Biology in Focus - Chapter 11
Transcript of Biology in Focus - Chapter 11
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CAMPBELL BIOLOGY IN FOCUS
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Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge
11Mendel and the Gene Idea
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Overview: Drawing from the Deck of Genes
What genetic principles account for the passing of traits from parents to offspring?
The “blending” hypothesis is the idea that genetic material from the two parents blends together (the way blue and yellow paint blend to make green)
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The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
Mendel documented a particulate mechanism through his experiments with garden peas
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Figure 11.1
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Concept 11.1: Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
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Mendel’s Experimental, Quantitative Approach
Mendel probably chose to work with peas because There are many varieties with distinct heritable
features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits
He could control mating between plants
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Figure 11.2
Parentalgeneration(P)
Stamens
First filialgenerationoffspring(F1)
Carpel
Technique
Results
1
2
3
4
5
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Mendel chose to track only characters that occurred in two distinct alternative forms
He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)
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In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization
The true-breeding parents are the P generation The hybrid offspring of the P generation are called
the F1 generation
When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced
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The Law of Segregation
When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation
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Figure 11.3-1
P Generation
Experiment
(true-breedingparents) Purple flowers White flowers
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Figure 11.3-2
P Generation
Experiment
(true-breedingparents)
F1 Generation(hybrids)
Purple flowers White flowers
All plants had purple flowers
Self- or cross-pollination
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Figure 11.3-3
P Generation
Experiment
(true-breedingparents)
F1 Generation
F2 Generation
(hybrids)
Purple flowers White flowers
All plants had purple flowers
Self- or cross-pollination
705 purple-floweredplants
224 white-floweredplants
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Mendel reasoned that in the F1 plants, the heritable factor for white flowers was hidden or masked in the presence of the purple-flower factor
He called the purple flower color a dominant trait and the white flower color a recessive trait
The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation
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Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits
What Mendel called a “heritable factor” is what we now call a gene
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Table 11.1
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Table 11.1a
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Table 11.1b
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Mendel’s Model
Mendel developed a model to explain the 3:1 inheritance pattern he observed in F2 offspring
Four related concepts make up this model
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First, alternative versions of genes account for variations in inherited characters
For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
These alternative versions of a gene are now called alleles
Each gene resides at a specific locus on a specific chromosome
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Figure 11.4
Allele for purple flowers
Pair of homologouschromosomes
Allele for white flowers
Locus for flower-color gene
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Second, for each character, an organism inherits two alleles, one from each parent
Mendel made this deduction without knowing about the existence of chromosomes
Two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation
Alternatively, the two alleles at a locus may differ, as in the F1 hybrids
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Third, if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant
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Fourth (now known as the law of segregation), the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the organism
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis
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Figure 11.5-1P Generation
Gametes:
Appearance:Genetic makeup:
Purple flowers PP
White flowerspp
P p
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Figure 11.5-2P Generation
Gametes:
Appearance:Genetic makeup:
F1 Generation
Purple flowers PP
White flowerspp
Gametes:
Appearance:Genetic makeup:
½ ½
Purple flowers Pp
P
P
p
p
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Figure 11.5-3P Generation
Gametes:
Appearance:Genetic makeup:
F1 Generation
F2 Generation
Purple flowers PP
White flowerspp
Gametes:
Appearance:Genetic makeup:
Eggs fromF1 (Pp) plant
Sperm fromF1 (Pp) plant
½ ½
Purple flowers Pp
P
P
p
p
P p
P
p
PP
pp
Pp
Pp
3 : 1
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Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele
For example, P is the purple-flower allele and p is the white-flower allele
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Useful Genetic Vocabulary
An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character
An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character
Unlike homozygotes, heterozygotes are not true-breeding
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Because of the effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes
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Figure 11.6Phenotype
1
Genotype
Purple
Purple
Purple
White
Ratio 3:1
PP (homozygous)
Pp (heterozygous)
Pp (heterozygous)
pp (homozygous)
Ratio 1:2:1
2
3
1
1
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The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous
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Figure 11.7 Technique
Predictions
Dominant phenotype,unknown genotype:
PP or Pp?
