SEX DETERMINATION

Prof Anju ChaudharyB.ed, M.sc, Phd

What is sex determination?The method by which distinction between male

and female is established in an organism under genetic control.

The sex chromosomes are responsible for determination of separate sexes.

It is a biological system that determines the development of sexual characteristics in an organisms.

Traditionally, the symbol ♂ designates male and the symbol ♀ designates female.

The XX-XY type of chromosomal sex determinationsSince the female are XX , as a result of meiosis during

the formation of gametes the egg cell contain only one type of egg.

The males consist of XY, and produce two type of sperm. Out of which half of the sperm consist of X chromosome and the other half consist of Y chromosome.

During fertilization, if the sperm containing X chromosomes fuses with the egg, the zygote XX develops into female.

While the sperm containing Y chromosomes fuses with an egg to produce a male with XY chromosomes.

Homogametic: The female is called the homogametic sex because only one type of gamete(X-bearing) is produced.

Heterogametic: The male is called as heterogametic sex because two different gametes(X and Y – bearing) are produced.

Sex ratio: The ratio of sex at fertilization is expected to be 1:1 because males produce equal number of X bearing and Y bearing sperm.

Sex determination in Various OrganismsGenotype of female Genotype of male

1. Insects XX XO

2. Drosophila XX XY3.Birds,fish and butterflies

WZ WW

4. Humans and other mammals

XX XY

Mechanism of Sex Determination:1. The X-O system of sex determination

operates in insects. The male have only one X chromosome designated as XO while the female have two X chromosomes(XX).

2. Drosophila melanogaster have four pairs of chromosomes. The female is XX and the male is XY. The Y is characteristically J shape.

For both insect and Drosophilsa the ratio of the X chromosome to the set of autosomes plays a key role in sex determination.

3. The Z-W system is a form of sex determination in birds and butterflies and some fishes. The genotype of the female is referred as WZ and WW for male. In this system the male is homogametic and female is heterogametic.

4. The XX/XY system is the most common sex determination system found in humans and mammals. The presence of Sry gene on the Y chromosome causes maleness.

C0NCLUSION:After the gametes fuse with each other

during fertilization, what factors determine whether the resulting zygote developed into male or female? Researchers have studied the process of sex determination in a wide range of species and discovered several mechanism of sex determination. They conclude that the difference in chromosome constitution is a mechanism for determining sex at the time of fertilization.

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P.A.C 2015