Chapter 9 Patterns of Inheritance...

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© 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko

PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey

Chapter 9 Patterns of Inheritance

 Dogs are one of man’s longest genetic experiments.

–  Over thousands of years, humans have chosen and mated dogs with specific traits.

–  The result has been an incredibly diverse array of dogs with distinct

–  body types and

–  behavioral traits.

Introduction

© 2012 Pearson Education, Inc.

Figure 9.0_1 Chapter 9: Big Ideas

Mendel’s Laws Variations on Mendel’s Laws

The Chromosomal Basis of Inheritance

Sex Chromosomes and Sex-Linked Genes

MENDEL’S LAWS


9.1 The science of genetics has ancient roots

 Pangenesis, proposed around 400 BCE by Hippocrates, was an early explanation for inheritance that suggested that –  particles called pangenes came from all parts of the

organism to be incorporated into eggs or sperm and

–  characteristics acquired during the parents’ lifetime could be transferred to the offspring.

 Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited.


9.1 The science of genetics has ancient roots

  The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was –  suggested in the 19th century by scientists studying

plants but

–  later rejected because it did not explain how traits that disappear in one generation can reappear in later generations.


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9.2 Experimental genetics began in an abbey garden

 Heredity is the transmission of traits from one generation to the next.

 Genetics is the scientific study of heredity.

 Gregor Mendel

–  began the field of genetics in the 1860s,

–  deduced the principles of genetics by breeding garden peas, and

–  relied upon a background of mathematics, physics, and chemistry.



  In 1866, Mendel –  correctly argued that parents pass on to their offspring

discrete “heritable factors” and

–  stressed that the heritable factors (today called genes), retain their individuality generation after generation.

 A heritable feature that varies among individuals, such as flower color, is called a character.

 Each variant for a character, such as purple or white flowers, is a trait.



  True-breeding varieties result when self-fertilization produces offspring all identical to the parent.

  The offspring of two different varieties are hybrids.

  The cross-fertilization is a hybridization, or genetic cross.

  True-breeding parental plants are the P generation.

 Hybrid offspring are the F1 generation.

 A cross of F1 plants produces an F2 generation.


Figure 9.2B

Stamen Carpel

Petal

Figure 9.2C_s1

Removal of stamens

Carpel

White

Stamens Transfer of pollen Purple Parents

(P)

2

1

Figure 9.2C_s2

Removal of stamens

Carpel

White

Stamens Transfer of pollen Purple

Carpel matures into pea pod

Parents (P)

2

3

1

3

Figure 9.2C_s3

Removal of stamens

Carpel

White

Stamens Transfer of pollen Purple

Carpel matures into pea pod

Seeds from pod planted

Offspring (F1)

Parents (P)

2

3

1

4

Figure 9.2D Character Traits

Dominant Recessive

Flower color

Purple White

Flower position

Axial Terminal

Seed color Yellow Green

Seed shape Round Wrinkled

Pod shape Inflated Constricted

Pod color Green Yellow

Stem length

Tall Dwarf

9.3 Mendel’s law of segregation describes the inheritance of a single character

 A cross between two individuals differing in a single character is a monohybrid cross.

 Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers.

–  The F1 generation produced all plants with purple flowers.

–  A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers.


Figure 9.3A_s1 The Experiment P generation (true-breeding parents)

Purple flowers

White flowers

×


F1 generation

Purple flowers

White flowers

All plants have purple flowers

×


F1 generation

F2 generation

of plants have purple flowers

of plants have white flowers

Purple flowers

White flowers

All plants have purple flowers

Fertilization among F1 plants (F1 × F1)

×

3 4

1 4

4


  The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis.

 Mendel needed to explain why

–  white color seemed to disappear in the F1 generation and

–  white color reappeared in one quarter of the F2 offspring.

 Mendel observed the same patterns of inheritance for six other pea plant characters.



 Mendel developed four hypotheses, described below using modern terminology.

1. Alleles are alternative versions of genes that account for variations in inherited characters.

2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different.

–  A homozygous genotype has identical alleles.

–  A heterozygous genotype has two different alleles.



3.  If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele.

–  The phenotype is the appearance or expression of a trait.

–  The genotype is the genetic makeup of a trait.

–  The same phenotype may be determined by more than one genotype.



4.  A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation.

 Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation.

–  The F1 hybrids all have a Pp genotype.

–  A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine.


