FKi "9

t^^l^'^a-

Sj

i'i'I

^'.

m

s

ESSENTIAL QUESTION

Mow are traitsiwherited?By the end of this lesson,youshould be able to analyzethe inheritance of traits Inindividuals.

ft, vm

s^'''!

m

•::gi

.^

-^iVii

y<!<";i.lrl;< -.Vv

•*^K^1'

"•sf-

•3'M^^'

l*^i^ ~:'l'v'*

^.-.r

'*•-&•

ffMi•^. K?.̂ (•Mf/

"^-a: •^

S^SSSK^k^s&^^

^

^'

•\

l'*:<>''^3;!fs£ite~tNN:^

•^^^~

Mem^^^e^ame.famil><^%i^rece^^^ c<a<;.?£iSi3^^%^>,ftt(,at%n-,i^

&''.'.~i*?

-^Shfrmergers fhatei <\>

:^^@1^' t»'

.^•i^^gl^rt^V,

.^

a»^

122 Unit 2 Reproduction and Heredity

»

W<SB

^UssonLabsQuick Labs• Dominant Alleles

• What's the Difference between a DominantTrait and a Recessive Trait?

<d»3

I Ewgafle Your Brain^-»'3 •Xit

"31 Predict Check T or F to show whether you think

each statement is true or false.2 Describe Do you know any identical twins?

How are they similar? How are they different?'a

3T F

sa a Siblings look similar

because they each havesome traits of their parents.Siblings always have thesame hair color.

Siblings have identical DNA.

3

•^

as D a•^

•^ a a•3

••^

I^ofe

^

-5

y '^•..-

\^.\a3

1•a5I

3wyw

.'Win

Active Reading'-^»

^ 3 Infer Use context clues to write your owndefinition for the words exhibit and investigate.

a

Vocabulary Termss

fa5Example sentenceA person with brown hair may also exhibit thetrait of brown eye color.

hereditygene

atlele

genotype

phenotype

dominant

recessive

incomplete

dominance

codominance

.»»-A

% exhibit:"5

4 Identify This list contains the key termsyou'll learn in this lesson. As you read,

"5

scircle the definition of each term.

"3Example sentenceGregor Mendel began to investigate thecharacteristics of pea plants.

sw9

59

ss

investigate:s

•^m^5

£5

•-3Lesson 4 Heredity 123

•5

n

I!

ill

Ili

11

.1

II

V.

.-s»g

What is heredity?Imagine a puppy. The puppy has long floppy ears like his motherhas, and the puppy has dark brown fur like his father has. How didthe puppy get these traits? The traits are a result of informationstored in the puppy's genetic material. The passing of geneticmaterial from parents to offspring is called heredity.

What did Gregor Mendeldiscover about heredity?The first major experiments investigatingheredity were performed by a monk namedGregor Mendel. Mendel lived in Austria inthe 1800s. Before Mendel became a monk, heattended a university and studied science andmathematics. This training served him wellwhen he began to study the inheritance of traitsamong the pea plants in the monastery's garden.Mendel studied seven different characteristics ofpea plants: plant height, flower and pod position,seed shape, seed color, pod shape, pod color, andflower color. A characteristic is a feature that hasdifferent forms in a population. Mendel studiedeach pea plant characteristic separately, alwaysstarting with plants that were true-breeding forthat characteristic. A true-breeding plant is onethat will always produce offspring with a certaintrait when allowed to self-pollinate. Each of thecharacteristics that Mendel studied had two

different forms. For example, the color of apea could be green or yellow. Thesedifferent forms are called traits.

Characferifiics of Peo Plant?

5 Apply Is flower color acharacteristic or a trait?

124 Unit 2 Reproduction and Heredity

C^ioracteriytic Trait?

Seed color

0Seed shape

^Pod color

r«Flower position

•A

'^ 5^^ ^^B^^^^^^^^^^^^^

^

Traits Depend on Inherited FactorsIn his experiments with seed pod color, Mendel took two setsof plants, one true-breeding for plants that produce yellow seedpods and the other true-breeding for plants that produce greenseed pods. Instead of letting the plants self-pollinate as theydo naturally, he paired one plant from each set. He did this byfertilizing one plant with the pollen of another plant. Mendel calledthe plants that resulted from this cross the first generation. All ofthe plants from this first generation produced green seed pods.Mendel called this trait the dominant trait. Because the yellow traitseemed to recede, or fade away, he called it the recessive trait.