Eggs
Sperm
½ offspring purple and½ offspring white
Recessive phenotype,known genotype:
pp
If purple-floweredparent is PP
If purple-floweredparent is Pp
Eggs
Sperm
All offspring purple
Results
or
or
p p
P
p
Pp
pp
Pp
pp
p p
P
P
Pp
Pp
Pp
Pp
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The Law of Independent Assortment
Mendel derived the law of segregation by following a single character
The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character
A cross between such heterozygotes is called a monohybrid cross
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Mendel identified his second law of inheritance by following two characters at the same time
Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
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Figure 11.8
YR yr
YR
yr
YYRR
yyrr
YyRr
YyRr
Experiment
Predictions
P Generation
F1 Generation
Predictedoffspring inF2 generation
Gametes
YYRR yyrr
YyRr
Results
EggsEggs
Sperm
Spermor
Hypothesis ofdependent assortment
Phenotypic ratio 3:1
Hypothesis ofindependent assortment
¾ ¼
½ ½
½
½
¼
¼
¼
¼
¼¼¼¼
YR yr
Phenotypic ratio approximately 9:3:3:1
Phenotypic ratio 9:3:3:1
YR yr
YR
yr
YYRR
yyrr
YYRr YyRr
Yr yR
Yr
yR
YyRR
YYRr YYrr YyrrYyRr
YyRR YyRr yyRryyRR
YyRr Yyrr yyRr
315 108 101 32
9 163 16
3 161 16
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Figure 11.8a
ExperimentP Generation
F1 Generation
Gametes
YYRR yyrr
YyRr
YR yr
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Figure 11.8b
YR yr
YR
yr
YYRR
yyrr
YyRr
YyRr
Predictedoffspring inF2 generation
Results
EggsEggs
Sperm
Sperm
Hypothesis ofdependent assortment
Phenotypic ratio 3:1
Hypothesis ofindependent assortment
¾ ¼
½ ½
½
½
¼
¼
¼
¼
¼¼¼¼
Phenotypic ratio approximately 9:3:3:1
Phenotypic ratio 9:3:3:1
YR yr
YR
yr
YYRR
yyrr
YYRr YyRr
Yr yR
Yr
yR
YyRR
YYRr YYrr YyrrYyRr
YyRR YyRr yyRryyRR
YyRr Yyrr yyRr
315 108 101 32
9 163 16
3 161 16
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The results of Mendel’s dihybrid experiments are the basis for the law of independent assortment
It states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
This law applies to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
Genes located near each other on the same chromosome tend to be inherited together
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Concept 11.2: The laws of probability govern Mendelian inheritance
Mendel’s laws of segregation and independent assortment reflect the rules of probability
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
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The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
This can be applied to an F1 monohybrid cross
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele
The Multiplication and Addition Rules Applied to Monohybrid Crosses
12
12
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Figure 11.9
R r
R
r
Rr
Segregation ofalleles into eggs
Eggs
Sperm
¼
½ ½
½
½
Segregation ofalleles into sperm
Rr Rr
RR
¼
rr
¼
Rr
¼
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The addition rule states that the probability that any one of two or more mutually exclusive events will occur is calculated by adding together their individual probabilities
It can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
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Solving Complex Genetics Problems with the Rules of Probability
We can apply the rules of probability to predict the outcome of crosses involving multiple characters
A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied
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For example, if we cross F1 heterozygotes of genotype YyRr, we can calculate the probability of different genotypes among the F2 generation
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Figure 11.UN01
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For example, for the cross PpYyRr Ppyyrr, we can calculate the probability of offspring showing at least two recessive traits
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Figure 11.UN02
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Concept 11.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Not all heritable characters are determined as simply as the traits Mendel studied
However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
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Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations When alleles are not completely dominant or
recessive When a gene has more than two alleles When a single gene influences multiple phenotypes
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Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
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Figure 11.10-1
P Generation
Gametes
WhiteCWCW
RedCRCR
CWCR
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Figure 11.10-2
½ ½
P Generation
F1 Generation
Gametes
Gametes
WhiteCWCW
PinkCRCW
RedCRCR
CWCR
CWCR
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Figure 11.10-3
Eggs
½ ½
½
½
P Generation
F1 Generation
Gametes
F2 Generation
GametesSperm
WhiteCWCW
PinkCRCW
RedCRCR
CWCW
CRCWCRCR
CWCR
CWCR
½ ½ CWCR
CW
CR
CRCW
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The Relationship Between Dominance and Phenotype Alleles are simply variations in a gene’s nucleotide
sequence When a dominant allele coexists with a recessive
allele in a heterozygote, they do not actually interact at all
For any character, dominant/recessive relationships of alleles depend on the level at which we examine the phenotype
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Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain At the organismal level, the allele is recessive At the biochemical level, the phenotype (i.e., the
enzyme activity level) is incompletely dominant At the molecular level, the alleles are codominant
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Frequency of Dominant Alleles Dominant alleles are not necessarily more common
in populations than recessive alleles For example, one baby out of 400 in the United
States is born with extra fingers or toes, a dominant trait called polydactyly
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Multiple Alleles
Most genes exist in populations in more than two allelic forms
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles of the gene: IA, IB, and i.
