Sex determination

In 1891 Henking, studying spermatogenesis in some insects, saw that half the sperms received a particular nuclear structure and half did not. He called it the X body; it was later recognised as a chromosome and named the X chromosome.

XO type: in many insects such as grasshoppers, males have one X besides the autosomes while females have two X. Males therefore make two kinds of sperm, with and without X.

XY type: in many insects and in mammals including humans (and Drosophila), males have one X and one smaller Y, while females are XX.

Chromosomes that decide sex are sex chromosomes; the rest are autosomes. In both XO and XY systems the male produces two kinds of gametes, so these are examples of male heterogamety.

In birds, female heterogamety is seen: females are ZW and produce two kinds of eggs, while males are ZZ and homogametic.

Humans have 23 pairs of chromosomes, 22 pairs of autosomes and one pair of sex chromosomes. Females make only X-bearing ova; males make X-bearing and Y-bearing sperms in equal proportion.

Because the sperm type decides the zygote's sex, there is an equal (50 per cent) chance of a boy or a girl in each pregnancy, and the father's gamete determines it.

Honey bees use a haplodiploid system based on the number of chromosome sets. A fertilised egg develops into a female (queen or worker) with 32 chromosomes (diploid), while an unfertilised egg develops by parthenogenesis into a male (drone) with 16 chromosomes (haploid).

Since drones are haploid, they make sperms by mitosis. A drone has no father and so cannot have sons, but it does have a grandfather and can have grandsons.