sex determination

In humans, sex is determined by the male. Sperm cells contain an X or a Y chromosome but egg cells contain only X chromosomes. If a sperm cell carrying an X chromosome fertilizes the egg the resulting baby will be female; if the sperm cell is carrying a Y chromosome then the baby will be male.

Process by which the sex of an organism is determined. In many species, the sex of an individual is dictated by the two sex chromosomes (X and Y) it receives from its parents. In mammals (except for the egg-laying mammals, for example the platypus), some plants, and a few insects, males are XY, and females XX; in birds, reptiles, some amphibians, and butterflies the reverse is the case. In bees and wasps, males are produced from unfertilized eggs, females from fertilized eggs.

Environmental factors can affect some fish and reptiles, such as turtles, where sex is influenced by the temperature at which the eggs develop.

In 1991 it was shown that maleness is caused by a single gene, 14 base pairs long, on the Y chromosome.

Many fish have a very flexible system of sex determination, which can be affected by external factors. For example, most members of the sea-bass family start as females and later may reverse their sex, depending on the local sex ratio, to become males.