Figure 9.3B_s1 The Explanation

P generation Genetic makeup (alleles) Purple flowers White flowers

Gametes All p

pp PP

P All


P generation

F1 generation (hybrids)

Genetic makeup (alleles) Purple flowers White flowers

Gametes All p

pp PP

P p Gametes

All Pp

2 1

2 1

P All

5


P generation

F1 generation (hybrids)

F2 generation

Genetic makeup (alleles) Purple flowers White flowers

Gametes P All p

pp PP

P

P

P

p

p

p

PP Pp

Pp pp

Eggs from F1 plant

Gametes

Fertilization

All Pp Alleles

segregate

Phenotypic ratio 3 purple : 1 white

Genotypic ratio 1 PP : 2 Pp : 1 pp

Sperm from F1 plant

2 1

2 1

All

Figure 9.3B_4

F2 generation P

P

p

p

PP Pp

Pp pp

Eggs from F1 plant

Phenotypic ratio 3 purple : 1 white

Genotypic ratio 1 PP : 2 Pp : 1 pp

Sperm from F1 plant

9.4 Homologous chromosomes bear the alleles for each character

 A locus (plural, loci) is the specific location of a gene along a chromosome.

  For a pair of homologous chromosomes, alleles of a gene reside at the same locus. –  Homozygous individuals have the same allele on both

homologues.

–  Heterozygous individuals have a different allele on each homologue.


Figure 9.4

P

P

a

a

B

b

PP aa Bb

Dominant allele

Recessive allele

Gene loci

Homologous chromosomes

Genotype: Heterozygous, with one dominant and one recessive allele

Homozygous for the recessive allele

Homozygous for the dominant allele

9.5 The law of independent assortment is revealed by tracking two characters at once

 A dihybrid cross is a mating of parental varieties that differ in two characters.

 Mendel performed the following dihybrid cross with the following results: –  P generation: round yellow seeds × wrinkled green seeds –  F1 generation: all plants with round yellow seeds –  F2 generation:

–  9/16 had round yellow seeds –  3/16 had wrinkled yellow seeds –  3/16 had round green seeds –  1/16 had wrinkled green seeds


Figure 9.5A

4 1

4 1

4 1

4 1

4 1

4 1

4 1

4 1

16 9

16 3

16 3

16 1

2 1

2 1

2 1

2 1

F1 generation

F2 generation

P generation

Gametes

Sperm

Eggs Yellow round

Green round Yellow wrinkled Green wrinkled

×

RRYY rryy

RY ry

RrYy

The hypothesis of dependent assortment Data did not support; hypothesis refuted

The hypothesis of independent assortment Actual results; hypothesis supported

RY

RY

ry

ry

Eggs

RY

RY

rY

rY

Ry

Ry ry

ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYy rrYy Rryy rryy

Sperm

6

Figure 9.5B

Phenotypes

Genotypes

Black coat, normal vision

B_N_

Black coat, blind (PRA)

B_nn

Blind

Chocolate coat, normal vision

bbN_

Blind

Blind Blind

Chocolate coat, blind (PRA)

bbnn

Mating of double heterozygotes (black coat, normal vision) BbNn BbNn ×

Phenotypic ratio of the offspring

9 Black coat,

normal vision

3 Black coat, blind (PRA)

1 Chocolate coat,

blind (PRA)

3 Chocolate coat, normal vision


 Mendel needed to explain why the F2 offspring –  had new nonparental combinations of traits and

–  a 9:3:3:1 phenotypic ratio.

 Mendel –  suggested that the inheritance of one character has no

effect on the inheritance of another,

–  suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and

–  called this the law of independent assortment.



  The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers:

–  black versus chocolate color,

–  normal vision versus progressive retinal atrophy.


9.6 Geneticists can use the testcross to determine unknown genotypes

 A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual.

 A testcross can show whether the unknown genotype includes a recessive allele.

 Mendel used testcrosses to verify that he had true-breeding genotypes.

  The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes.


Figure 9.6

What is the genotype of the black dog?

Two possibilities for the black dog:

× Testcross

Genotypes

Gametes

Offspring All black 1 black : 1 chocolate

or

B_? bb

Bb BB

B B

b b

b

Bb Bb bb

9.7 Mendel’s laws reflect the rules of probability

 Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected –  the segregation of allele pairs during gamete formation

and

–  the re-forming of pairs at fertilization.

  The probability scale ranges from 0 to 1. An event that is –  certain has a probability of 1 and

–  certain not to occur has a probability of 0.


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9.7 Mendel’s laws reflect the rules of probability

  The probability of a specific event is the number of ways that event can occur out of the total possible outcomes.

 Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event.

  The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition.


Figure 9.7

F1 genotypes

Formation of eggs

Formation of sperm

Bb female Bb male

Sperm

F2 genotypes Eggs

B

B B B B

B

b

b

b b b b

×

2 1

2 1

2 1

2 1

2 1

2 1

4 1

4 1

4 1

4 1

( )

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees

  In a simple dominant-recessive inheritance of dominant allele A and recessive allele a,

–  a recessive phenotype always results from a homozygous recessive genotype (aa) but

–  a dominant phenotype can result from either

–  the homozygous dominant genotype (AA) or

–  a heterozygous genotype (Aa).

 Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles.


Figure 9.8A Dominant Traits Recessive Traits

Freckles No freckles

Widow’s peak Straight hairline

Free earlobe Attached earlobe

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees

  The inheritance of human traits follows Mendel’s laws.

 A pedigree

–  shows the inheritance of a trait in a family through multiple generations,

–  demonstrates dominant or recessive inheritance, and

–  can also be used to deduce genotypes of family members.


Figure 9.8B

First generation (grandparents)

Second generation (parents, aunts, and uncles)

Third generation (two sisters)

Female Male Attached Free

Ff Ff Ff ff

Ff Ff ff ff ff

ff

FF or Ff

FF or Ff

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9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene

  Inherited human disorders show either

1.  recessive inheritance in which

–  two recessive alleles are needed to show disease,

–  heterozygous parents are carriers of the disease-causing allele, and

–  the probability of inheritance increases with inbreeding, mating between close relatives.

2.  dominant inheritance in which

–  one dominant allele is needed to show disease and

–  dominant lethal alleles are usually eliminated from the population.


Figure 9.9A

Parents

Offspring

Sperm

Eggs

Normal Dd

Normal Dd ×

D

D

d

d

DD Normal

Dd Normal

(carrier)

Dd Normal

(carrier)

dd Deaf

9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene

  The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is

–  recessive and

–  carried by about 1 in 31 Americans.

 Dominant human disorders include

–  achondroplasia, resulting in dwarfism, and

–  Huntington’s disease, a degenerative disorder of the nervous system.


Table 9.9

 New technologies offer ways to obtain genetic information –  before conception,

–  during pregnancy, and

–  after birth.

 Genetic testing can identify potential parents who are heterozygous carriers for certain diseases.

9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy


 Several technologies can be used for detecting genetic conditions in a fetus. –  Amniocentesis extracts samples of amniotic fluid

containing fetal cells and permits –  karyotyping and

–  biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease.

–  Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests.



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Figure 9.10A Amniocentesis

Ultrasound transducer

Fetus

Placenta Uterus

Cervix

Amniotic fluid extracted

Centrifugation

Amniotic fluid Fetal cells

Cultured cells

Several hours

Several weeks

Several weeks

Biochemical and genetics tests

Several hours

Several hours

Fetal cells

Cervix Uterus

Chorionic villi

Placenta Fetus

Ultrasound transducer

Tissue extracted from the chorionic villi

Chorionic Villus Sampling (CVS)

Karyotyping

 Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects.

  Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus.

 Newborn screening can detect diseases that can be prevented by special care and precautions.



 New technologies raise ethical considerations that include –  the confidentiality and potential use of results of

genetic testing,

–  time and financial costs, and

–  determining what, if anything, should be done as a result of the testing.



VARIATIONS ON MENDEL’S LAWS


9.11 Incomplete dominance results in intermediate phenotypes

 Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance.

  For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which

–  neither allele is dominant over the other and

–  expression of both alleles occurs.


Figure 9.11A P generation

F1 generation

F2 generation

2 1

2 1

2 1

2 1

2 1

2 1

Gametes

Gametes

Eggs

Sperm

×

Red RR

White rr

Pink hybrid Rr

R

R

R

R

r

r

r

r

RR rR

Rr rr

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9.11 Incomplete dominance results in intermediate phenotypes

  Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F2 generation.

 One example of incomplete dominance in humans is hypercholesterolemia, in which

–  dangerously high levels of cholesterol occur in the blood and

–  heterozygotes have intermediately high cholesterol levels.


Figure 9.11B

Normal Mild disease Severe disease

Phenotypes

Cell

LDL receptor

LDL

HH Homozygous

for ability to make LDL receptors

hh Homozygous

for inability to make LDL receptors

Genotypes Hh

Heterozygous

9.12 Many genes have more than two alleles in the population

 Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population.