The enzyme (I) adds specific carbohydrates to the surface of blood cells
The enzyme encoded by IA adds the A carbohydrate, and the enzyme encoded by IB adds the B carbohydrate; the enzyme encoded by the i allele adds neither
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Figure 11.11
Carbohydrate
(b) Blood group genotypes and phenotypes
Allele
Red blood cellappearance
Genotype
noneBA
IB
Phenotype(blood group)
iIA
IAIB iiIAIA or IAi IBIB or IBi
BA OAB
(a) The three alleles for the ABO blood groups and theircarbohydrates
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Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
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Extending Mendelian Genetics for Two or More Genes
Some traits may be determined by two or more genes
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Epistasis
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
For example, in Labrador retrievers and many other mammals, coat color depends on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
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Figure 11.12
¼
¼
¼
¼
¼ ¼¼¼ BE Be
BE
be
BBEE
bbee
BbEE BbEe
bE be
bE
Be
BBEe
BbEE bbEE bbEeBbEe
BBEe BbEe BbeeBBee
BbEe bbEe Bbee
9 : 4: 3
Eggs
Sperm
BbEe BbEe
![Page 64: Biology in Focus - Chapter 11](https://reader031.fdocuments.in/reader031/viewer/2022012310/587a4d001a28ab00148b6a99/html5/thumbnails/64.jpg)
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Polygenic Inheritance
Quantitative characters are those that vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
Skin color in humans is an example of polygenic inheritance
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Figure 11.13
Eggs
Sperm
AaBbCc AaBbCc
Phenotypes:
0Number ofdark-skin alleles: 1 2 3 4 5 6
1 64 1 646 64 6 64 1 6415 64 15 6420 64
1 8
1 8
1 8
1 8
1 8
1 8
1 8
1 8
1 81 8 1 8 1 8 1 81 81 8 1 8
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Nature and Nurture: The Environmental Impact on Phenotype
Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype
The norm of reaction is the phenotypic range of a genotype influenced by the environment
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The phenotypic range is generally broadest for polygenic characters
Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype
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Integrating a Mendelian View of Heredity and Variation
An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
An organism’s phenotype reflects its overall genotype and unique environmental history
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Concept 11.4: Many human traits follow Mendelian patterns of inheritance
Humans are not good subjects for genetic research Generation time is too long Parents produce relatively few offspring Breeding experiments are unacceptable
However, basic Mendelian genetics endures as the foundation of human genetics
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Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees
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Pedigrees can also be used to make predictions about future offspring
We can use the multiplication and addition rules to predict the probability of specific phenotypes
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Figure 11.14
WWor
Ww
ww
ww ww
ww ww
WwWw
Ww Ww
No widow’s peakWidow’s peak
wwWw
1st generation(grandparents)
3rd generation(two sisters)
2nd generation(parents,aunts, anduncles)
Affected male
Affected female
Male FemaleKey
Mating
Attachedearlobe
Freeearlobe
Offspring, inbirth order(first-born on left)
FForFf
ff
ff ff
Ff ff
FfFF orFf
Ff Ff
ffFf
(a) Is a widow’s peak a dominant or recessive trait? (b) Is an attached earlobe a dominantor recessive trait?