Then Mendel let the first-generation plants self-pollinate.He called the offspring that resulted from this self-pollination thesecond generation. About three-fourths of the second-generationplants had green seed pods, but about one-fourth had yellowpods. So the trait that seemed to disappear in the first generationreappeared in the second generation. Mendel hypothesized thateach plant must have two heritable "factors" for each trait, onefrom each parent. Some traits, such as yellow seed pod color, couldonly be observed if a plant received two factors—one from eachparent—for yellow pod color. A plant with one yellow factor andone green factor would produce green pods because producinggreen pods is a dominant trait. However, this plant could still passon the yellow factor to the next generation of plants.

Mmt'-'l Active Readiwg

6 Identify As you read, underlineMendel's hypothesis about howtraits are passed from parents tooffspring.

I

^ Visualize It!7 Apply Which pod color is recessive?

x

i

^ ^^ i 4. ^

Parent plants Mendel jcrossed true-breeding tgreen-pod plants withtrue-breeding yellow-pod ^plants. |

-^^^-^S^-i^W^-.s ^—*

First generation All of thefirst generation plantshad green pods. Mendel |let these plants self- fpollinate. |?

Second generation Aboutthree-fourths of the isecond generation had |green pods, and one- jfourth had yellow pods. f

Lesson 4 Heredity 125

Ill

'Ti

!i!

I

I

I

t

Ill

II

II

i

I

)

I

(

! II

Ill

Y>

rHow are traits inherited?Mendel's experiments and conclusions have been the basis formuch of the scientific thought about heredity. His ideas can befurther explained by our modern understanding of the geneticmaterial DNA. What Mendel called "factors" are actually segmentsofDNA known as genes!

Genes Are Passed from Parents to OffspringGenes are segments ofDNA found in chromosomes that giveinstructions for producing a certain characteristic. Humans, likemany other organisms, inherit their genes from their parents.Each parent gives one set of genes to the offspring. The offspringthen has two versions, or forms, of the same gene forevery characteristic—one version from each parent. Thedifferent versions of a gene are known as alleles (uh'LEELZ).Genes are often represented by letter symbols. Dominantalleles are shown with a capital letter, and recessive allelesare shown with a lowercase version of the same letter. An

organism with two dominant or two recessive alleles is saidto be homozygous for that gene. An organism that has onedominant and one recessive allele is heterozygous.

(yeney are ma^eyp of DNA.

ft

^ ^

it It tk i< 11

It <K(:In humans, cells contain pairs ofchromosomes. One chromosome ofeach pair comes from each of twoparents. Each chromosome containssites where specific genes are located.

A gene occupies aspecific location on bothchromosomes in a pair.

19 -:;a§20

^Visualize It!Apply Circle a gene pairfor which this person isheterozygous.

^'jiiB

c cc c

m£dd d

a,e

F..EFF

&e

GWG

w

Humans have 23 pairsof chromosomes.

Alleles arealternateforms of the

same gene.

•"•^^•^ss-?

126 Unit 2 Reproduction and Heredityn«n»S*B'»W!8a«<»l-»!m>a>»--»-.»,sa-?^

I

"w!iSt:.T^fS^^S^S^Ss:^

a

rhis irl hase?.

^s^ •K-ar^

s^ ?-^I

Km's

f

3

?

<:assI

r

Ii

9 Apply The girls in thisphotograph have different typesof hair. Is hair type a genotype ora phenotype?

This girl does not^oveJliwple?.

