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.
Unlike sex-linked hybrids, such as 'red sex-links' or 'black sex-links', the Legbar is an auto-sexing breed. Several other auto-sexing breeds or auto-sexing varieties of breeds exist, such as Plymouth Rock, Bielefelder Kennhuhn, Niederrheiner, and Norwegian Jærhøns. Most breeds that end with -bar, such as Welbar, Rhodebar, Brussbar or Wybar, are auto-sexing as well. The importance that auto-sexing plays in the Legbar breed is also reflected in the fact that, next to a standards for the adult birds, the down colour and patterns are also standardised. Day-old male chicks can be distinguished from day-old female chicks by the down colour and the pattern they form. Female Legbar chicks in general have a broad very dark brown stripe extending over the head, neck and rump and a clear eye barring. The edges of the stripe are clearly defined and should not be blurred and blending with the ground colour, which is dark brown. A light head spot should be visible but is usually small. The male Legbar chicks in contrast have a much paler down shade and the pattern is blurred and washed out from head to rump.
The marked difference between male and female chicks is due to gene dosage of the sex-linked barring gene ('barring' (B), 'nonbarring' (b+)). This gene is located on the Z-Chromosome of birds. Birds have different sex-chromosomes (Z and w) and a different sex-determination system compared to mammals (X and Y). Male birds have therefore two Z-chromosomes while female birds have a Z- and a dwarfed w-chromosome. This means that phenotypically barred cocks can either have the B/B or the B/b+ genotype, while a barred hen always has to have a B/- genotype. The colour-sexing of Legbar chicks, however, is only possible because the male chicks have a double dose of the sex-linked barring gene (genotype B/B), while the female chicks only have a single dose (genotype B/-), resulting in the observed down colours.
The concept of an auto-sexing breed of chicken is due to the geneticist Reginald Punnett, who during the First World War had already proposed the technique of cross-breeding chickens carrying the barred gene (B) with others to produce sex-linked chicks with plumage differences that could easily be distinguished.
Working at the Genetical Institute of Cambridge University, he and Michael Pease cross-bred Golden Campines with barred Plymouth Rocks, resulting in the creation of the Cambar in 1928. About ten years later they produced the Legbar by crossing brown Leghorns with barred Plymouth Rocks.
Other "Cambridge" breeds later developed were:
Many other breeds were created in the same way, all making use of barred Plymouth Rocks to impart the barred gene: