Genetics of sex
The chromosomes in a cell are divided into autosomes (the same in both sexes) and sex chromosomes. Humans have 46 chromosomes: 44 autosomes and 2 sex chromosomes. A woman is XX, a man XY. Women make one kind of egg, all with X (homogametic sex), men make X and Y sperm (heterogametic sex). The child’s sex is set by the fertilising sperm: with X it gives XX (girl), with Y it gives XY (boy); the probability is about 1 : 1. Other types: in birds, butterflies and some fish the male is ZZ and the female ZW (female heterogametic); grasshoppers are XO (the male has one X); in honeybees an unfertilised egg gives a male (drone) – haploid – and a fertilised egg a female. By the time sex is fixed, three types are distinguished: progamic (sex is set in the egg before fertilisation, e.g. in rotifers), syngamic (sex is set at fertilisation – the most common type) and epigamic (sex is set after fertilisation by the environment; e.g. in some reptiles sex depends on incubation temperature). Sex chromosomes carry other genes besides those for sex. The Y chromosome carries a gene that determines the male sex (SRY).
“Parent cards”: students build a Punnett square from XX and XY cards, find the sex ratio and compare it with ZW–ZZ.