 Human ABO blood group phenotypes involve three alleles for a single gene.

  The four human blood groups, A, B, AB, and O, result from combinations of these three alleles.

  The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance.


9.12 Many genes have more than two alleles in the population

  In codominance,

–  neither allele is dominant over the other and

–  expression of both alleles is observed as a distinct phenotype in the heterozygous individual.

–  AB blood type is an example of codominance.


Figure 9.12

Blood Group (Phenotype) Genotypes

Carbohydrates Present on Red Blood Cells

Antibodies Present in Blood

A

B

AB

O

IAIA or IAi

IBIB or IBi

IAIB

ii

Carbohydrate A

Carbohydrate B

Carbohydrate A and Carbohydrate B

Neither

Anti-B

Anti-A

Anti-B

Anti-A

None

No reaction Clumping reaction

O A B AB

Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left

9.13 A single gene may affect many phenotypic characters

 Pleiotropy occurs when one gene influences many characteristics.

 Sickle-cell disease is a human example of pleiotropy. This disease

–  affects the type of hemoglobin produced and the shape of red blood cells and

–  causes anemia and organ damage.

–  Sickle-cell and nonsickle alleles are codominant.

–  Carriers of sickle-cell disease are resistant to malaria.


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Figure 9.13A Figure 9.13B An individual homozygous for the sickle-cell allele

Produces sickle-cell (abnormal) hemoglobin

The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped

Sickled cell

The multiple effects of sickled cells

Damage to organs Other effects Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis)

Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections

9.14 A single character may be influenced by many genes

 Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes.

 Human skin color is an example of polygenic inheritance.


Figure 9.14 P generation

F1 generation

F2 generation

Eggs

Sperm

Skin color

Frac

tion

of p

opul

atio

n

aabbcc (very light)

AABBCC (very dark)

AaBbCc AaBbCc

×

×

8 1

64 15

64 20

64 6

64 1

64 15

64 6

64 1

8 1

8 1 8 1 8 1

8 1 8 1

8 1

8 1

8 1

8 1

8 1

8 1

8 1

8 1

8 1

9.15 The environment affects many characters

 Many characters result from a combination of heredity and the environment. For example,

–  skin color is affected by exposure to sunlight,

–  susceptibility to diseases, such as cancer, has hereditary and environmental components, and

–  identical twins show some differences.

 Only genetic influences are inherited.


THE CHROMOSOMAL BASIS OF INHERITANCE


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9.16 Chromosome behavior accounts for Mendel’s laws

  The chromosome theory of inheritance states that

–  genes occupy specific loci (positions) on chromosomes and

–  chromosomes undergo segregation and independent assortment during meiosis.


9.16 Chromosome behavior accounts for Mendel’s laws

 Mendel’s laws correlate with chromosome separation in meiosis. –  The law of segregation depends on separation of

homologous chromosomes in anaphase I.

–  The law of independent assortment depends on alternative orientations of chromosomes in metaphase I.


Figure 9.16_s1 F1 generation All yellow round seeds

(RrYy)

Meta- phase I

of meiosis Y y

R r r

r R

R

Y

Y

y

y

Figure 9.16_s2 F1 generation All yellow round seeds

(RrYy)

Meta- phase I

of meiosis

Anaphase I

Metaphase II R

y

r

Y Y

R r

y

Y

Y

y

y

R

R

r

r r

r

r R

R

R

Y

Y

Y

y

y

y

Figure 9.16_s3 F1 generation

4 1

4 1

4 1

4 1

All yellow round seeds (RrYy)

Meta- phase I

of meiosis

Anaphase I

Metaphase II

Fertilization

Gametes

F2 generation 9 :3 :3 :1

RY ry rY Ry

R

R

R

y y

y

r r

r

Y Y

Y

Y Y

Y

R

R R

r

r r

y

y y

Y

Y

y

y

R

R

r

r r

r

r R

R

R

Y

Y

Y

y

y

y

Figure 9.16_4

Sperm

Eggs

Yellow round

Green round

Yellow wrinkled

Green wrinkled

RY

RY

rY

rY

Ry

Ry ry

ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYy rrYy Rryy rryy

4 1

16 9

16 3

16 3

16 1

4 1

4 1

4 1

4 1

4 1

4 1

4 1

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9.17 SCIENTIFIC DISCOVERY: Genes on the same chromosome tend to be inherited together

 Bateson and Punnett studied plants that did not show a 9:3:3:1 ratio in the F2 generation. What they found was an example of linked genes, which –  are located close together on the same chromosome and

–  tend to be inherited together.


