Chapter 15Chromosomal Basis of Inheritance

Concept 15.1: Mendelian inheritance has its physical basis in the behavior of chromosomes

• Mitosis and meiosis were first described in the late 1800s

• The chromosome theory of inheritance states:– Mendelian genes have specific loci (positions)

on chromosomes

– Chromosomes undergo segregation and independent assortment

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Figure 15.2a

P Generation Yellow-roundseeds (YYRR)

Green-wrinkledseeds (yyrr)

Meiosis

Fertilization

Gametes

Y

YR R

YR

y

yr

y r

r

Figure 15.2b

F1 Generation

All F1 plants produceyellow-round seeds (YyRr).

Meiosis

Metaphase I

Anaphase I

Metaphase II

R R

R R

R R

R R

R R R R

r r

r r

r r

r r

r r r r

Y Y

Y Y

Y Y

Y Y

Y Y Y Y

y y

y y

y y

y y

yy y y

Gametes

LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation.

LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.

1

2 2

1

1/41/4

1/41/4YR yr Yr yR

Figure 15.2c

F2 Generation

3Fertilization recombines the R and r alleles at random.

Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.

An F1 F1 cross-fertilization

9 : 3 : 3 : 1

LAW OF SEGREGATION LAW OF INDEPENDENTASSORTMENT

3

Morgan’s Experimental Evidence: Scientific Inquiry

• -specific gene / specific chromosome came from Thomas Hunt Morgan

• Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors

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• Worked with fruit flies- why?

– wild type, or normal, phenotypes that were common in the fly populations

– mutant phenotypes -alternative to the wild type

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Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair

• In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)

– The F1 generation all had red eyes

– The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes

• Morgan determined that the white-eyed mutant allele must be located on the X chromosome

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Figure 15.4b

F2

Generation

PGeneration

Eggs

Eggs

Sperm

Sperm

Xw

CONCLUSION

XX Y

w

ww w

w

ww w

w

w

w

w

w

ww w

F1

Generation

Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance

• In humans other animals, there is a chromosomal basis of sex determination

• Only ends of Y chromosome have regions that are homologous with regions of the X chromosome

• The SRY gene on the Y chromosome– Sex determining Region of Y

– If absent gonads develop into ovaries

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Figure 15.5

X

Y

• Females are XX, and males are XY• Each ovum contains an X chromosome, while

a sperm may contain either an X or a Y chromosome

• Other animals have different methods of sex determination

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• A gene that is located on either sex chromosome is called a sex-linked gene

• Genes on the Y chromosome are called Y-linked genes; there are few of these

• Genes on the X chromosome are called X-linked genes– Fathers- x-linked to all daughters/no sons

– Mothers-x-linked to sons and daughters

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Recessive Sex-Linked TraitFemale must be homozygous in

order to express the trait• Fewer females with sex-linked

disordersMales are hemizygous• a condition where only one

copy of a gene is present in a diploid organism

• More males than females have x-linked disorders

Examples• Color blindness, hemophilia,

Duchenne muscular dystrophy

Sex Linkage ProblemColor Blindness - recessive

A man who is color blind is married to a female that carries the color blindness allele.

What is the probability that a son would be color blind?

Daughter?A daughter that is a carrier?

Sex Linkage ProblemHemophilia- recessive

A man with hemophilia marries a woman with normal clotting factors but is heterozygous.

What is the probability that they will have a … daughter with hemophilia?

What is the probability that they will have a… son with hemophilia?

What is the probability that their son… will have hemophilia?

X Inactivation in Female Mammals

• one of the two X chromosomes in each cell is randomly inactivated during embryonic development

• The inactive X condenses into a Barr body– Reactivated in gonad cells

– Females are a “mosaic” of cells

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Figure 15.8

Early embryo:

X chromosomesAllele fororange fur

Allele forblack fur

Two cellpopulationsin adult cat:

Cell division andX chromosomeinactivation

Active XInactive X

Active X

Black fur Orange fur

• Each chromosome has hundreds or thousands of genes (except the Y chromosome)

• Genes located on the same chromosome that tend to be inherited together are called linked genes

Concept 15.3: Linked genes tend to be inherited together because they are located near each other on the same chromosome

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How Linkage Affects Inheritance

• Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters

• Saw higher # of parental types than expected if it was a dihybrid cross- determined wing/body color inherited together– these genes do not assort independently, and

reasoned that they were on the same chromosome

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Figure 15.9-4P Generation (homozygous)

Wild type(gray body, normal wings)

F1 dihybrid(wild type)

Testcrossoffspring

TESTCROSS

b b vg vg

b b vg vg

b b vg vg

b b vg vg

Double mutant(black body,vestigial wings)

Double mutant

Eggs

Sperm

EXPERIMENT

RESULTS

PREDICTED RATIOS

Wild type(gray-normal)

Black-vestigial

Gray-vestigial

Black-normal

b vg b vg b vg b vg

b b vg vg b b vg vg b b vg vg b b vg vg

965 944 206 185

1

1

1

1

1

0

1

0

If genes are located on different chromosomes:

If genes are located on the same chromosome andparental alleles are always inherited together:

:

:

:

:

:

:

:

:

:

b vg

Genetic Recombination

• nonparental phenotypes produced• the production of offspring with combinations

of traits differing from either parent– Recombinants

– 50% recombinants expected for genes on different chromosomes- ie independent assortment

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Figure 15.UN02

Gametes from green-wrinkled homozygousrecessive parent (yyrr)

Gametes from yellow-rounddihybrid parent (YyRr)

Recombinant offspring

Parental-type

offspring

YR yr Yr yR

yr

YyRr yyrr Yyrr yyRr

Recombination of Linked Genes: Crossing Over

• Morgan discovered that genes can be linked, but the linkage was incomplete-some recombinant phenotypes were observed

• He proposed:– some process must occasionally break the

physical connection between genes on the same chromosome

• That mechanism was the crossing over of homologous chromosomes

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Figure 15.10b

Testcrossoffspring

965Wild type

(gray-normal)

944Black-

vestigial

206Gray-

vestigial

185Black-normal

Sperm

Parental-type offspring Recombinant offspring

Recombinationfrequency

391 recombinants2,300 total offspring

100 17%

b vg b vgb vgb vg

b vg b vg b vg b vg

b vg

Eggs

Recombinantchromosomes

bvg b vg b vg b vg

Recombinant FrequencyProblem

• Total offspring – 1566• Parental types – 1472• Recombinants – 94• Frequency = (# of recombinants/Total) * 100

= %

=(distance between the two alleles

aka map units)

Recombinant FrequencyProblem

• A wild type fruit fly (heterozygous for gray body color and red eyes) was mated with a black fruit fly with purple eyes. What is the recombinant frequency for these genes?

• Wild type – 721• Black purple – 751• Gray purple – 49• Black red - 45

Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry

• Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome

• Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

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• A linkage map is a genetic map of a chromosome based on recombination frequencies

• Distances between genes can be expressed as map units; one map unit, represents a 1% recombination frequency

• Map units indicate relative distance and order, not precise locations of genes

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Errors and ExceptionsAlterations of chromosome

number• Increase or decrease in the

number of chromosomes from the normal diploid number

• Occurs during anaphase of meiosis

Alterations of chromosome structure

• Actual chromosome is altered

Alterations of Chromosome Number

Nondisjunction• Certain homologous chromosomes fail to

separate during meiosis• May occur in anaphase I or anaphase II• Results in aneuploidy

Meiosis I

Meiosis II

Nondisjunction

Non-disjunction

Gametes

Number of chromosomes

Nondisjunction of homo-logous chromosomes inmeiosis I

(a) Nondisjunction of sisterchromatids in meiosis II

(b)

n 1 n 1n 1 n 1 n 1n 1 n n

Figure 15.13-3

• Aneuploidy results from the fertilization of gametes in which nondisjunction occurred– Offspring have an abnormal number of a

particular chromosome

• monosomic zygote has only one copy of a particular chromosome

• trisomic zygote has three copies of a particular chromosome

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• Polyploidy is a condition in which an organism has more than two complete sets of chromosomes

– Triploidy (3n) is three sets of chromosomes

– Tetraploidy (4n) is four sets of chromosomes

– common in plants, but not animals

– more normal in appearance than aneuploids

– Ex. Downs Syndrome

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Figure 15.15

Alterations of Chromosome StructureDeletion• Chromosomes lose a

fragment of DNA• Cri du chat syndrome (#5)

– A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood

Inversion• Fragments break off then

reattach in reverse orientation

Alterations of Chromosome StructureTranslocation• Fragments reattach

to nonhomologous chromosomes

• Chronic mylegenous leukemia (22 switches with 9)

Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance

• There are two normal exceptions to Mendelian genetics– genes located in the nucleus

– genes located outside the nucleus

• In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance

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Mother v. Father Inheritance

For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits

Genomic imprinting• Induces intrinsic changes in chromosomes

inherited from males and females• Silencing of certain genes that are “stamped”

with an imprint during gamete production• Prader-Willi Syndrome (paternal)• Angelman Syndrome (maternal)Fragile X Syndrome• consequence of maternal genomic imprinting

Figure 15.17b

Mutant Igf2 alleleinherited from mother

Mutant Igf2 alleleinherited from father

Normal-sized mouse (wild type) Dwarf mouse (mutant)

Normal Igf2 alleleis expressed.

Mutant Igf2 alleleis expressed.

Mutant Igf2 alleleis not expressed.

Normal Igf2 alleleis not expressed.

(b) Heterozygotes

• It appears that imprinting is the result of the methylation (addition of —CH3) of cysteine nucleotides

• Genomic imprinting is thought to affect only a small fraction of mammalian genes

• Most imprinted genes are critical for embryonic development

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Inheritance of Organelle Genes• Extranuclear genes (or cytoplasmic

genes) are found in organelles in the cytoplasm

• Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules

• Extranuclear genes are inherited MATERNALLY because the zygote’s cytoplasm comes from the egg

• The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant

• defects in mitochondrial genes prevent cells from making enough ATP– Make protein complexes of electron transport and

ATP synthase

– diseases that affect the muscular and nervous systems

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