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Figure 11.14a
WWor
Ww
ww
ww ww
ww ww
WwWw
Ww Ww
Widow’s peak
wwWw
1st generation(grandparents)
3rd generation(two sisters)
2nd generation(parents, aunts, and uncles)
Affected maleAffected female
Male
Female
KeyMating
Offspring, inbirth order(first-born on left)
(a) Is a widow’s peak a dominant or recessive trait?No widow’s peak
![Page 74: Biology in Focus - Chapter 11](https://reader031.fdocuments.in/reader031/viewer/2022012310/587a4d001a28ab00148b6a99/html5/thumbnails/74.jpg)
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Figure 11.14aa
Widow’s peak
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Figure 11.14ab
No widow’s peak
![Page 76: Biology in Focus - Chapter 11](https://reader031.fdocuments.in/reader031/viewer/2022012310/587a4d001a28ab00148b6a99/html5/thumbnails/76.jpg)
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Figure 11.14b
Affected maleAffected female
Male
Female
KeyMating
Offspring, inbirth order(first-born on left)
1st generation(grandparents)
3rd generation(two sisters)
2nd generation(parents, aunts, and uncles)
Attached earlobe Free earlobe
FForFf
ff
ff ff
Ff ff
FfFF orFf
Ff Ff
ffFf
(b) Is an attached earlobe a dominant or recessive trait?
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Figure 11.14ba
Attached earlobe
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Figure 11.14bb
Free earlobe
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Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner
These range from relatively mild to life-threatening
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The Behavior of Recessive Alleles
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal
Most people who have recessive disorders are born to parents who are carriers of the disorder
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Figure 11.15
Parents
Sperm
NormalAa
Normal Aa
EggsAA
Normal Aa
Normal(carrier)
Aa Normal(carrier)
aa Albino
A
a
A a
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Figure 11.15a
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If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low
Consanguineous (between close relatives) matings increase the chance of mating between two carriers of the same rare allele
Most societies and cultures have laws or taboos against marriages between close relatives
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Cystic Fibrosis
Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
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Sickle-Cell Disease: A Genetic Disorder with Evolutionary Implications
Sickle-cell disease affects one out of 400 African-Americans
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
Symptoms include physical weakness, pain, organ damage, and even paralysis
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Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms
About one out of ten African-Americans has sickle-cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes
Heterozygotes are less susceptible to the malaria parasite, so there is an advantage to being heterozygous
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Dominantly Inherited Disorders
Some human disorders are caused by dominant alleles
Dominant alleles that cause a lethal disease are rare and arise by mutation
Achondroplasia is a form of dwarfism caused by a rare dominant allele
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Figure 11.16
Parents
Sperm
DwarfDd
Normaldd
Eggs Dd
Dwarf dd
Normal
Dd Dwarf
dd Normal
d
d
D d
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Figure 11.16a
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The timing of onset of a disease significantly affects its inheritance
Huntington’s disease is a degenerative disease of the nervous system
The disease has no obvious phenotypic effects until the individual is about 35 to 45 years of age
Once the deterioration of the nervous system begins the condition is irreversible and fatal
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Multifactorial Disorders
Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer, have both genetic and environmental components
Lifestyle has a tremendous effect on phenotype for cardiovascular health and other multifactorial characters
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Genetic Counseling Based on Mendelian Genetics
Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings
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Figure 11.UN03
Phenotypes:
20 1Number ofdark-skin alleles: 53 4 6
1 64 6 64 15 646 6420 64 15 64 1 64
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Figure 11.UN04
PP (homozygous)
Pp (heterozygous)
Pp (heterozygous)
pp (homozygous)
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Figure 11.UN05
In the whole population,some genes have morethan two alleles
Pleiotropy
Relationship amongalleles of a single gene Description Example
Codominance
Multiple alleles
Incomplete dominanceof either allele
Complete dominanceof one allele
One gene is able to affectmultiple phenotypiccharacters
Both phenotypesexpressed inheterozygotes
Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes
Heterozygous phenotypesame as that of homo-zygous dominant
ABO blood group alleles
Sickle-cell disease
IAIB
IA, IB, i
CRCR CRCW CWCW
PP Pp
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Figure 11.UN06
Polygenic inheritance A single phenotypiccharacter is affected bytwo or more genes
The phenotypicexpression of one geneaffects the expressionof another gene
Epistasis
Relationship amongtwo or more genes Description Example
AaBbCc AaBbCc
BbEe BbEe
BEBE bE
bE
be
be
Be
Be
9 : 3 : 4
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Figure 11.UN07
No widow’s peakWidow’s peak
WWor
Ww
ww
ww ww ww
ww ww
WwWwWw
Ww Ww
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Figure 11.UN08
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Figure 11.UN09
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Figure 11.UN10
George Arlene
TinaCarla
Daniel Alan
Sandra Tom Sam Wilma Ann Michael
Christopher