Genes Influence TraitsThe alternate forms of genes, called alleles, determine thetraits of all living organisms. The combination ofallelesthat you inherited from your parents is your genotype(JEEN*uh*typ). Your observable traits make up yourphenotype (FEEN*uh*typ). The phenotypes of some traitsfollow patterns similar to the ones that Mendel discovered inpea plants. That is, some traits are dominant over others. Forexample, consider the gene responsible for producing dimples,or creases in the cheeks. This gene comes in two alleles: one fordimples and one for no dimples. If you have even one copy of theallele for dimples, you will have dimples. This happens becausethe allele for producing dimples is dominant. The dominant allelecontributes to the phenotype if one or two copies are present inthe genotype. The no-dimples allele is recessive. The recessiveallele contributes to the phenotype only when two copies of it arepresent. If one chromosome in the pair contains a dominant alleleand the other contains a recessive allele, the phenotype will bedetermined by the dominant allele. If you do not have dimples, itis because you inherited two no-dimples alleles—one from eachparent. This characteristic shows complete dominance, becauseone trait is completely dominant over another. However, not allcharacteristics follow this pattern.

^.@@2S3@11 Identify What is the phenotype of an individual

with one allele for dimples and one allele for nodimples?

1

St.1

y

\

8

s,.

'MfWWW"

rwkOvw^i^so^ ry.

10 Imagine Write a short storyabout a world in which youcould change your DNA andyour traits. What would be theadvantages? What would be thedisadvantages?

I

^

127

&»..„„

11'

Ill4

11

•i:

lljW

ISlEllliliiii•

llii:!.!

\^

i.i

1;;;

11:1

;il'l

Many Genes Can Influence a Single TraitSome characteristics, such as the color of your skin, hair, andeyes, are the result of several genes acting together. Differentcombinations of alleles can result in different shades of eye color.Because there is not always a one-to-one relationship betweena trait and a gene, many traits do not have simple patterns ofinheritance.

A Single Gene Can Influence Many TraitsSometimes, one gene influences more than one trait. For example,a single gene causes the tiger shown below to have white fur. If youlook closely, you will see that the tiger also has blue eyes. The genethat affects fur color also influences eye color.

Many genetic disorders in humans are linked to a single genebut affect many traits. For example, the genetic disorder sickle cellanemia occurs in individuals who have two recessive alleles for a

certain gene. This gene carries instructions for producing a proteinin red blood cells. When a person has sickle cell anemia alleles, thebody makes a different protein. This protein causes red blood cellsto be sickle or crescent shaped when oxygen levels are low. Sickle-shaped blood cells can stick in blood vessels, sometimes blockingthe flow of blood. These blood cells are also more likely to damagethe spleen. With fewer healthy red blood cells, the body may notbe able to deliver oxygen to the body's organs. All of the traitsassociated with sickle cell anemia are due to a single gene.

Tjhi? single gene affect? tfietiger's fur color and eye color.

t

^ Visualize It?

a

I

ss

Sl

m13n§

mE-a*-s

^13

12 Identify How many genes areresponsible for eye color in thisexample?

iVN-

fSi!«.-

^ '«<r'%M if- .)r.

( ^,^^ ?s^^ 9

^<

sisy

y..*"

1»?•^m "^, •^

»>•»•*»,<

f.•<>

/ ^r^

•--..

'I. m

»-

s?^WK

-«"%;

ill 128 Unit 2 Reproduction and Heredity

IIm

II5:^

ililIS3

iiilil8.1

^11

II

TheEnvironment Can Influence TraitsSometimes, the environment influences an organism's phenofrype.For example, the arctic fox has a gene that is responsible for coatcolor. This gene is affected by light. In the winter, there are fewerhours of daylight, and the hairs that make up the arctic fox's coatgrow in white. In the summer, when there are more daylight hours,the hairs in the coat grow in brown. In this case, both genes andthe environment contribute to the organisms phenotype. Theenvironment can influence human characteristics as well. Forexample, your genes may make it possible for you to grow to betail, but you need a healthy diet to reach your full height potential.

Traits that are learned in ones environment are not inherited.For example, your ability to read and write is an acquired trait—askill you learned. You were not born knowing how to ride a bike,and if you have children, they will not be born knowing how to doit either. They will have to learn the skill just as you did.

^GSSSSSQ13 Identify Give an example of an acquired trait.

:^

isf;RS"i"I'IIs ElIl^s'Ft

fls,"i-s.

Inthe^^et,fhearc1-fotvncoat.

