away from you.Wash your hands with soap and waterafter working with plant material.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION

YOUR OWNDESIGN

YOUR OWNDESIGN

336

Time AllotmentInitial session: one class periodfor planting of P1 generation,then 5 minutes daily for watering;one class period for cross-pollina-tion; 10 days later, one classperiod for collecting and plantingof F1 seeds; 10 days later: oneclass period for examination of F2plants.

Process Skillsform a hypothesis, observe andinfer, collect data

Safety PrecautionsHave students use caution whenhandling, using, and plugging inlight fixtures or light banks.Students should use cautionwhen using a razor blade.

■ Seeds of normal and varie-gated Brassica rapa (WisconsinFast Plants) can be orderedfrom biological supply houses.The variegated gene is carriedon chloroplast DNA.

■ It is critical to provide light influorescent banks (cool-white,40 watts/bulb) in order toachieve the complete life cyclein such a short time. Lightbanks should be adjustable sothat they remain about 5-8 cmabove the plants’ growing tipsat all times. Lights remain oncontinuously for 24 hours eachday.

■ Soil must be kept constantlymoist, especially during thegermination period.

Possible HypothesesIf the trait is inherited throughthe cytoplasm, then the femaleparent contributes the trait.

If the trait is inherited throughthe cytoplasm, then the male par-ent contributes the trait.

PREPARATIONPREPARATION

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Teaching Strategies■ Variegated plants tend to grow a littleslower than nonvariegated plants.Therefore, these seeds should be startedabout 4 days earlier.■ Review the process of fertilization andmake sure students realize that the egg con-tributes most of the cytoplasm andorganelles to the zygote. The sperm is

almost all nucleus.■ Students should work in groups.■ Brassica will not self-pollinate. Therefore,keep the two plant types separate from eachother to avoid random cross-pollination, orhave students remove flowers that were notused in cross-pollination.

Possible Procedures■ Both variegated and normal seed types


Chapter 12 AssessmentChapter 12 Assessment

SUMMARYSUMMARY

Section 12.1

Section 12.2

Section 12.3

Main Ideas■ A pedigree is a family tree of inheritance.■ Most human genetic disorders are inherited

as rare recessive alleles, but a few are inher-ited as dominant alleles.

Vocabularycarrier (p. 316)fetus (p. 318)pedigree (p. 315)

MendelianInheritance ofHuman Traits

Main Ideas■ Alleles can be incompletely dominant or

codominant. ■ There may be many alleles for one trait or

many genes that interact to produce a trait.■ Inheritance patterns of genes located on sex

chromosomes are due to differences in thenumber and kind of sex chromosomes inmales and in females.

■ The expression of some traits is affected bythe internal and external environments ofthe organism.

Vocabularyautosome (p. 324)codominant alleles

(p. 323)incomplete dominance

(p. 321)multiple alleles (p. 323)polygenic inheritance

(p. 326)sex chromosome (p. 324)sex-linked trait (p. 325)

Main Ideas■ The majority of human traits are controlled

by multiple alleles or by polygenic inheri-tance. The inheritance patterns of thesetraits are highly variable.

■ Sex-linked traits are determined by inheri-tance of sex chromosomes. X-linked traitsare usually passed from carrier females totheir male offspring. Y-linked traits arepassed only from male to male.

■ Mistakes in meiosis, usually due to nondis-junction, may result in an abnormal numberof chromosomes. Autosomes or sex chromo-somes can be affected.

Vocabulary karyotype (p. 335)

CHAPTER 12 ASSESSMENT 339

1. If a trait is X-linked, males pass the X-linkedallele to ________ of their daughters.a. all c. noneb. half d. 1/4

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 2. Stem length demonstrates a range of pheno-types. This is an example of ________.a. autosomal dominantb. autosomal recessivec. sex-linkaged. polygenic inheritance

When HeredityFollowsDifferent Rules

ComplexInheritance ofHuman Traits

339

Main IdeasSummary statements can be used bystudents to review the major con-cepts of the chapter.

Using the VocabularyTo reinforce chapter vocabulary, usethe Content Mastery Booklet andthe activities in the Interactive Tutorfor Biology: The Dynamics of Life onthe Glencoe Science Web Site.www.glencoe.com/sec/science

1. a2. d

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS


All ChapterAssessment

questions and answers have beenvalidated for accuracy and suitabil-ity by The Princeton Review.


Chapter Assessment, pp. 67-72MindJogger VideoquizzesComputer Test BankBDOL Interactive CD-ROM, Chapter 12

quiz

One of the most famous examples of a pedigree demonstrating inheritance

of a sex-linked trait is the family of QueenVictoria of England and hemophilia.

Queen Victoria had four sons and five daugh-ters. Her son Leopold had hemophilia and

died as a result of a minor fall. Two of her daugh-ters, Alice and Beatrice, were carriers for the traitand passed the disorder to royal families in Spain,Prussia, and Russia over four generations.

The Spanish royal family Victoria’s daughterBeatrice, a carrier for the trait, married PrinceHenry of Battenberg, a descendent of Prussianroyalty. Two of their sons inherited the trait,both dying before the age of 35. Her daughter,Victoria, was a carrier and married King AlfonsoXIII of Spain, thus transmitting the allele to theSpanish royal family. Two of their sons died ofhemophilia, also by their early thirties.

The Prussian royal family Alice, another ofVictoria’s daughters, married Louis IV of Hesse,part of the Prussian royal family and related toPrince Henry of Battenberg. One of Alice’s sons,Frederick, died at the age of three from hemo-philia. One of her daughters, Irene, continued topass the trait to the next generation of Prussianroyalty by giving it to two of her sons.

The Russian royal family Irene’s sister and Queen Victoria’s granddaughter, Alix(Alexandra), married Czar Nicholas II of Russia.Four healthy daughters were born, but the firstmale heir, Alexis, showed signs of bleeding andbruising at only six weeks of age. Having abrother, an uncle, and two cousins who had suf-fered from the disorder and died at early ages,you can imagine the despair Alix felt for her sonand the future heir. In desperation, the familyturned to Rasputin, a man who claimed to havehealing abilities and used Alexis’ illness for his

own political power. The series of events sur-rounding Alexis and his hemophilia played a rolein the downfall of the Russian monarchy.

The British throne today Queen Elizabeth II,the current English monarch, is descended from Queen Victoria’s eldest son, Edward VII.Because he did not inherit the trait, he could notpass it on to his children. Therefore, the Britishmonarchy today does not carry the recessiveallele for hemophilia, at least not inherited fromQueen Victoria.


ConnectionSocial StudiesSocial Studies

Connection Queen Victoria andRoyal Hemophilia

If you were the child of a female carrier for a sex-linked trait such as hemophilia, what wouldbe your chances of carrying the trait?

To find out more about hemo-philia, visit the Glencoe Science

Web Site. www.glencoe.com/sec/science

CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY

Queen Victoria and her family

338

Purpose Students will learn why hemo-philia has been called the royaldisease.

Teaching Strategies■ Inform students that about 1in 10 000 males has hemophilia,whereas about 1 in 100 000 000females inherits this disorder.Make sure students realize thatthe reason for this is the fact thathemophilia is an X-linked disor-der.■ Have students hypothesizehow Queen Victoria became acarrier of the disease when nei-ther of her parents had the disor-der. Students should be able todeduce that one of Victoria’s par-ents developed a spontaneousmutation in his or her X chromo-some, which was then passed toher. The mutation could not haveoccurred during the productionof one of Victoria’s egg cellsbecause so many of her childrenwere affected.

Connection to BiologyFour genotypes are possible inthe offspring. Half the girls willbe carriers and half will be nor-mal. Among the boys, half willhave hemophilia and half will benormal. Thus, one-fourth of allchildren of a female carrier and anormal male will be carriers of asex-linked trait.

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ConnectionSocial StudiesSocial Studies

Connection

Internet Address Book

Note Internet addresses that you find useful in the spacebelow for quick reference. VIDEOTAPE

MindJogger VideoquizzesChapter 12: Patterns of Heredity andHuman GeneticsHave students work in groups as they playthe videoquiz game to review key chapterconcepts.




CHAPTER 12 ASSESSMENT 341

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

The following graph illustrates the numberof flowers produced per plant by a certainplant population.

Interpreting Data Use the graph to answerthe questions that follow.1. How many flowers are produced by

plants that have only dominant genes forflower production?a. 4 c. 16b. 12 d. 28

2. How many flowers are produced byplants that have half the possible num-ber of dominant genes for flower pro-duction?a. 4 c. 16b. 12 d. 28

3. What pattern of inheritance is suggestedby the graph?a. multiple allelesb. incomplete dominancec. polygenic inheritanced. sex-linkage

4. Observing and Inferring From theabove graph, estimate the number ofgene pairs that control the number offlowers in these plants.

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web Site.www.glencoe.com/sec/science

CD-ROM

which determine the trait of sickle-

cell anemia by

and height by

4.

6.

which determine the

3.

5.

of colorblindness

and

shows

2.

A

1.

22. If a child has type O blood and its mother hastype A, could a man with type B be the father?Why couldn’t a blood test be used to provethat he is the father?

23. Why do certain human genetic disorders,such as sickle-cell anemia and Tay-Sachs dis-ease, occur more frequently among one eth-nic group than another?

24. How can a single gene mutation in a proteinsuch as hemoglobin affect several body systems?

25. Recognizing Cause and Effect Explain why amale with a recessive X-linked trait usuallyproduces no female offspring with the trait.

26. Comparing and Contrasting Compare mul-tiple allelic with polygenic inheritance.

27. Concept Mapping Complete the conceptmap by using the following vocabulary terms: sex-linked trait, autosomes, karyotype,sex chromosomes, polygenic inheritance,codominant alleles.

THINKING CRITICALLYTHINKING CRITICALLY

Numbers of flowers per plant

Num

ber o

f pla

nts

4 8 12 16 20 24 28

100

300

200

400

500

Number of Flowers Produced by Plants

341

22. The man could be IBi and be thefather. A blood test can showonly that he could possibly bethe father, but not that he isthe father.

23. People tend to marry withintheir own ethnic groups, allow-ing recessive alleles to show upmore frequently in the popula-tions.

24. The protein may be one that istransported or used in many sys-tems. Malfunctions of one bodysystem usually affect others.

25. All X chromosomes from themale parent are contributed tofemale offspring. If the femaleparent is homozygous domi-nant for the trait, all femaleoffspring will be heterozygousand will not show the trait. Thefemale parent would have tobe heterozygous for that traitin order for half her female off-spring to show the trait.

26. In multiple allelic inheritance,many forms of a trait may befound in a population, but theyare based on only two allelesfor the trait in each individual.In polygenic inheritance, theforms of the trait appear con-tinuous, and individuals havemany genes that add to theinheritance of the trait.

27. 1. Karyotype; 2. Autosomes; 3. Sex chromosomes; 4. Codom-inant alleles; 5. Sex-linked trait;6. Polygenic inheritance

THINKING CRITICALLYTHINKING CRITICALLY


1. d2. c3. c4. There are three gene pairs governing

the number of flowers produced perplant.


3. Two parents with normal phenotypes have adaughter with a genetically inherited disor-der. This is an example of a(n) ________ trait.a. autosomal dominant c. sex-linkedb. autosomal recessive d. polygenic

4. Which of the follow-ing disorders would be inherited accord-ing to the pedigree shown here?a. Tay-Sachs diseaseb. sickle-cell anemiac. cystic fibrosisd. Huntington’s disease

5. Which of the following disorders is likely tobe inherited by more males than females?a. Huntington’s diseaseb. Down syndromec. hemophiliad. cystic fibrosis

6. Infants with PKU cannot break down theamino acid ________.a. tyrosine c. methionineb. lysine d. phenylalanine

7. A karyotype reveals ________.a. an abnormal number of genesb. an abnormal number of chromosomesc. polygenic traitsd. multiple alleles for a trait

8. A mother with blood type IBi and a fatherwith blood type IAIB have children. Which ofthe following genotypes would be possiblefor their children?a. AB c. Bb. O d. a and c are correct

9. Normally, lethal autosomal dominant traitsare eliminated from a population becausethey ________.a. have a late onsetb. have an early onsetc. don’t produce phenotypes that affect a

carrier’s healthd. aren’t dominant

10. Whose chromosomes determine the sex ofoffspring in humans?a. mother’s c. both parents’b. father’s d. neither parents’

11. A single individual carries ________ alleles for a trait.

12. ________ is a disorder that results from trisomyof chromosome 21.

13. Most sex-linked traits are passed from motherto ________.

14. The normal sex chromosomes of human malesare ________, and the normal sex chromo-somes of females are ________.

15. To analyze ________, geneticists make a chartof chromosomes called a(n) ________.

16. ________ during meiosis might result inmonosomy or trisomy.

17. The inheritance pattern that occurs equallyin both sexes and skips generations is________.

18. The genotype of the individ-ual represented by this pedigree symbol is ________. Use the letters Y and y to represent alleles.

19. Feather colors in pigeons are produced by________ inheritance.

20. If a trait has three different phenotypes, thetrait is inherited by ________ or ________.

21. The brother of a woman’s father has hemo-philia. Her father was unaffected, but sheworries that she may have an affected son.Should she worry? Explain.

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

340 CHAPTER 12 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Get to the Root of Things If you don’t know a word’s meaning, you can stillget an idea of its meaning if you focus on its roots,prefixes, and suffixes. For instance, words that startwith non-, un-, a-, dis-, and in- generally reversewhat the rest of the word means.

340

3. b4. d5. c6. d7. b8. d9. b

10. b11. two12. Down syndrome13. son14. XY, XX15. numbers of chromosomes,

karyotype16. Nondisjunction17. autosomal recessive18. Yy19. multiple allelic20. incomplete dominance,

codominance

21. If the woman’s father had theallele on his X chromosome, hewould have had hemophilia,but he did not. Thus, the Xchromosome she received fromhim is free of the allele. It ispossible, but unlikely, that shereceived an X chromosomethat carries the allele from hermother. In that case, she couldpass it to her son.

APPLYING MAIN IDEASAPPLYING MAIN IDEAS



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