Natural gene / phase 1
Sooty — the dark that lies on top
The countershading over a horse’s topline, crest, shoulder and croup that makes it look darker than its base colour says it should be. A muddy palomino, a smutty buckskin, a liver-leaning chestnut and a dark bay that people keep calling black are all this, and it is the commonest single reason a coat cannot be identified from a photograph.
What it does
What to look for: a horse that looks darker than its colour ought to be, shaded along the topline, crest, shoulder and croup. A muddy palomino, a smutty buckskin and a dark bay that everyone keeps calling black are all this.
It is the single commonest reason a coat is hard to identify, so if a horse's colour is not adding up, suspect this first.
Crossing two of them
This gene has no effect on the horse’s health.
Its gene carrot
- Gene key
- horsegenetics.sooty
- Alleles
- S2 S1 s
- Outcomes
- wild,
faint-sooty,sooty,heavy-sooty,extreme-sooty - Default allele
- s
- Priority
SootyGene.PRIORITY— after dun, before the dilutions- Founder draws
- 1 × nextFloat()
- Deterministic
- no when expressed
- Epi draws
- 1 × nextFloat — the expression roll
- Health
- none established
Covered by SootyGeneTest and the golden file, and the range has
been eyeballed as side views over five bases. Not seen in a game. See
verification §0-T.
Sooty is a failure to restrict
Phase 1 only ever pushes pigment down. Every texel starts at a full load of both pigments and each gene takes some away; a gene that adds black cannot exist in this model.
Sooty does not add any. It declines to remove what agouti and the dilutions were about to take — the eumelanin was always on that texel, and this locus is the horse not switching it off along its topline. The painter walks the black level back up toward, and never past, the load the texel began the pipeline with.
That is the honest mechanical description, and it happens to be what the
biology is thought to be: the chromosome-22 region the evidence points at
sits beside ASIP, whose entire job is telling a follicle to stop
making black. SootyGeneTest asserts the ceiling directly,
because if it ever breaks the gene has stopped being “a failure to
restrict” and become “invents pigment”, and the
pipeline’s downward-only guarantee goes with it.
The dosage
| Combination | Dosage | Outcome | What shows |
|---|---|---|---|
s/s | 0 | wild type | the base colour reads cleanly |
S1/s | 1 | faint-sooty | a dark haze along the spine, a little at the croup |
S1/S1, S2/s | 2 | sooty | an obviously dark topline over a clearer belly |
S2/S1 | 3 | heavy-sooty | a dark cape over shoulder, back and croup |
S2/S2 | 4 | extreme-sooty | dark enough that a bay or buckskin reads nearly black |
faint-sooty draws a dark line down the spine, and that is
exactly the thing a horse gets misidentified as a
dun for. AQHA is explicit that a dorsal stripe
alone does not establish dun. In this mod the difference is visible and
structural: a dun brings leg bars, a shoulder bar and a face
mask with its stripe and dilutes the body; sooty brings
a diffuse band and darkens. A horse can be both, and sooty runs after dun
precisely so a sooty dun keeps its primitive markings and gains a cape
around them.
The same score looks different on every base
The genotype sets the intensity; what darkening looks like is whatever pigment is there to keep. That falls out of the mechanism rather than needing a per-colour table, which is the nice part:
| Base | What sooty does to it |
|---|---|
| Bay | The kept eumelanin is black, so it caps the shoulder and back. A dark bay, a mahogany, or something that reads seal-brown. |
| Buckskin | A brown-to-near-black cape over the gold. Frequently mistaken for a dark bay. |
| Palomino | The most dramatic, because of the contrast: gold dragged toward bronze, with muddy lower legs. |
| Chestnut | The chart’s warm edge stays brown a long way down, so it reads as a liver overlay rather than as black hairs — which is the right answer, since a chestnut cannot make black hair. |
| Black | Nothing. There is no removed pigment to keep. That is not a special case in the code; it is arithmetic, and it is exactly what the reference says about trying to identify sooty on a black horse. |
| Grey | Drawn, then progressively hidden as grey lightens over it — grey runs later. |
Where it sits
- Countershading is the core: full along the topline, smoothly fading to nothing at the belly. The barrel’s own height fraction does it, so it does not depend on the horse’s size.
- The croup and the shoulder take extra — two soft humps along the barrel. They are the two places the reference names as darkest.
- The crest takes most of the body’s share, thinning down the neck; the face a little.
- Muddy lower legs, but only on a chestnut-based coat. On a bay the legs are already black-pointed and there is nothing to add.
- The mane, tail and ears are excluded outright. Sooty is a body-coat modifier; a dark mane on a sooty horse is the base colour’s doing, and the reference puts mane and tail changes with silver, flaxen, cream and sun bleaching instead.
- Mottled by a fixed noise field, so it reads as hairs rather than as an airbrush.
Health
There is no established disorder, welfare problem or performance
consequence from sooty. The mod gives it no
TraitContribution at all.
The nuance worth recording is that the chromosome-22 region sits near ASIP, and the study’s authors suggested future work on whether regulatory variation there might have health implications. That is a research hypothesis, not a finding: they identified no causal mutation and demonstrated no health effect. Meanwhile grey really does carry melanoma risk, silver really does carry an eye defect, frame really is a homozygous lethal and LP/LP really does carry night blindness — and none of that belongs to sooty because a sooty horse happens to be dark.
What is actually known
Not much, and nothing testable. There is no validated commercial sooty test and no demonstrated inheritance model — not dominant, not recessive, not codominant, not incomplete dominant. Saying otherwise would be inventing a fact.
What exists is a 2020 genome-wide association study of 126 bay horses (Quarter Horses, Arabians, Persian Horses, two Standardbreds and one Thoroughbred) finding a very strong association between a region on chromosome 22 near ASIP and RALY and how far black spreads over a bay. The lead marker’s effect was roughly additive — which is the one structural fact worth building on, and is why this locus is a dosage. The causal mutation was not identified; a candidate RALY indel was found and the lead marker still predicted the phenotype better.
So: a dose-dependent modifier with a per-horse roll, the same shape flaxen and the bay shade score take, and labelled as a modelled modifier rather than as settled science.
Why it is not the shade locus, given they are the same paper
Shade models the same chromosome-22 signal. Keeping them apart is deliberate:
| Shade | Sooty (this gene) | |
|---|---|---|
| Scope | Bays only | Every base with pigment to keep |
| Justified by | The study, which measured bay horses and ranked bay shade. That is the claim it supports. | The phenotype. A muddy palomino and a smutty buckskin are real and common and obviously not bay shade. |
| Does | Decides how far black spreads from the points — blood bay to seal brown | Lays a dark cape over the topline of anything |
| Shows on a chestnut | No | Yes — as a liver overlay |
Folding them into one locus would have claimed a great deal more than the paper does: that one mutation causes both bay shade and every dirty palomino. Two abstractions of overlapping evidence, each honest about its own scope. A horse carries both, and a dark bay can be dark for either reason or for both at once.
common/genetics/genes/SootyGene.java