Chromosomal sex determination : The chromosomes, which determine the somatic characters of an individual are known as autosomes. These chromosomes do not differ in morphology and number in male and female sex. Those chromosomes, which? differ in morphology and number In male and female sex and contain genes responsible for the determination of sex are known as allosomes or sex chromosomes. There arc two types of sex chromosomal mechanisms
a) Heterogametic male and
b) Heterogametic female
a) Heterogametic male: In this mechanism the female sex has two X chromosomes, while the male sex has only a single X chromosomes.
The heterogametic male may be of the following two types:
(i) XX-XO
(ii) XX-XY
i) XX-XO type : In certain Insects belonging to orders Herniptera (true bugs), Orthaptera (grass hoppers) and Dictyoptera (cockroaches) female has two IC chromosomes (XX) and are, thus homogametic, while male has only single .X. chromosome (XO). The male being heterogametic sex produces, two types of sperms, half with X chromosome and half without X chromosome in equal proportions. The sex of the offspring depends upon the sperm that fertilises the egg, each of which carries a single X chromosome.. Thus fertilisation between male and female gametes always produced zygotes With one IC chromosome from the female, but only 50% of the zygotes have an additional X Chromosome from the .male in this way, .XO. and .XX. types would be formed in equal proportions, the former being males and former bitter being females.
(ii) XX-XY type: XX - XY type : In man, other mammals, certain insects including Drosphila, thefemales possess two. X chromosomes (CX) and are thus homogametic, produce one kind of RES, each one with one X chromosome. While the males posséss one X and one Y chromosome (XY) and are hence, heterogametic. They produce two kinds of sperms, half with X chromosome and half with Y chromosome. The sex of embryo depends on the kind of sperm. An egg fertilised by a X bearing sperm, produces a female, but if fertilised by a Y bearing sperm, a male is produced. "
b) Heterogametic female ,In this method of sex determination, the male produces similar type of gametes, while female produce dissimilar gametes. The heterogametic females may be of following two types. (i) ZO-ZZ (ii) ZW-ZZ
i) ZO-ZZ: This mechanism is found in certain moths and butterflies.. this case, female .possesses one single 2. chromosome and hence is heterogametic, producing two kinds of eggs half with Z chromosome and .another half without any Z chromosome. Male possess. two Z chromosomes and thus homogametic, producing single .type of sperms, each carries single Z chromosome. The sex of the offspring depends on the kind of egg.
ii) ZW-ZZ: This system is found in certain insects (gypsy moth) and vertebrates such as fishes, reptiles and birds. In this system, the .female is heterogametic and produces two types of garnet., one with .Z. chromosome and the other with .W. chromosome. On the other hand, male is homogametic and produces all sperms of same type carrying one Z chromosome. The sex of. the offspring depends on the .kind of egg being. fertilised . The chromosome bearing eggs produce males, but theta chromosome bearing eggs produces females.
