Many genes, one gene, and sex

Polygenic inheritance. Some traits are not either-or but come in a smooth range: human height and skin colour are examples. They are controlled by three or more genes, each adding a little to the effect (additive effect), and the environment also plays a part. That is why people are not just 'tall' or 'short' but every height in between.

Pleiotropy. The opposite case: one gene with several effects. In phenylketonuria, a mutation in the gene for the enzyme phenylalanine hydroxylase means the amino acid phenylalanine cannot be broken down. It builds up and causes mental retardation, and also reduced hair and skin pigmentation.

Sex determination. The first clue came in 1891, when Henking saw that half the sperms of a bug carried a small extra body; he called it the X body. It was later found to be a chromosome, and chromosomes linked to sex were called sex chromosomes; the rest are autosomes. Different animals use different systems:

• XO type (grasshopper and many insects): females have XX; males have only one X (XO). Half the sperms carry X, half carry none.
• XY type (humans, Drosophila): females are XX; males are XY.
• ZW type (birds): here it is reversed. Females are ZW and males ZZ.

The rule: the sex that makes two kinds of gametes decides the sex of the offspring. In the XO and XY types the male does this (male heterogamety), so in humans it is the father's sperm that decides. In birds the female does (female heterogamety), so the egg decides the sex of the chick.

Honey bees use yet another system, haplodiploidy. A fertilised egg (32 chromosomes) becomes a female: a queen or a worker. An unfertilised egg develops on its own by parthenogenesis into a male, the drone, with only 16 chromosomes. So a drone has a mother but no father, though it does have a grandfather.