1C

ss

*«(?-

s**,1,

(^ &;%

9W•»

..S%S^^•rtf?

d—W^,a.,-

•'9»-

^^.^l

1:1,•«(•«V-ai.^i""'"'';'""''-^:^

•l''^l&ih»8aA,i,»afa..i.B.u.,s,;*.'sai»-i»Sg»

tl!,^ «"EiN

as-y.s^^

s% 14 Predict What advantage does white fur give thearctic fox in winter?

^s<f-e.

K^¥.

a.«•"« •sm9.

B

a

m

sS^.'^WI^

"•ys^'

In^eWin^',^ea^ticfox has a W^ite '

•fWt

m

-^^ys^^''''^^^:':":'^ft^%:y!&l5ft:;''¥'''S^S^.^":'":/::'''^^

'^yi^g:'^,.'.;.?i':

..ffif':'

.l%/SiS»3ae«fe;aN''*"*8 ;a«*^Lesson 4 Heredity 129

WSK

llilil

Illi'SS

S'!

I'll

li:i

i

e^gthe R^What are the exceptions to completedominance?The characteristics that Mendel chose to study demonstratedcomplete dominance, meaning that heterozygous individualsshow the dominant trait. Some human traits, such as freckles and

dimples, follow the pattern of complete dominance, too. However,other traits do not. For traits that show incomplete dominance orcodominance, one trait is not completely dominant over another.

Incomplete DominanceIn incomplete dominance, each allele in a heterozygous individualinfluences the phenotype. The result is a phenotype that is a blendof the phenotypes of the parents. One example of incompletedominance is found in the snapdragon flower, shown below. Whena true-breeding red snapdragon is crossed with a true-breedingwhite snapdragon, all the offspring are pink snapdragons. Bothalleles of the gene have some influence. Hair texture is an exampleof incomplete dominance in humans. A person with one straight-hair allele and one curly-hair allele will have wavy hair.

Actwe Reading

15 Identify As you read, underlineexamples of incompletedominance and codominance.

^t Visualize If!16 Analyze How can you tell that

these snapdragons do notfollow the pattern of completedominance?

m

I

Pink snQp^ragonf ore proJuced bya croff between a ce<! ?nap<Jrag<?nand a vh'tte ynop^ragon.

x

130

ll

3

•?»••»

m

t.

»,.

a

SS-SL

s

'S3

^

<vs

I

81

I

I!IIli

§1

I:

is

Ix

?

gIIiIII-9;gi

IIii1;ii0

i%-

CodominanceFor a trait that shows codominance, both of the alleles in aheterozygous individual contribute to the phenotype. Instead ofhaving a blend of the two phenotypes, heterozygous individualshave both of the traits associated with their two alleles. An exampleof codominance is shown in the genes that determine humanblood types. There are three alleles that play a role in determininga person's blood type: A, B, and 0. The alleles are responsiblefor producing small particles on the surface of red blood cellscalled antigens. The A allele produces red blood cells coated withA antigens. The B allele produces red blood cells coated with Bantigens. The 0 allele does not produce antigens. The A and Balleles are codominant. So, someone with one A allele and one Ballele will have blood cells that are coated with A antigens and B

antigens. This person would have type AB blood.

Active R&ading18 Identify What antigens coat the red blood cells ofa person with type AB blood?

MII III il!l;l!ltMllffl

ai^^i^ii^,^'i^aSi&g'MK.SS

17 Research Blood type is animportant factor when peoplegive or receive blood. Researchthe meanings of the phrases"universal donor" and "universalrecipient." What are thegenotypes of each blood type?

)

i;'

^ Visualize Iti19 Predict The color of these imaginary fish is controlled by a single gene.

Sketch or describe their offspring if the phenotypes follow the patternof complete dominance, incomplete dominance, or codominance. I

x

kr

K-

fe

Coiviplete dominancelue i? dominant to

yellolv.)Incomplete dominance CoJominonce

fs.

