15323_SEX DIFFERENTIATION AND SEX – INFLENCED DOMINANCE

8/6/2019 15323_SEX DIFFERENTIATION AND SEX INFLENCED DOMINANCE

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SEX DIFFERENTIATION AND SEX

INFLENCED DOMINANCE


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SEX CHROMATIN BODIES

M.L. Barr observed chromatin bodies in nerve

cells of female cats


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Barr Bodies

Darkly staining region found in nuclei of normalfemales (46,XX) and not in normal males (46,XY)

Also called sex chromatin body

XXY males have one, XXX females have two,others more

Why are individuals with abnormal karyotypesinvolving sex chromosomes generally completely

normal?

Perhaps inactivation isnt complete

Perhaps damage is done developmentally beforeinactivation occurs


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Barr Bodies Female cells shown to

right at top, male cell

to right at bottom


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Karyotypes

and BarrBodies


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Mechanism of Inactivation

X-chromosome encoded X-inactivation center(Xic) is major control unit

About 1 million bp in size

Four genes, one encodes Xist (X-inactive specific

transcript) No ORF on transcript

Structural RNA, acting in mechanism of inactivation

Expression ofXic leads to inactivation of

chromosome expressing the Xist locus (cis-acting) All X chromosomes express at low level, one at high and

is inactivated

Deletion of region makes chromosome unable to beinactivated


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Dosage Compensation

In mammals, males have 1 X while females

have 2. Having only 1 copy of any other

chromosome would be lethal. How can the X

be present in 1 copy or 2 copies and produce

normal offspring in either case?

Basic answer: only 1 X is active in each female

cell.


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Lyon Hypothesis

It has long been known that femalecells contain Barr bodies, blobs ofchromatin located on the inside ofthe nuclear membrane. Each femalecell has 1 Barr body; male cells donthave Barr bodies.

Mary Lyon proposed that Barr bodiesare inactive X chromosomes, and thatmammalian cells inactivate all butone of their Xs, converting the extrasinto Barr bodies.

Proof: XXY individuals are male, buthave a Barr body; XO individuals are

female but have no Barr bodies; XXXindividuals are female with 2 Barrbodies in each cell.


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Lyon Hypothesis

1961, Mary Lyon and Liane Russel, independently

Inactivation of X chromosome is random (which

one), occurs in somatic cells at an early stage ofembryonic development and is then passed on to

progeny cells by mitosis

Female mice and coat color genes, calico cats and fur

color/patterns

Also human G6PD (glucose 6-phosphate

dehydrogenase

No fibroblast clone expresses more than one isozyme


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Calico Cats

Orange and black patches reflect X chromosome

inactivation, white patches due to another gene


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Fragile X Chromosome and Mental

Retardation

Inactivation of one of the two sex

chromosome in female mammals

Reactivation of heterochromatic X

chromosomes.

Fragile X-chromosomes contains fragile site.Near end of long arm Xq27.


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Hormones and Sex differentiation

External or secondary sexcharacteristics.

Hormones synthesized in ovary and testis are

controlled by pituitary glands.


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Sex Influenced Dominance

Dominance of Alleles may differ in

heterozygotes of two sexes.

Examples: Sheep

Two breeds: 1. dorset sheep (both sexes

horned) (h+h+).autosomal chromosomes. 2. suffolk sheep(hh)-hornless

Cross between h+h+ X hh


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h+h (heterozygous). Males horned and female

hornless.

F2- 3:1F2 feamle 1 horned and 3 hornless.


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Sex-Influenced Traits

A sex-influenced trait is an autosomal traitthat is dominant in one sex and recessive inthe other. Good examples: male patternbaldness in humans and horns in sheep.

Pattern baldness is found in both sexes, but is

rarer in females. Females usually get very thinhair all over, instead of the classic recedinghairline and bald spot on top that men get.

Baldness is autosomal, but it is dominant inmales and recessive in females. Thus, maleheterozygotes are bald but femaleheterozygotes have normal hair.

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15323_SEX DIFFERENTIATION AND SEX – INFLENCED DOMINANCE