Figure 9.17_1

The Experiment

Purple flower

Long pollen × PpLl PpLl

Phenotypes Observed offspring

Prediction (9:3:3:1)

Purple long Purple round Red long Red round

284 21 21 55

215 71 71 24

Figure 9.17_2 The Explanation: Linked Genes

Parental diploid cell PpLl

Meiosis

P L

P L

p l

p l Most gametes

Fertilization

Sperm

Most offspring Eggs

3 purple long : 1 red round Not accounted for: purple round and red long

P L P L

P L

P L

PL

PL p l

p l

p l

p l

pl

pl

9.18 SCIENTIFIC DISCOVERY: Crossing over produces new combinations of alleles

 Crossing over between homologous chromosomes produces new combinations of alleles in gametes.

  Linked alleles can be separated by crossing over, forming recombinant gametes.

  The percentage of recombinants is the recombination frequency.


Figure 9.18A

P L

Tetrad (pair of

homologous chromosomes)

p l

p l p L

p L P l Crossing over

Parental gametes

Recombinant gametes

Figure 9.18C_1

The Experiment

Female Male

ggll GgLl

Black body, vestigial wings

Gray body, long wings (wild type)

Offspring: Gray long Black vestigial Gray vestigial Black long

Recombinant phenotypes

Parental phenotypes

Recombination frequency = = 0.17 or 17% 391 recombinants 2,300 total offspring

965 944 206 185

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Figure 9.18C_2

Offspring

Parental Recombinant

Eggs Sperm

Crossing over

G L g l g l

g l g l g l G l g L

G L g l G l g L g l

g l g l

g l The Explanation

GgLl Female

ggll Male

G L

9.19 Geneticists use crossover data to map genes

 When examining recombinant frequency, Morgan and his students found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur.

 Recombination frequencies can thus be used to map the relative position of genes on chromosomes.


Figure 9.19A

Section of chromosome carrying linked genes

Recombination frequencies

17%

9% 9.5%

g c l

Figure 9.19B

Mutant phenotypes Short aristae

Black body (g)

Cinnabar eyes (c)

Vestigial wings (l)

Brown eyes

Red eyes

Normal wings (L)

Red eyes (C)

Gray body (G)

Long aristae (appendages on head)

Wild-type phenotypes

SEX CHROMOSOMES AND SEX-LINKED GENES


9.20 Chromosomes determine sex in many species

 Many animals have a pair of sex chromosomes, –  designated X and Y,

–  that determine an individual’s sex.

  In mammals, –  males have XY sex chromosomes,

–  females have XX sex chromosomes,

–  the Y chromosome has genes for the development of testes, and

–  an absence of the Y allows ovaries to develop.


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Figure 9.20A

X

Y

Figure 9.20B

Parents (diploid)

Gametes (haploid)

Offspring (diploid)

Female

Female

Male

Male

Egg Sperm

44 +

XY 44 +

XX

22 + X

22 + Y

22 + X

44 +

XX 44 +

XY

Figure 9.21A Figure 9.21A_1

Figure 9.21A_2 Figure 9.21B Male Female

Sperm

Eggs

R = red-eye allele r = white-eye allele

XR

Xr

XrY XRXR

XRXr XRY

Y

16

Figure 9.21C


Male Female

Sperm

Eggs XR

xR

XRY XRXr

Y

Xr XrY

XRY XRXR

XrXR

Figure 9.21D


Male Female

Sperm

Eggs

XrY

XRXr

XrXr Xr

XR

Xr Y

XrY

XRY

XRXr

9.22 CONNECTION: Human sex-linked disorders affect mostly males

 Most sex-linked human disorders are

–  due to recessive alleles and

–  seen mostly in males.

 A male receiving a single X-linked recessive allele from his mother will have the disorder.

 A female must receive the allele from both parents to be affected.


9.22 CONNECTION: Human sex-linked disorders affect mostly males

 Recessive and sex-linked human disorders include

–  hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting,

–  red-green color blindness, a malfunction of light-sensitive cells in the eyes, and

–  Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination.


Figure 9.22

Female Male Hemophilia

Carrier

Normal Alexis

Alexandra Czar Nicholas II of Russia

Queen Victoria

Alice Louis

Albert

9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution

  The Y chromosome provides clues about human male evolution because

–  Y chromosomes are passed intact from father to son and

–  mutations in Y chromosomes can reveal data about recent shared ancestry.


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