Chocolate

Chocolate is a sex-linked gene in chickens and ducks (5)(6) that causes the bird to be a deep, chocolate brown color on a dark base. It is thought to be a similar mutation, though different inheritance, to the dark brown color seen in mice, cattle, sheep, pigs, cats, rabbits, sheep, and other mammals. (3)

It works by diluting the overall eumelanin pigmentation in the bird, and is unlike the sex-linked silver/gold gene in that it is a TYRP1 gene (1), putting it in the same category as the roux mutation in quail. It also is very similar to some of the mutations that can make humans have blonde hair (3).

Chocolate is sexlinked recessive (1) meaning that it can be used to create a foolproof, one-off batch where the chicks/ducklings can be sexed immediately at hatch. This is useful in hard-to-sex breeds like silkies, where it would take half a year to be certain of sex. The easiest way to do this is to cross a chocolate rooster over any confirmed non-chocolate hen. The female offspring will have one copy of the chocolate gene and show it, while the males will carry one copy from the father and hide the chocolate gene, making all females appear chocolate while all males appear to have whatever base pattern the parents had.

Chocolate is not currently recognized by the APA in any chicken breeds, though it is recognized in Call, Muscovy, and Runner ducks. (4)

Breeding with Chocolate:

Chocolate Male x Chocolate Female = 100% Chocolate chicks

Chocolate Male x Clean Female = Chocolate females, het. Chocolate males

Clean Male x Chocolate Female = 100% clean female chicks, 100% het. Roux male chicks

Het. Chocolate male x Chocolate Female = 50% Chocolate female chicks, 50% clean female chicks, 50% het. Chocolate male chicks, 50% Chocolate male chicks

Het. Chocolate male x Clean Female = 50% clean female chicks, 50% Chocolate female chicks, 50% het. Chocolate male chicks, 50% clean male chicks.

Note: it is impossible to have a heterozygous female chick, as females can only have one Z chromosome, and the "pair" or the W chromosome is short, meaning it forces the recessive mutation to show even with only one copy. This is called hemizygous. Refer to the genetics series if you would like to read more.