Sex-Linked Colour Genes in Chickens · Chicken Color Standards

A few colour genes sit on the sex chromosomes, so they are inherited differently by male and female chicks. That one fact explains a genetics party trick with real commercial value: crosses whose chicks can be told apart by colour the day they hatch. This page covers how it works and the three sex-linked colour genes this site models. For the wider system, see the Genetics Guide.

The Z/W system

Birds do not use the X/Y system mammals do. Chickens have Z and W sex chromosomes, and — the key reversal — it is the female who carries two different ones:

A rooster is ZZ — two Z chromosomes, so two copies of every Z-linked gene.
A hen is ZW — a single Z, plus a W that carries almost no colour genes. She is hemizygous: whatever single allele is on her one Z is what she shows, with no second copy to mask it.⁴

Two consequences follow, and both matter at the breeding pen:

A hen passes her single Z only to her sons. Her daughters get their Z from the father and a W from her.
A recessive sex-linked trait shows up far more readily in hens, because a hen needs only one copy to show it while a rooster needs two.

Rooster (ZZ) vs hen (ZW)

Rooster — two Z chromosomes, two copies of every Z-linked gene

Hen — one Z, one W (few colour genes) — hemizygous for Z-linked genes

The three sex-linked colour loci

Of the genes this site works with, three are Z-linked — and all three have now been traced to specific genes on the Z chromosome:

Locus	What it does	Gene
Silver / Gold (`S`)	The dominant `S` allele suppresses pheomelanin, giving a silver/white ground in place of gold.	SLC45A2¹
Barring (`B`)	The dominant `B` allele lays transverse white bars across each feather — the cuckoo/barred look.	CDKN2A/B²
Chocolate (`Choc`)	A recessive allele that dilutes eumelanin (black) to a warm chocolate brown.	TYRP1³

Every other colour locus on the site — Extension, Columbian, Blue, Lavender and the rest — is autosomal, inherited identically by both sexes. Only these three carry the sex-linked behaviour below. (For the full list, see the glossary.)

Auto-sexing: telling chicks apart at hatch

Here is the payoff. Because a father passes a Z to both sons and daughters while a mother passes hers only to sons, you can set a cross up so that sons and daughters inherit different alleles at a sex-linked locus — and so hatch in different colours. Get the direction right and the chicks sex themselves. For the brooder-side view — what the down actually looks like — see sexing chicks by down colour.

Gold / silver sexing

Silver (S) is dominant to gold (s+). Cross a gold rooster (s+/s+) over a silver hen (S/W):

Chick	Gets from father	Gets from mother	Result
Sons (ZZ)	`s+`	`S`	`S/s+` → silver
Daughters (ZW)	`s+`	`W`	`s+/W` → gold

Silver sons, gold daughters — visible in the down at hatch. Reverse the cross (silver father × gold mother) and it does not work: every chick inherits the father's silver and all look alike. Direction is everything.

Barring sexing (the "black sex-link")

Barring (B) is dominant and works the same way. For what the gene does to the plumage — barred vs cuckoo, and why cocks look paler — see Barred & Cuckoo genetics. Cross a non-barred rooster (b+/b+) over a barred hen (B/W):

Chick	Gets from father	Gets from mother	Result
Sons (ZZ)	`b+`	`B`	`B/b+` → barred (a pale head spot as a chick)
Daughters (ZW)	`b+`	`W`	`b+/W` → non-barred (solid)

This is how commercial "black sex-link" hybrids work. A solid-coloured father over a barred mother gives barred sons (they show a head spot and grow up lighter) and solid daughters — sortable by eye on hatch day, with no vent-sexing skill required. The same logic drives "auto-sexing" breeds, which were bred to carry barring so that purebred chicks could be sexed at hatch.

Chocolate (Choc) is sex-linked too, but recessive, so it can be used for sexing only in the reverse arrangement (a chocolate father over a non-chocolate mother makes chocolate daughters). You can set any of these up and read the offspring off in the Breeding Outcomes tool, which handles the sex-linked ratios automatically.

Why the tools split results by sex

For an autosomal gene, "half the offspring" means half the sons and half the daughters alike. For a sex-linked gene it does not — sons and daughters can inherit entirely different alleles from the same mating, as the tables above show. That is why Compare Standards flags sex-linked loci, and why Breeding Outcomes reports frequencies on a rooster basis and notes that hen ratios differ wherever a sex-linked gene is in play. For the broader picture, see the sex-linkage section of the Genetics Guide.

References

Gunnarsson U et al. (2007) Mutations in SLC45A2 cause plumage color variation in chicken and Japanese quail. Genetics 175:867–877 — the sex-linked Silver/Gold (S) locus.
Hellström AR et al. (2010) Sex-linked barring in chickens is controlled by the CDKN2A/B tumour-suppressor locus. Pigment Cell & Melanoma Research 23:521–530.
Li J et al. (2019) A missense mutation in TYRP1 causes the chocolate plumage color in chicken and alters melanosome structure. Pigment Cell & Melanoma Research 32:381–390.
Sex-linked (Z-linked) inheritance and hemizygous females: classical basis, Bateson W, Punnett RC (1911). See also the Genetics Guide references.

Try it: set up a sex-linked cross · read the Genetics Guide · browse the glossary