Flaxen — the chestnut’s pale mane

Crossing two of them

Not the look-alikes

Five things give a horse pale long hair. Telling them apart is most of what identifying flaxen consists of, and one of them matters for a reason that is not cosmetic.

Breeds

Because the cause is unknown, “the flaxen allele occurs in breed X” can only ever mean “the visible phenotype is common there”. Two breeds here name the locus:

• Haflinger — the breed the numbers are calibrated against, and its whole look: fixed e/e for chestnut, with the flaxen pool pushed to the top of the range. A Haflinger without a white mane is not much of a Haflinger.
• Morgan — the breed flaxen pedigrees were actually studied in, at a moderate share.

The reference also names the Black Forest Horse, Breton, Frederiksborger, Jutland and the South German Coldblood; none of those is in the breed list yet. Every other breed is f/f, because a breed that does not name a coat locus gets it wild — the feral population is where the full spread lives.

Its gene carrot

Gene key

horsegenetics.flaxen

Alleles

Fl2 Fl1 f

Outcomes

wild, mild-flaxen, flaxen, strong-flaxen, extreme-flaxen

Shows when

dosage above FlaxenGene.PLAIN_MAX — and only on a chestnut

Default allele

f

Priority

FlaxenGene.PRIORITY — immediately after silver

Founder draws

1 × nextFloat()

Deterministic

no when expressed

Epi draws

2 × nextFloat — the mane’s score, then the tail’s offset

Health

none — and that matters; see below

The dosage

One strong copy and two mild ones are the same horse. That is what an additive dosage means, it is the whole reason for three alleles where the folklore model has two, and it is what lets two mild carriers throw something paler than either of them.

On top of the dosage each horse rolls an expression offset — FlaxenGene.EXPRESSION_RANGE either way — standing in for the season, age, sun and condition variation that is one of the reasons a single-locus model never convinced anyone. It is deliberately narrower than a band, so a horse never contradicts the name its genotype was given. Like every other per-horse number here it rides on the expressing copy’s stored values, so it is fixed for life and inherited with the allele.

Chestnut only, and carried by everything

Flaxen is a red-pigment trait: it shows on a horse that makes no black hair at all, which is e/e at extension. FlaxenGene.expressionIn reports the wild type on anything else — the same suppression agouti does on a chestnut, in the opposite direction.

A bay or a black carries the whole dosage silently and hands it on, and that is the satisfying part of the gene: two plain-looking chestnuts can throw a flaxen foal, and a black stallion can be the reason a line has it at all.

How it paints

• The mane and the tail are rolled separately. Real flaxen manes and tails are frequently not the same shade, and one being paler than the other is the commonest thing to notice about a mixed-hair chestnut. FlaxenGene.TAIL_OFFSET is how far apart they can land.
• Strand mixing. A fixed noise field spares some hairs, so a flaxen mane keeps darker strands in it. A uniform wash reads as paint rather than as hair.
• It takes red away and leaves black alone, which on the chestnut it only ever runs on is the whole move. The chart’s top row runs from a warm brick red to white without passing through grey, so a mane at the pale end lands blond with no tint term at all. That is exactly what silver cannot do — it works on black pigment, off a column where scaling alone leaves a point jet — and why silver has a blackTint term and this does not.
• Nothing else moves. Not the body, the legs, the ear rims, the skin, the eyes or the hooves. The reference is unusually firm about it: flaxen is a long-hair modifier, and every gene that also lightens a body is a different gene.

It is a phenotype, and this is a simulation of one

There is no identified flaxen mutation and no DNA test for it. That is the single most important fact about this gene and the reason it is shaped the way it is.

Breeders have modelled flaxen as a simple recessive for a century, written f/f or fl/fl, on pedigree evidence — flaxen bred to flaxen tends to give flaxen, and Morgan pedigrees in particular were studied for exactly this. That is a useful breeding observation and it is not proof of a locus. The veterinary-genetics resources list no flaxen variant and no commercial test, and they are explicit that a great deal of equine shade variation remains unexplained.

So this locus is a simulated polygenic modifier, and it says so in its own description in the gene browser. It is the same shape the bay shade score takes, and for the same reason: a graded, heritable, additive quantity is what the evidence describes, and one locus with a dosage plus an expression roll says that in a genotype code a player can actually read. Three to six separate polygenes would say it too, at the cost of six segments in every horse’s code and six loci nobody could reason about.