LETTER:Genetic duck-pond

Sir: The explanation of the preponderance of Mallard males on the Barbican pond (Letters, 23 February) lies in the sex chromosome make- up of the ducks. Humans and other mammals have two X chromosomes in the female, one X and a Y in the male. A deleterious gene on one X is commonly countered by a normal one on the other, so that the females have a better survival rate than the males.

In mallards and other birds, however, there is a converse arrangement: here it is the males that have the identical pair of sex chromosomes, WW, while the females have a differing pair WZ. This results in a better survival-rate for, and a substantial predominance of, males.

Denys W Tucker

London, SW19