The essentials
Henking, working with insects, observed a nuclear structure present in only half the sperm and called it the X body; it was later named the X chromosome. Chromosomes that determine sex are called sex chromosomes; the rest are autosomes.
XX–XO type: in grasshoppers and many other insects, females are XX and males have only one X (XO). XX–XY type: in humans and Drosophila, females are XX and males are XY. In both these types, the male is heterogametic (produces two kinds of gametes) and the female is homogametic. ZW–ZZ type: in birds, the female is heterogametic (ZW) and the male is homogametic (ZZ).
Humans: there are 23 pairs of chromosomes, 22 pairs of autosomes and one pair of sex chromosomes. Females produce only X-bearing ova; males produce 50% X-bearing and 50% Y-bearing sperm. So each pregnancy has an equal chance of a male or female child, and the sex is decided by the sperm.
Honey bee: sex is determined by the number of chromosome sets an individual receives (haplodiploid system). A fertilised egg (diploid, 32 chromosomes) develops into a female (queen or worker), while an unfertilised egg develops into a male (drone) by parthenogenesis and is haploid (16 chromosomes). Males produce sperm by mitosis. Males have no father and cannot have sons, but they have a grandfather and can have grandsons.
Key points
- Heterogametic male: XO (grasshopper), XY (humans, Drosophila).
- Heterogametic female: ZW (birds).
- Honey bee female 2n = 32, male n = 16.
Common mistakes
- In birds, it is the female, not the male, whose gametes decide the sex of the offspring.