B) — and the difference between them is not a colour gene at all. For the wider system, see the Genetics Guide.
The barring gene
The Barring gene (B) switches pigment production off at regular intervals as each feather grows, so the feather comes in as an alternating ladder of coloured and colourless bands.1 It needs a solid eumelanin (black) base to write on, and a silver ground helps keep the pale bands clean rather than leaking gold. A typical black barred/cuckoo genotype, on a rooster basis, is E/E · S/S · B/B:
| Gene | Genotype | What it contributes |
|---|---|---|
| Extension — black | E/E | A solid black base for the bars to interrupt. |
| Silver | S/S | A silver ground, so the pale bands stay white rather than leaking gold. |
| Barring | B/B | The sex-linked gene that switches off pigment in regular bands.1 |
Because B sits on the Z sex chromosome, it is a sex-linked gene: a rooster carries two copies (B/B) and a hen only one (written B/–, because she has a single Z). That single fact drives both of the surprises below.
Barred vs cuckoo — a feathering gene, not a colour gene
Barred and cuckoo carry the very same Barring gene. What sets them apart is the rate of feather growth, controlled by the slow-feathering gene K. Slow-growing feathers spend longer under each on/off cycle, so the bands land closer together — narrow, sharp barring. Fast-growing feathers spread the same cycles over more length, so the bands sit further apart — the wider, fuzzier cuckoo look.2
| Look | Feathering | Result |
|---|---|---|
| Barred | Slow (K) | Narrow, crisp, tightly-ruled bars — the Barred Plymouth Rock, Dominique. |
| Cuckoo | Fast (k+) | Wider, softer, less regular bands — the Cuckoo Marans, Coucou de Rennes, Bielefelder. |
B.
Why cocks look paler than hens
Because the gene is sex-linked, a rooster carries two copies of B and a hen only one — and that second copy switches pigment off for longer, making the colourless band about twice as wide.1 So barred/cuckoo roosters read distinctly paler than hens of the same breed: more white in every feather. It is the clearest everyday sign of a sex-linked gene at work — the two sexes are not just different in ratio, they are different to look at.
That same sex-linkage is what makes barring so useful for auto-sexing: cross a barred hen with a non-barred rooster and the barring lands only on the sons, so cockerels and pullets can be told apart at hatch. The full cross tables are worked through on the sex-linked genes page.
On other bases
Barring can be laid over any base, not just black. On a gold duckwing (e+) base it produces crele — the barred-partridge look of a Cream Legbar or a Crele Old English, where the bars cross gold and cream rather than black and white. Add blue and you get blue cuckoo; add lavender and you get lavender cuckoo. In every case the barring gene is the same; only the background it writes on has changed. See the pattern genes page for how barring sits alongside the other pattern-making genes.
- Sex-linked barring maps to the CDKN2A/B locus and acts by periodically halting melanocyte activity during feather growth: Hellström AR et al. (2010) Sex-linked barring in chickens is controlled by the CDKN2A/B tumour suppressor locus. Pigment Cell & Melanoma Research 23(4):521–530. General sex-linkage framework: Smyth JR Jr (1990), in Crawford RD (ed.) Poultry Breeding and Genetics — see the Genetics Guide references.
- The barred/cuckoo distinction as an effect of feather-growth rate (the
Kslow-feathering gene) rather than a separate barring allele follows the standard-data description used by the Compare tool; genotypes shown here match that standard data.