lecture07 chromosomal basis inheritance sv.ppt

8/17/2019 Lecture07 Chromosomal Basis Inheritance Sv.ppt

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Lecture 7

CHROMOSOMAL BASIS OF

INHERITANCE

Outline

• Chromosome theory of inheritance

• Discovery of sex-linked genes

• Chromosomal basis of sex

• Sex-linked inheritance in humans

• Inactivation of X chromosomes

• Inheritance of linked genes

• Alteration of chromosome numberand structure

Mendelian InheritanceReview

• Chromosomes (and genes) are present

in diploid cells

•

Homologous chromosomes separateduring meiosis

• Allelles segregate during meiosis

• Fertilization restores 2n condition for

chromosomes and genes


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Chromosome Theory of Inheritance

Mendelian genes have specific loci (positions)

along chromosomes and it is the chromosomes

that undergo segregation and independent

assortment

Behavior of

homologous

chromosomes during

meiosis

2 genes:Y = yellow

y = green

R = round

r = wrinkled

Morgan’s Experimental Evidence

First experimental evidence showing an association betweengenes and chromosomes came from experiments done by Dr.

Thomas Morgan

Fruit fly: Drosophila melongaster :

Ideal study animal:

Fast breeder, large number off offspring, large sample size

4 pairs of chromosomes; females = XX, males = XY

Morgan was looking for variation:Eye color: red = wild type, white = mutant phenotype


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All flies have red eyes:

Conclusion: red eye color dominant towhite eye color

Female flies have red eyes, male flies

have white eyes

Conclusion: if only males show white

eyes, then eye color gene must be

located on X chromosome

Sex-linked InheritanceThe fruit fly Drosophila melanogaster

Eye color is a variable trait.

1 mm

A trait

determined by

genes on the

sex

chromosomes

is said to be

_________

Reciprocal Crosses Confirm That Eye Color in

Drosophila Is an X-Linked Trait

One half of reciprocal cross

FemaleMale gametes

Male

MalesFemales

F e m a l e

g a m e t e s

w+ = red eyes, w = white eyes


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Eye Color in Drosophila Is an X-Linked Trait

MalesFemales

Female

MaleOther half of reciprocal cross

Male gametes

F e m a l e

g a m e t e s

Terminology

X and Y: sex chromosomes

X-linked inheritance: a gene resides on the X chromosome

Y-linked inheritance: a gene resides on the Y chromosome

sex-linked inheritance: a gene is on either sex chromosome

autosomes: non sex chromosomes

autosomal inheritance: shown by genes on non-sex

chromosomes

Chromosomal Basis of Sex

The X and Y chromosomes are sex

chromosomes—they determine the sex

of offspring

(XX = female; XY = male)

X and Y chromosomes are different in

size and shape, but act like ________

during meiosis


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Chromosomal Basis of Sex

-

each egg contains one X chromosome-

half the sperm contain an X

chromosome and half contain the Y

chromosome

Sex determination is a 50-50 chance

form gametes

Various chromosomal

systems of sex

determination


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Inheritance of X-linked Genes(color-blindness)

Figure 10.26

Figure 10.26

Red-green color blindness is a sex-linked trait inhumans

Hemophilia: A Sex-Linked Trait

• Hemophilia – proteinsrequired for blood clottingare absent

• blood clots slowly or not

at all

• Two genes that encodeblood-clotting proteins areon the X chromosome

• X-linked


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- Queen Victoria – mutant

allele

- 3/9 children received

defective allele

- Allele transferred by

marriage to other royal

families

Escaped the disorder

In all, 10 of Victoria’s male descendants had hemophilia

Patterns in X-linked Recessive

Phenotypes

• One copy of allele is needed for expression in males, ___________ In females

• Daughters who receive one mutant X chromosomeare ___________

• Male with mutation passes it to his daughters; sonsget _______chromosome


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X Inactivation in Female Mammals

XX = females, XY = males

In females, one X is inactivated, thusfemales and males now have one

copy of X-linked genes

- the inactive X condenses to form a

Barr body

Which X forms the Barr body occurs

randomly and independently in

embryonic cells

X Inactivation in Female Mammals

In ovaries, Barr-body chromosomes

are re-activated in the cells that give

rise to eggs, resulting in every female

gamete having an active X

How Linkage Affects Inheritance

Morgan looked at fruit flies of 2 differentphenotypes:

• Wild-type flies: grey body, normal wings

• Mutant flies: black bodies, vestigial wings

•

Mutant alleles are recessive to wild-type and

are not sex-linked


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How Linkage Affects Inheritance

Most offspring had a parental phenotype

suggesting the body

color and wing size

are linked on the

same chromosome

and are thus inherited

together

Linked Genes: Crossing Over

Alteration of Chromosome Number or

Structure Cause Genetic Disorders


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Abnormal Chromosome Number

Aneuploidy: an abnormal number of chromosomes

Monosomic: a missing chromosome;

Trisomic: an extra chromosome;

Polyploidy: more than 2n chromosomes in all somatic

cells; fairly common in plants

Nondisjunction:

Abnormal

Chromosome

Number

Down Syndrome

Most cases of trisomy 21 result from nondisjunction during meiosis 1


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Nondisjunction of

the X chromosome

1/1000

1/1000 1/2500

Alteration of Chromosome Structure

Deletions and duplications are

likely to occur during meiosis

eg. in crossing over, one can have

an exchange of unequal-sized

segments of DNA

Translocations and inversions can

alter phenotype because gene

expression can be affected by the

location of neighboring genes

Disorders Caused by Structurally

Altered Chromosomes

Some are

implicated in

cancers

Cri du chat: deletion in chromosome 5

lecture07 chromosomal basis inheritance sv.ppt

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