SEX DETERMINATION

The living organisms are two types. They are bisexual or monoecious and unisexual or dioecious. Bisexual organism contains both testis and ovary. The same tissue in bisexual organism is able to produce sperms in testis and ova in ovary. In dioecious organsims, the male and female gametes are produced by seperate individuals. The question of sex determination arises in unisexual organisms only.

SEX DETERMINATION

1. CHROMOSOMES

Normally organisms are diploid (male set, female set). The chromosomes are of two types. They are autosomes and sex chromosomes.

A. Autosomes :

The chromosomes that are identical in male and female are called autosomes. For example Drosophila contains '4' pairs (8) of chromosomes. The first '3' pairs in male and female are identical.. So, the first '3' pairs of chromosomes are called autosomes.

B. Sex chromosomes:

The chromosomes that differ in male from female or vice versa are called sex chromosomes. These are also called allosomes. For example, the female Drosophila 4th pair is 'XX'. Whereas the male 4th pair is 'Xy'. As the male and female differ in 4,h pair of chromosomes, tha4t,h pair in Drosophila is considered as sex chromosomes.

2. CHROMOSOMAL MECHANISM OF SEX DETERMINATION

It is the formation of sex based on the presence of sex chromosomes. The chromosomal mechanism of sex determination is mainly on the basis of heterogamesis (formation of different types of gametes).

According to chromosomal theory, the one sex among male and female is heterogametic. Whereas other sex is homogametic. The heterogametic individual produces two types of gametes. For example, male Drosophila (2A + Xy) is heterogametic where 'A' indicates autosomes and '2' indicates diploid. 'Xy' are sex chromosomes. The gametes produced from male Drosophila are two types. They are A + X and A + y. Female Drosophila (2A + XX) is homogametic. All the ova are identical. Every ovum contains A + X.

A. Sex determination in Drosophila

It was first explained by Bridges.

Drosophila melanogaster (Fruit fly) contains '4' pairs of chromosomes (3 pairs of autosomes + 1 pair of sex chromosomes). The female contains 3 pairs of autosomes + 'XX' sex chromosomes. So female is homogametic. Each ovum contains '3' autosomes + 'X' sex chromosome. The male fly contains 3 pairs of autosomes + 'Xy' sex chromosomes. The male is heterogametic. It produces two types of sperms. 50% of the sperms contain '3' autosomes + X. The remaing 50% of the sperms contain 3 autosomes + 'y' sex chromosome.

If the ovum of Drosophila is fertilized by 'X' carrying sperm, the chromosomal component of formed zygote is '3' pairs of autosomes + XX sex chromosomes. This zygote develops into female.

SEX DETERMINATION-DROSOPHILA

If the ovum of Drosophila is fertilized by 'y' carrying sperm, the chromosomal component of formed zygote is '3' pairs of autosomes + 'Xy' sex chromosomes. This zygote develops into male.

NON DISJUCTION:

Normally during meosis of gametogenesis,-the two homologous chromosomes seperate and each one enters into one gamete. It is called disjunction. Sometimes both the homologous chromosomes may enter into same gamete. This leads to chromosomal number change in the gamete. It is called NON DISJUNCTION.

Due to non disjunction, the female may produce abnormal ova. They are AXX, AO, AAX, OX (O -no chromosome).

Due to non disjunction the male fly may produce abnormal sperms. They are AAX, Oy, AXy, AO.

BRIDGES GENE BALANCE THEORY OF SEX DETERMINATION:

According to this theory, the female sex determining genes lie on 'X' chromosomes. Male sex determining genes lie on autosomes. So 'y' chromosome has no importance in sex determination. Hence sex determination depends upon the ratio of 'X' and autosomes. Bridges gave sex values to the autosomes and sex chromosomes. They are

i. Haploid autosomal set -1 (A - value of maleness)

ii. 'X' sex chromosome value -1.5 (x - value of femaleness)

iii. 'y' sex chromosome value - 0 (y - no sex deciding value)

The chromosomes of individuals, ratios of male and female values and also sex are represented in the following table.

Super male and super females are sterile. Inter sex individual contains both male and female characters.

BRIDGES GENE BALANCE THEORY SEX DETERMINATION

Always the probability of formation of male child is 50%. Likewise the formation of female child is also 50%. Hence in any population the number of males is always equal to the number of females.

B. SEX DETERMINATION IN THE HUMAN BEING

Human being contains '23' pairs of chromosomes (46). They are 22 pairs of autosomes and one pair of sex chromosomes. The female human being contains '22' pairs of autosomes + 'XX' sex chromosomes (AA + XX). Where 'A' indicates one set of autosomes. So female human being is homogametic. Every ovum contains 22 autosomes + X sex chromosome (A + X).

Male human being contains '22' pairs of autosomes + 'Xy' sex chromosomes (AA + Xy). So male human being is heterogametic. Male human being produces two kinds oi sperms in the equal proportion. They are 'A + X' and 'A + y'.

If human ovum is fertilized by 'A + X' sperm, the chromosomal component of zygote is 'AA + XX'. This zygote develops into female.

If human ovum is fertilized by 'A + y' sperm, the chromosomal component of zygote is 'AA + XY'. This zygote develops into male. chrmosomes-sex

In the 'human beings 'y' chromosome decides the sex. Presence of 'y' indicates male and absence indicates female.

sex-detemination

C. SEX DETERMINATION IN THE GRASS HOPPERS

Female Grass hopper contains 12 pairs of chromosomes. They are 11 pairs of autosomes + 1 pair of sex chromosomes (AA + XX). So female fly is homogametic. Each ovum contains 11 + X or A + X (A - autosomes).

Male Grass hopper contains 11 pairs of autosomes and one sex chromosome (AA + XO). Male has one sex chromosome only. So male fly is heterogametic. It produces two kinds of sperms. They are 'A + X' and 'A + O'.

sex-detemination-grass hopper

The above chromosomal nature indicates that two 'X' chromosomes are required for femaleness. Where as one 'X' chromosome is required for maleness.

D. SEX DETERMINATION IN THE FOWLS

The sex chromosome of Fowls are Z' and 'W.

Female Fowl contains 'AA + zw' chromosomes. So female is heterogamtic. Female Fowl produces two types of ova. They are 'A + Z' and 'A + w'.

Male Fowl contains 'AA + ZZ' chromosomes. So male is homogametic. Cock produces one type of sperms only. Each sperm contains 'A + Z'.

If the sperm fuses with 'A + Z' ovum, the chromosomal nature of zygote is 'AA + ZZ'. This zygote develops into cock.

If the sperm fuses with 'A + w' ovum, the chromosomal nature of zygote is 'AA + Zw'. This zygote develops into hen.

sex-detemination-birds

The above chromosomal nature indicates that the presence of 'W indicates femaleness and absence indicates maleness. So in Fowls sex determining chromosome is 'W

E. SEX DETERMINATION IN THE HONEY BEES

The sex of a honey bee depends upon whether it developed from fertilized zygote or unfertilized ovum.

The fertilized zygote develops into female. The female that eats royal Jelly develops into Queen Bee. The female that does not eat royal jelly develops into sterile female worker. So both Queen Bee and workers are diploid.

The unfertilized ova develop into male bees or drones. So drones are always haploid.

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