mo/mo, that pauses pigment at the very start of each feather's growth — so the tip comes in white, then colour resumes underneath. What that tip looks like on the finished bird depends entirely on what other genes are already running underneath it: on a solid black bird it is a neat white V; on a gold mille fleur it is the last of three coloured bands; on a laced Tollbunt it sits on top of black edging. Same gene, very different standards. For the wider system, see the Genetics Guide.
What mottling actually does
Each new feather starts to grow without pigment cells in place, so the very tip is always white. Once pigment cells catch up, colour resumes — eumelanin (black) first, then pheomelanin (red/gold) if the bird carries it, which is why a gold-based mottled bird often shows a thin dark band just under the white tip before the ground colour appears. Because pigment-cell renewal slows with age, that white tip grows larger with every moult — a mottled bird whitens visibly as it gets older.1
| Gene | Genotype | What it contributes |
|---|---|---|
| Mottling | mo/mo | Pauses pigment at the start of each feather's growth, leaving the tip white; the white area expands with each successive moult. |
The molecular basis is known: mottling is a substitution (Arg332His) in the EDNRB2 gene, which disrupts endothelin‑3/EDNRB2 signalling and impairs the differentiation, proliferation and migration of pigment cells into the feather bulb — hence the temporary gap in pigment supply at each feather's start.2 It is a timing mutation, not a dilution — it does not reduce how much pigment a feather can carry, only delays when it starts.
Same gene, different standards
mo/mo is recessive and behaves identically wherever it sits — but the base pattern it lands on decides what the finished variety looks like. Here are established standards built on the same gene:
| Standard | Base genotype | What mottling adds |
|---|---|---|
| Black Mottled (Ancona) | E/E · Ml/Ml · mo/mo | Neat, organised white V-tips on an otherwise solid black feather. |
| Exchequer | same base, bred for extreme expression | Roughly equal black and white in large, asymmetric patches — the same genotype as Black Mottled, pushed to its far end by selection. |
| Random Mottled (e.g. Houdan) | Ml/Ml · mo/mo on any black base | Irregular, unorganised white — less uniform than the Ancona pattern. |
| Speckled / Red Mille Fleur (Sussex Speckled) | e+ · Mh · Pg · Ml · mo | A red/mahogany duckwing base instead of gold — same three-zone spangle-and-tip structure as gold mille fleur, in a deeper colour. |
| Mille Fleur (gold) | eb · Db · Pg · Ml · mo | Black spangle over a gold ground — see Mille Fleur genetics. |
| Porcelain | eb · Db · Pg · Ml · mo · lav | Mille fleur softened by lavender to cream and grey — see Porcelain genetics. |
| Tollbunt | eb · Co · Pg · Ml · mo | Black lacing (from Columbian) instead of a spangle, plus the white tip — see Tollbunt genetics. |
| Buff Columbian Mottled | eb · Co · mo | White tips added over a Buff Columbian bird's black hackle/tail/wing-tip markings. |
Mottling also shows up on a mahogany/partridge base, where the white fleck tips sit over deep red-and-black patterning instead of a solid ground:
How it can drift toward "spangled"
Genuine spangling — a Hamburg-type spangle — comes from a different set of genes entirely (Pattern, Dark Brown and Melanotic together, with no mottling involved). But because mottling's white-tipped, dark-banded structure can look spangle-like at a glance, the word "spangled" gets applied loosely in some standards to birds that are genetically mottled, not spangled. Old English Game "Spangled" is a well-known example: it is genetically mottled, and has nothing to do with a real Pg/Db/Ml spangle.3 The same visual overlap is part of why the amount of white in any mottled bird keeps changing with age and selection — more white per moult can push a mottled bird's appearance further toward what a casual eye calls "spangled," even though no spangle gene is present.
How it breeds
Because mo/mo is fully recessive, an outcross to a non-mottled bird hides it completely in the F1 — those chicks look plain but carry one copy — and the white tips reappear in about a quarter of the F2, the standard recessive pattern. Two mottled parents (both mo/mo) breed the tipping true every time; what still needs selecting each generation is how much white each bird shows, since expression grows with age and drifts without active selection pressure in either direction. You can plan mottling crosses in the Breeding Outcomes tool.
- Somes RG Jr (1980) The mottling gene, the basis of six plumage color patterns in the domestic fowl. Poultry Science 59(7):1370–1374.
- Kinoshita K, et al. (2014) PLoS ONE — identifies a substitution (Arg332His) in the EDNRB2 gene disrupting endothelin‑3/EDNRB2 signalling as the molecular basis of the mottling phenotype. doi.org/10.1371/journal.pone.0086361.
- Genotypes and the Old English Game "Spangled" naming note match the Compare tool's standard data.