The gene: TYRP1
Chocolate (Choc) is a dilution of eumelanin — the black pigment — shifting it to a warm chocolate brown. It leaves pheomelanin (red/gold) essentially untouched, so on a gold-based bird only the black parts turn brown. Its gene has been identified as TYRP1 (tyrosinase-related protein 1), an enzyme in the black-pigment pathway; a single missense mutation changes the pigment granule's structure and gives the brown result.1
Sex-linked recessive inheritance
The important part: TYRP1 lies on the Z chromosome, so chocolate is sex-linked, and it is recessive. Because roosters are ZZ and hens are ZW:
- A hen carries only one copy (she is hemizygous), so a single chocolate allele on her one Z is enough to make her chocolate.
- A rooster needs two copies to show chocolate; with one copy he is a carrier that looks normal.
So chocolate appears more readily in hens, and a normal-looking rooster can carry it hidden — a common source of "where did that come from?" chocolate chicks.
choc/choc) over a non-chocolate hen gives chocolate daughters and normal-looking (carrier) sons — the daughters inherit their single Z, carrying chocolate, from their father. It is the recessive mirror-image of the gold/silver and barring sexing crosses on the sex-linked genes page, and you can set it up in the Breeding Outcomes tool.
Chocolate vs the other browns
Several genes can make a bird "brown", and they are not interchangeable. Chocolate is specifically the sex-linked TYRP1 dilution of black:
| Brown effect | Gene / locus | How it differs from chocolate |
|---|---|---|
Chocolate (Choc) | TYRP1, Z-linked, recessive | — |
| Dun | An allele of the Dominant White locus (PMEL17), autosomal, incompletely dominant | Different gene and chromosome; not sex-linked, and one copy already shows. |
Dark Brown ground (Db) | Dark Brown locus, autosomal | Deepens the red/gold ground colour, not the black — a different pigment entirely. |
If a "brown" trait is passing differently to sons and daughters, chocolate is the likely candidate; if it shows with a single copy and isn't sex-linked, look at dun instead.
- 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. Dun as a PMEL17 allele: Kerje S et al. (2004) Genetics 168:1507–1518 (see the Genetics Guide references).