* SEX DETERMINATION "👇🏻

The sex is hereditary difference between two individuals of the same species. Sex is one of the most conspicuous and interesting kinds of hereditary differences observed among individuals of the same species. Determination of sex is determined at the time of fertilization, when the male and female gametes fuse together. In most of organisms, sex is determined by one or one pair of chromosomes called sex chromosomes or allosomes.

A. Male heterogamety :--

Males form two types of gametes. One gamete possesses X chromosome and other lacks it. In some cases male may possess a Y chromosome. Such males are known as heterogametic Females in such cases form only one type of gamete which contain X chromosome Male heterogamy is of two types

1. 'XX (9) - XY' (ở) type :-👇🏻

By 1900, when microscope techniques had become quite well developed and chromosome behaviour was understood, it was noticed that there was one pair of chromosomes that differed from others. In females, the members of this pair were similar, but differ in appearance in other sex (males). The two chromosomes, that were alike (in female) were the same as one of the members of the unlike pair in male. The chromosome which was present in pair in female and single in male was identified as X chromosome. In males, the other chromosome was called as Y. So, the two sexes can be characterized as under.

Female XX

Male = XY

X-chromosomes were first identified by Wilson and Stevens in 1905. The so called XY system occurs in wide variety of animals including Drosophila and man, as well as at least in some plants (e.g. Lychnis an angiosperm) Melandrium X and Y chromosomes are called sex chromosomes (allosomes), the remaining ones of a given complement, which are same in both sexes are called autosomes. The type of system discussed above is called XX-XY system.

Show of Role of X and Y chromosomes in Determination of sex :👇🏻

2. 'XX (9) -XO' (ơ) type :-

Another system reported is XX-XO. In 1902 an American McClung reported that the somatic cells of female grasshopper bears 24 chromosomes, whereas those of males had only 23. Thus in many insects there is a chromosomal difference between the sexes, female being referred to as XX having two X chromosomes and males as XO (with one X chromosome). As a result of meiosis all the eggs of such species carry one X chromosome, whereas only half of the sperms have one, the other half having none. Since the males produce two types of gametes X or O in XO type and X and Y in XY type, they are called heterogametic. Females are homogametic producing only one type of gamete with X chromosome.

B. Female heterogamety. In such cases females produce two types of gametes. One egg contains X and other lacking it (X) or containing Y chromosome. Thus the male is AAXX and female is AAXO or AAXY. For the sake of convenience and to avoid confusion the standard practice is that in these organisms X is denoted to as Z and Y as W. Female heterogamy is also of two types.

Show Of 'XX-XO' Determination of sex in Cockroach and Grasshopper: 👇🏻

1. 'ZZ (♂) -ZW' (9) type :-👇🏻

Another interesting ZZ-ZW system has been found in some birds including domestic fowl and some fishes. In this case females are heterogametic and males are homogametic. Sex chromosomes here have been designated as Z and W to avoid confusion with instances where female is homogametic. Females here are ZW and males ZZ .

2. 'ZO (9)-ZZ' (in) types --👇🏻

In ZO-ZZ type of sex determination which occurs in some butterflies and moths. Here females have odd sex chromosome (AA + Z) while the males have two homomorphic sex chromosomes (AA + ZZ). The females are heterogametic. They produce two types of eggs, male forming with one sex chromosome (A + Z) and female forming without the sex chromosome (A + O). The males are homogametic, forming similar types of sperms (A + Z). The two sexes are obtained in the progeny in equal ratio as both the types of eggs are produced in equal ratio.

'ZO-ZZ'determination of sex in a butterfly :👇🏻

* Note /--👇🏻 

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