Chromosomal Basis of Inheritance

Genes reside on chromosomes.Sex Chromosomes and Autosomes

Sex chromosomes contain genes that determine an organism’s sex (gender). The remaining chromosomes that are

not directly involved in determining the sex of an individual are called

autosomes.

Karyotypes: Male and Female

Chromosomal Inheritance

• Sex Determination– In mammals, an individual carrying two X

chromosomes is female. – An individual carrying an X and a Y chromosome is

male.– Sex chromosomes pair during meiosis I. Meiosis

proceeds, paired chromosomes separate = move to different cells.• Sperm has equal chance getting X or a Y chromosome• Egg only gets an X

Sex chromosomes

• Male mammals, Y chromosome contains a gene called – SRY– Sex-determining Region Y– Codes for protein that causes the gonads of

embryo to develop as testes.– Females don’t have SRY gene – develop ovaries

Effects of Gene Location

• Sex-Linked Genes and Traits– Genes found on the X chromosome are X-linked

genes. – A sex-linked trait is a trait whose allele is located

on a sex chromosome.– Because males have only one X chromosome, a

male who carries a recessive allele on the X chromosome will exhibit the sex-linked trait.

Gene location

• Linked Genes– Pairs of genes that tend to be inherited together

are called linked genes. • Chromosome Mapping– The farther apart two genes are located on a

chromosome, the more likely a cross-over will occur.

– Researchers use recombinant percentages to construct chromosome maps showing relative gene positions.

Genes on Chromosomes

• Thomas Hunt Morgan– Provided convincing evidence that chromosomes

are the location of Mendel’s heritable factors

• Morgan worked with fruit flies– Because they breed at a high rate – A new generation can be bred every two weeks– They have only four pairs of chromosomes

• Morgan first observed and noted– Wild type, or normal, phenotypes that were

common in the fly populations

• Traits alternative to the wild type– Are called mutant phenotypes

Figure 15.3

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

Figure 15.4

The F2 generation showed a typical Mendelian 3:1 ratio of red eyes to white eyes. However, no females displayed the white-eye trait; they all had red eyes. Half the males had white eyes,and half had red eyes.

Morgan then bred an F1 red-eyed female to an F1 red-eyed male toproduce the F2 generation.

RESULTS

PGeneration

F1

Generation

X

F2

Generation

Morgan mated a wild-type (red-eyed) female with a mutant white-eyed male. The F1 offspring all had red eyes.

EXPERIMENT

• Morgan determined – That the white-eye mutant allele must be located on

the X chromosome

CONCLUSION Since all F1 offspring had red eyes, the mutant white-eye trait (w) must be recessive to the wild-type red-eye trait (w+). Since the recessive trait—white eyes—was expressed only in males in the F2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here.

PGeneration

F1

Generation

F2

Generation

Ova(eggs)

Ova(eggs)

Sperm

Sperm

XX X

XY

WW+

W+

W

W+W+ W+

W+

W+

W+

W+

W+

W

W+

W W

W

Gene on Chromosomes

• Morgan’s discovery that transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait– Was the first solid evidence indicating that a specific

gene is associated with a specific chromosome• Genes on X chromosome X-linked genes• Genes on the Y chromosome Y-linked (SRY in humans)

– Sex-linked

• Morgan determined that– Genes that are close together on the same

chromosome are linked and do not assort independently

– Unlinked genes are either on separate chromosomes of are far apart on the same chromosome and assort independently

Parentsin testcross

b+ vg+

b vg

b+ vg+

b vg

b vg

b vg

b vg

b vg

Mostoffspring

X

or

• Sex-linked genes– Follow specific patterns of inheritance

Figure 15.10a–c

XAXA XaY

Xa Y

XAXa XAY

XAYXAYa

XA

XA

Ova

Sperm

XAXa XAY

Ova XA

Xa

XAXA XAY

XaYXaYA

XA YSperm

XAXa XaY

Ova

Xa Y

XAXa XAY

XaYXaYa

XA

Xa

A father with the disorder will transmit the mutant allele to all daughters but to no sons. When the mother is a dominant homozygote, the daughters will have the normal phenotype but will be carriers of the mutation.

If a carrier mates with a male of normal phenotype, there is a 50% chance that each daughter will be a carrier like her mother, and a 50% chance that each son will have the disorder.

If a carrier mates with a male who has the disorder, there is a 50% chance that each child born to them will have the disorder, regardless of sex. Daughters who do not have the disorder will be carriers, where as males without the disorder will be completely free of the recessive allele.

(a)

(b)

(c)

Sperm

• Some recessive alleles found on the X chromosome in humans cause certain types of disorders– Color blindness– Duchenne muscular dystrophy– Hemophilia

Inheritance of Sex-Linked Genes

– Muscular dystrophy• Absence of a key muscle protein – dystrophin• Gene locus on the X chromosome

– Hemophilia• Sex-linked recessive disorder• Absence of one or more proteins used for clotting

X inactivation in Female Mammals

• In mammalian females– One of the two X chromosomes in each cell is randomly

inactivated during embryonic development • Barr body• Genes not expressed• In ovaries, Barr body chromosomes are reactivated to give rise

to ova– Female gamete has an active X

• Large-scale chromosomal alterations– Often lead to spontaneous abortions or cause a

variety of developmental disorders• Chromosome Mutations– Chromosome mutations are changes in the

structure of a chromosome or the loss or gain of an entire chromosome.

