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Sex chromosomes and Sex determination

In animal kingdom, sex is perhaps the most conspicuous

phenotype.

Animals with distinct male and female are sexually dimorphic.

Sometimes, this dimorphism is established by environmental

factors.

In many other species, sex is determined by the pair of

sex chromosomes.

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In some cold-blooded vertebrates, some fishes, many reptiles

(e.g. certain snakes, lizards, turtles, and all crocodiles andalligators) as well as in some invertebrates (e.g. certaincrustaceans),

sex is determined after fertilization not by sexchromosomes deposited in the egg.

The choice is usually determined by thetemperature atwhich early embryonic development takes place.

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e.g. :- In one species of turtle, if eggs are incubatedabove 30C, they hatch as females. Eggs that havebeen incubated at lower temperatures, they hatch asmales !!!

In some cases (e.g. many turtles and lizards), ahigher temperature during incubation favors the

production of females.

In other cases (e.g., alligators), a higher temperaturefavors the production of males.

Even in cases (e.g. some lizards) where there are sexchromosomes, a high temperature can convert agenotypic male (ZZ) into a female.

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In humans and other mammals, sex determination generally proceeds in

the direction of female developmentunless genes involved in testis

determination are activated. The SRY geneis believed to be the switch

that initiates the testis development.

SRY

SRY(for sex-determining region Y)is a gene

located on the short (p) arm just outside the

pseudoautosomal region. It is the master

switch that triggers the events that converts the

embryo into a male. Without this gene, you get a

female instead.

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In human being and other mammals malenessis due tothe dominant effect of Y chromosome. This is evident

from the study of individuals with abnormal number of sexchromosomes.

XOanimals develop as females

XXYanimals develop as male

Early in the embryonic stage, the Y chromosome directsthe primordial gonads to develop into testes.

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Zygote

X X X Y

Ovary

Female sexualcharacteristics

In the absence ofY chromosome,

no TDFis

produced.

Step

1

The lack of TDF

allows the cortex of

the embryonic gonads

to develop into

ovaries.

Step

2

In the absence

of testosterone,

the embryo

develops

female

characteristics.

Step

3

TDF induces the

medulla of the

embryonic gonads

to develop into

testes

Step

2

The testes produces

testosterone, a

hormone that initiates

the development of

male sexualcharacteristics.Male sexualcharacteristics

Step

3

cortex Medulla

Embryonic

gonads

The testis-determining

factor (TDF) is produce

by a gene on the Y

chromosome

Step

1

SRY

gene

TDF

Testis

Differentiatedgonads

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X X X Y X Y

XX male Normal XYmale

XY female

SRY

geneMissing segmen

that contains th

SRYgene

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Testisis formed

Testis secretes testosterone

Testosterone binds with receptorsin many kind of cells

Once bound,

the hormone-receptor complextransmits a signalto thenucleus, instructing the cell in how to differentiate.

The concerted differentiation of many types of cells lead to the

development of distinct male characteristics, such as

heavy musculature, beard and deep voice.

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If the testosterone signalingsystem fails

The individual develops into a female

One reason for this failure is the inabilityto make the testosterone

receptor.

XY individualswith this biochemical deficiency initially developsas males-----------testesare formed and testosteroneis

produced.

However, testosterone dont show any effect as there is no receptor for it on

the cells. So, it cannot transmit signal into the cells instructing it todifferentiate as a male cell.

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These people without testosterone receptor acquire female

sexual characteristics. Although, they do not have any

ovariesand they are sterile.

This syndrome is called testicular feminization,

results from a mutation in an X-liked gene, Tfm,which encodes the testosterone receptor.

The tfmmutation is transmitted from mothers to their

hemizygous XY offspring (who are phenotypically

female) in a typical X-linked pattern.

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X

Y

SRY TDF testis Testosterone

Tfm Testosterone

receptor

Testosterone

Receptor complex

Signals male

differentiation

Male secondary

sexual

characteristics

X

Y

SRY TDF testis Testosterone

Tfm

mutation

No

Testosterone

receptor

No Testosterone

Receptor complex

Female secondary

sexual

characteristics

No signal

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Sex determination in Drosophila

The Y chromoosome in Drosophila do not play any role in sex determination.

The sex of the fly is determined by the ratio of X chromosomes to

autosomes (first discovered by Bridges in 1921).

Normal flies have a pair of sex chromosomes (either XX or XY) and

three pairs of autosomes (AA).

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Bridges worked with flies having abnormal number of

chromosomes and observed that

whenever the ratio X:Awas 1 or greater, the fly wasfemale.

And whenever it was 0.5 or less, the fly was a male.

Flies with an X:A ratiobetween 0.5 and 1developed

characteristics of both the sexes. Thus, Bridges calledthem intersexes.

In none of these fliesY chromosomehad any effect on

the sex.

However, Y is required for male fertility.

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Ratio of X Chromosomes to Autosomes and the

corresponding phenotype in Drosophila.

X Chromosomes(X) and set of

Autosomes (A)

X:A ratio Phenotype

1X 2A 0.5 Male

2X 2A 1.0 Female

3X 2A 1.5 Metafemale

4X 3A 1.33 Metafemale

4X 4A 1.0 Tetraploid Female

3X 3A 1.0 Triploid Female

3X 4A 0.75 Intersex

2X 3A 0.67 Intersex

2X 4A 0.5 Tetraploid Male

1X 3A 0.33 Metamale

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Sex Determination in Other Animals

Males of humans and Drosophila are referred to asHETEROGAMETIC SEXas they have two types of gametes (X

and Y). Females are homogametic sex. (X and X).

In birds, butterflies and some reptiles:

this situation is reversed.

Males are homogametic (ZZ) and females are

heterogametic (ZW).

However, little is known about the mechanism of sex

determination in the Z-X sex chromosome system.

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In a haplo-diplo system of sex determination,

## eggs are produced through meiosis in the queen,

##spermsare produced through mitosisin the male.This system ensures that fertilized eggs will have the diploid

chromosome number and that unfertilized eggs will have

the haploid number.

In this system a queencan control the ratio of malesto

females by regulating the proportion of unfertilized eggsthat she lays. Because this number is small, most of the

progeny are female, albeit sterile, and serve as workers for

the hive.

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Z W

ZZ Z

ZZ

W

x

Sex Determination in Birds

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x

Diploid

Unfertilized egg

Haploid

Fertilized egg

Haploid

Diploid

Sex

Determination

in

Honeybees