Auto-sexing breeds of poultry are those in which the sex of newly-hatched chicks can be determined from the colour and markings of the down. Some breeds of chicken, of goose and of domestic pigeon have this characteristic. The idea of such a breed is due to Reginald Punnett, who created the first auto-sexing chicken breed, the Cambar, at the Genetical Institute in Cambridge in 1928.

Reginald Crundall Punnett FRS was a British geneticist who co-founded, with William Bateson, the Journal of Genetics in 1910. Punnett is probably best remembered today as the creator of the Punnett square, a tool still used by biologists to predict the probability of possible genotypes of offspring. His Mendelism (1905) is sometimes said to have been the first textbook on genetics; it was probably the first popular science book to introduce genetics to the public.