• Gene Mutations– Gene mutations are changes in one or more of the

nucleotides in a gene.

Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders

Abnormal Chromosome Number• When nondisjunction occurs– Pairs of homologous chromosomes do not separate

normally during meiosis– Gametes contain two copies or no copies of a

particular chromosome

Figure 15.12a, b

Meiosis I

Nondisjunction

Meiosis II

Nondisjunction

Gametes

n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes

Nondisjunction of homologouschromosomes in meiosis I

Nondisjunction of sisterchromatids in meiosis II

(a) (b)

Aneuploidy

– Results from the fertilization of gametes in which nondisjunction occurred

– Is a condition in which offspring have an abnormal number of a particular chromosome

• If a zygote is trisomic– It has three copies of a particular chromosome

• If a zygote is monosomic– It has only one copy of a particular chromosome

Alterations of Chromosome Structure

• Breakage of a chromosome can lead to four types of changes in chromosome structure– Deletion– Duplication– Inversion– Translocation

Alterations of chromosome structure

Figure 15.14a–d

A B C D E F G HDeletion

A B C E G HF

A B C D E F G HDuplication

A B C B D EC F G H

A

A

M N O P Q R

B C D E F G H

B C D E F G HInversion

Reciprocaltranslocation

A B P Q R

M N O C D E F G H

A D C B E F HG

(a) A deletion removes a chromosomal segment.

(b) A duplication repeats a segment.

(c) An inversion reverses a segment within a chromosome.

(d) A translocation moves a segment fromone chromosome to another,nonhomologous one. In a reciprocal

translocation, the most common type,nonhomologous chromosomes exchangefragments. Nonreciprocal translocationsalso occur, in which a chromosome transfers a fragment without receiving afragment in return.

Human Disorders Due to Chromosomal Alterations

• Alterations of chromosome number and structure– Are associated with a number of serious human

disorders

Down Syndrome• Down syndrome– Is usually the result of an extra chromosome 21,

trisomy 21

Figure 15.15

Aneuploidy of Sex Chromosomes

• Nondisjunction of sex chromosomes– Produces a variety of aneuploid conditions

• Klinefelter syndrome– Is the result of an extra chromosome in a male,

producing XXY individuals

• Turner syndrome– Is the result of monosomy X, producing an X0

karyotype

Genetics Problems

• Pattern baldness is an example sex influenced trait that is dominant in males and recessive in females. A nonbald couple produced a bald son. What are the phenotypes of the parents?

• Father is not bald – homozygous bb• Nonbald female can be Bb or bb • If son is bald then she must be Bb

• Hemophilia is a recessive X-linked trait in humans. If a heterozygous woman has children with a normal man, what are the odds of the following combinations of children?

• A) an affected son• B) an unaffected child• B) four unaffected offspring in a row

• 1) Set up Punnett square• 2) ¼• 3) ¾• 4) 3/4x3/4x3/4x3/4 = 81/256

• In humans, colorblindness (cc) is a recessive In humans, colorblindness (cc) is a recessive sex-linked trait.sex-linked trait.

• Remember:Remember: XX - femaleXX - femaleXY - maleXY - male

• Two normal people have a colorblind son.Two normal people have a colorblind son.

• What are the What are the genotypesgenotypes of the parents? of the parents?

• XXCCXX_?_? x x X XCCYY

• What are the What are the genotypesgenotypes and and phenotypesphenotypes possible among their other children?possible among their other children?

XXCC Y Y parents parents

XXCC X XCCXXCC X XCCYY

XXcc X XCCXXcc X XccYY

50%: female (one normal, one a carrier)50%: female (one normal, one a carrier)50%: male (one normal, one colorblind)50%: male (one normal, one colorblind)

• A couple has a colorblind daughter.A couple has a colorblind daughter.

• What are the possible genotypes and What are the possible genotypes and phenotypes of the parents and the daughter?phenotypes of the parents and the daughter?

XXcc Y Y

XXCC X XCCXXcc X XCCYY

XXcc X XccXXcc X XccYY

parents: Xparents: XccY and XY and XCCXXc c or Xor XccXXcc

father colorblindfather colorblind mother carrier or colorblindmother carrier or colorblind

daughter: Xdaughter: XccXXcc - colorblind - colorblind

X-Linked Gene

• Genes that are physically located on the X chromosome

• Hemizygous– Used to describe single copy of an X-linked gene in

the male

X-Linkage Recessive Trait

• An affected male has a carrier mother• An affected female has a carrier or affected

mother and an affected father• A carrier (female) has a carrier mother or an

affected father

X-Linked Dominant Inheritance

• X-linked conditions and traits are rare• Expressed in female in one copy– Has the associated trait/illness– High rate miscarriage rate for male babies– Affected female has an affected mother Males have more severe effects Passed from male to all daughters but to no sons

Problem

• An unaffected woman who is heterozygous for the X-linked allele causing Duchenne muscular dystrophy has children with a normal man. What are the probabilities of the following?– An unaffected son– An unaffected son or daughter– A family of three children, all whom are affected

• Hemophilia is a recessive X-linked trait in humans. If a heterozygous woman has children with a normal man, what are the odds of the following combinations of children?– An unaffected son– Four unaffected offspring in a row