Sex Determination

The biological mechanism by which the sex (male or female) of an individual is established based on genetic or chromosomal factors, is called sex determination.

Henking's Observation (1891):

In 1891, the German scientist H. Henking studied spermatogenesis in squash bugs (insects) and made a critical observation:

• He traced a specific nuclear structure through the entire process of spermatogenesis.
• 50% of sperms received this structure after spermatogenesis; the other 50% did not.
• He named it the "X body" but could not explain its significance​
• Later scientists confirmed that this "X body" was in fact a chromosome → renamed the X chromosome.

Sex determination systems vary widely across the animal kingdom. Four major systems are recognised:

• Sex determination: It is the mechanism by which an organism develops into a male or a female based on genetic factors.
• Types of organisms: Organisms may be bisexual (hermaphrodite), having both sex organs, or unisexual (dioecious), like humans, with separate sexes.
• Discovery: Henking (1891) discovered the X-body, later identified as the X chromosome involved in sex determination.
• XX–XY system: Females are XX (homogametic) and males are XY (heterogametic), seen in humans and Drosophila.
• ZW–ZZ system: Females are ZW (heterogametic) and males are ZZ (homogametic), seen in birds and some reptiles.
• Haplodiploidy: In honeybees, unfertilized eggs develop into haploid males and fertilised eggs into diploid females.