Non-Mendelian genetics

In chickens, the sex-determining chromosomes are called Z and W. Male roosters have two Z chromosomes, and female hens have one Z chromosome and one W chromosome. A gene located on the Z chromosome controls the speed of feather growth in young chicks. The allele for slow-feathering is dominant to the allele for fast-feathering.

During the first few days after hatching, chicks with different feathering speed phenotypes have visually different feather lengths. Chicken breeders often take advantage of this visual difference to quickly determine the sex of newly-hatched chicks. To do this, the breeders have to use parents with specific feathering speed genotypes.