Asked by: William Spurr, Colchester
It depends on how you define gender. The chromosomes that normally control the physical differences between male and female are fixed at the moment of fertilisation and cannot change. But the sex chromosomes work by coding for enzymes that affect the bird’s development in the egg and during its life. These enzymes are sensitive to temperature and if eggs fertilised with male chromosomes are cooled by a few degrees for three days after laying, the relative activity of the sex hormones will favour development of female characteristics. (In reptiles, temperature is entirely responsible for determining sex.) In about 10 per cent of cases, this cooling will produce a chicken with a fully functioning and reproductively fertile female body-type; even though the chicken is genetically male.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.