Sex-linked barring

Sex-linked barring is a plumage pattern on individual feathers in chickens, which is characterized by alternating pigmented and apigmented bars. The pigmented bar can either contain red pigment (phaeomelanin) or black pigment (eumelanin) whereas the apigmented bar is always white. The locus is therefore often referred to as an ‘eumelanin diluter’ or ‘melanin disruptor’. Typical sex-linked barred breeds include the Barred Plymouth Rock, Delaware, Old English Crele Games as well as Coucou de Renne.

The presence of a white bar on a dark background is distinguishing sex-linked barring from Autosomal barring, another plumage pattern in chickens which is created by a black bar on a light color background (white/ beige or brown) as exemplified by the breed Egyptian Fayoumi. The absence of pigment in the white bar has been attributed to a lack of pigment producing cells (melanocytes) in the feather follicle during feather growth. Initially it was proposed that this lack was the result of cell death as a consequence of the B locus mutation but later research demonstrated that the lack is the result of premature cell differentiation rather than apoptosis.

Male chicken of traditional sex-linked barred breeds like the Barred Plymouth Rock usually show much wider and clearer white bands than females of the same breed. Further characteristics of sex-linked barred chickens are the dilution of skin pigment in the legs as well as a white dot at the top of the head of freshly hatched chicks which can be used for autosexing: homozygous males have a much bigger spot than hemizygous females.

Sex-linked barring has been established as the dominant locus B by traditional mendelian genetics in the beginning of the 20th century. The responsible gene was predicted to be located on the Z chromosome and since male birds are homogametic (ZZ), they can be either hetero- or homozygous for sex-linked barring. Females are always hemizygous at this locus (ZW). In 2010 Swedish scientist have identified four mutations located in or around the tumor suppressor locus CDKN2A, which appear to be associated with sex-linked barring. The four mutations are organized in 3 different alleles named B0, B1 and B2. All alleles carry two non-coding mutation located in regulatory regions of the gene (the promoter and intron) but only B1 and B2 carry two additional missense mutations in a functional important domain of the protein. The B1 allele is causing the typical sex-linked barring phenotype/ appearance and is present in most modern sex-linked barred chicken breeds. Females or male chickens carrying the B2 allele in the heterozygous condition show a defined barring pattern but in the homozygous condition, males are essentially white with very little pigmentation. This phenotype has been initially described as a distinct but closely related mutation, however, it was later assigned to the same gene and termed ‘Sex-linked Dilution'.

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