Sex Determination

• Sex determination is the method by which the distinction between male and female is established in a species.
• Sex chromosomes determine the sex of the individual in dioecious or unisexual organisms.
• The chromosomes other than the sex chromosomes of an individual are called autosomes.
• Sex chromosomes may be similar (homomorphic) in one sex and dissimilar (heteromorphic) in the other.
• Individuals having homomorphic sex chromosomes produce only one type of gametes (homogametic), whereas heteromorphic individuals produce two types of gametes (heterogametic).

Henking (1891) could trace a specific nuclear structure all through spermatogenesis in a few insects, and he also observed that 50% of the sperm received this structure after spermatogenesis. In contrast, the other 50% of sperm did not receive it. Henking named this structure the X body, but he could not explain its significance. Further investigations by other scientists led to the conclusion that the ‘X body’ of Henking was a chromosome, which is why it was given the name X-chromosome.

1. Heterogametic Sex Determination: In heterogametic sex determination, one of the sexes produces similar gametes, and the other sex produces dissimilar gametes. The sex of the offspring is determined at the time of fertilisation.
   
   
2. Heterogametic Males: In this method of sex determination, the males are heterogametic producing dissimilar gametes, while females are homogametic producing similar gametes. It is of two kinds XX-XO type (e.g. Bugs, cockroaches, and grasshoppers) and XX-XY type (e.g. Human beings and Drosophila).
   
   
3. Heterogametic Females: In this method of sex determination, the females are heterogametic producing dissimilar gametes, while males are homogametic producing similar gametes. To avoid confusion with the XX-XO and XX-XY types of sex determination, the alphabets ‘Z’ and ‘W’ are used here instead of X and Y, respectively. Heterogametic females are of two types, ZO-ZZ type (e.g. Moths, butterflies, and domestic chickens) and ZW-ZZ type (e.g. Gypsy moths, fishes, reptiles, and birds).

Y-chromosome: The human Y chromosome is only 60 Mb in size with 60 functional genes whereas X chromosomes are 165 Mb in size with about 1,000 genes.