^

I]

e^^s?.^ '^•-^^s-ysi '^^^"Sffi^^^i-.^ig"^

Lesson 4 Heredity 131

!jiII

I

Ill•";! <iii

'w!;:'i

ffiillEillilllii!

m-oan.t.n.,

-.i; ^- il.&.--

illIll

31 Ki^:yfl.

Hs'

"^^;-is"s'

ummary isa;a-?is&a

;'i

I:'

m

^

s:

^iSiSSiffSititSyi'^ • to com.ptetethis summary, circle the correct word or phrase. Then.li"^|^^^||^|^^J^^^:^,i\ ..• use thekey below to.checkyourlanswers. You can use this page' ;^ti|t^fiS|i|J|$.t^®^^^SJ" ' to review the main'concepts of the lesson. . ,. .,' .^ . . .• 1;'1'^:?^^^S%I^|S

^vs,ff^s^ss^^^igh!MM

' x:ft^:%&^l?'j^yte% ?5®fc:sS'yyffiff'-s'^s "?%ssi

Heredity

I'.

I;s^'^y^L'''-

'"-:. .;.••?..<•-, ' •••Ky-Bxi^^^^'.^%:::€Sf%Sai?'ms-& .£W'stii&f.?*.- *€

ms ^^s«>& *-a- K:IAJ Sssfe-j* m WPs?

•£Sift3; tfi

SMB^

Gregor Mendel studied patterns of heredityin pea plants.

e"

m20 Traity that seemeA to disappear in

Meodel;$' •fif'yt-genef-ation croyye? l^eredominant/ recessive traits.

.1,

E rs^

3^y

ii}'

ft^-^,*:'

KK'^y^

^•autt.' ...••»*•.,

Ky'^^i/Q^

Inherited genes influencethe traits of an individual.

21 /An individL/al l^itAi

the genotype 60 iyheterozy^ous I^tOmoZygoo?.

•s

!!

m

^fji^? »•^»

•v6:w y^^.

«& '^i'^!i^

E-

V- ^

"Kl

K.

esv%; *K

Phenotypes can follow complete dominance,incomplete dominance, or codominance.

22 W^ien tAieye imaginary •fiy/i croyy, t^eir o'f'fypfingare all gf-een. T^ii? ;$• an example o-f

inco»r>p!ete dominance.

x

1;

^

Sa

s <

®

^®

fKii;9^-

r;m^

isia^•^-

g*

y-iSKy?

^|?^^,(%MeUttt^p^3i<^o3ui.zc ssrtba^ot.^.^is^KIy?Si.'^.r^^' ':-:-: '?'•:'-'-~: ^"3^':.: '••^^'•' ' - ••aMS^S%^SS^^^y%^-^"K~A''-'-^-•'---''-•'' ' ' .-.-m.snTOW-.wW'BB^

•I

11

^

s

ii

23 Apply If a child has blonde hair and both of her parents have brownhair, what does that tell you about the allele for blonde hair?

132 Unit 2 Reproduction and Heredity

f"ws^'-^ L^:f

'<?~iW'!'Sai!afca^K

4LessonLesson ReviewI Vocabularyp Draw a line to connect the following terms to

their definitions.

1 heredity A an organisms appearance orother detectable characteristic

3 phenotype B a section of DNA that containscharacteristic

C the passing of genetic materialfrom parent to offspring

Key Concepts4 Describe What did Mendel discover about

genetic factors in pea plants?V

5 Describe What is the role of DNA indetermining an organism's traits?

Use this diagram to answer the followingquestions.

F«

sg

tffi

Identify What is the genotype at the Q gene?

9 Apply For which genes is this individualheterozygous?

Critical Thinking10 Describe Marfan syndrome is a genetic

disorder caused by a dominant allele. Describehow Marfan syndrome is inherited.

E

!!1

II

:j}

NI

I

1:1

^

B-

II

6 Apply Imagine that a brown horse and a whitehorse cross to produce an offspring whosecoat is made up of some brown hairs and somewhite hairs. Which pattern of dominance is thisan example of?

7 Identify Give an example of a trait that iscontrolled by more than one gene.

11 Describe Jenny, Jenny's mom, and Jenny'sgrandfather are all good basketball players.Give an example of an inherited trait and anacquired trait that could contribute to theirskill at basketball.

e

>

Lesson 4 Heredity 133