#there's so much factors in culture surrounding equine coat colours that it's difficult to gauge

[thunderboltfire]

Hey I'm NOT an animal coat color genetics expert, but I dabbled in it a little bit.

horses have two pigments determining their coat color - red and black.

They also have two genes controlling their base color - Extension ("E") and Agouti ("A"). Extension determines if a horse produces the black pigment at all, and agouti tells us where it's located.

Chestnuts are at the bottom of epistatic hierarchy, meaning that any other genetic combination makes it not occur, essentially "covers" it.

Chestnut is the rarest of the basic coats, because it requires two copies of recessive extension alleles (marked as 'E' - any horse that has ee in its genotype will be chestnut-based). It can have any agouti alleles, though.

Black is actually a similar case, because it only occurs if a horse has two recessive agouti alleles (aa). It takes only one dominant extension allele to make a black horse, so it can be either Ee or EE.

Add a single dominant Agouti allele to a black horse and you have a bay, because agouti restricts black pigment only to the points, mane and tail creating a bay pattern.

Technically, the plainest and naturally the most prolific basic color would be bay - so the OP is right saying that chestnuts are definitely not the most plain even out of the basic coat colours.

in short:

ee - no black pigment (only red pigment)

Ee, EE - black pigment present

aa - no restriction of black pigment

Aa, AA - restriction of black pigment (bay pattern)

ee + aa, Aa or AA - a horse doesn't produce a black pigment at all, agouti can be there or not, it does nothing bc there's nothing to restrict. A horse is chestnut!

Ee or EE + aa - a horse produces black pigment, but there's no agouti to restrict it. A horse is black!

Ee or EE + Aa or AA - a horse produces black pigment and it's restricted to the points and manes. A horse is bay!

It makes chestnut technically the rarest out of them all, BUT also it's the easiest to maintain in breeding. If You cross a chestnut and a chestnut, it's ee x ee - there's no dominant E allele to inherit, so the offspring HAS to be chestnut. That's why there's quite a few all-chestnut breeds - haflingers, suffolk punches, and a vast majority of gidrans are chestnut.

The situation differs a bit when You want to restrict a breed's color to be only black - your horse can be Ee and still be black, so if You unknowingly cross Ee x Ee, You have a chance to get an ee - or a chestnut. That's why every so often a chestnut friesian pops up, depite its parents and all recorded ancestors being black.

So, while chestnut is rarer than bay and black, it's next to impossible to completely get rid of a chance of getting a chestnut. AFAIK chestnut color is not approved in hucul ponies, but they still pop up sometimes, despite of the fact that no chestnut horse has been permitted in breeding for decades.

As far as the cultural approach goes, coat colors are always a matter of fashion and personal taste. If I were to guess, I'd say that the current proliferation of the 'basic coat colours' in sport horses is the result of them being used in the military in the past. Some armies preferred to have cavalry units on horses of uniform colour, there was also a period when painted horses were looked down upon as less noble looking. If You have a herd of chestnuts, blacks and bays, the results of breeding are pretty easy to predict and will mostly be constrained to just those colours - chestnut, bay and black. Nowadays diluted colors and patterns are fashionable, people are fascinated by the diversity of equine coats and often deem the basic colors to be boring, but it was not always this way!

This is completely off the wall, and idk TOO much abt horse genetics or even history, but from what I DO know, chestnut is a super common, considered p basic color but it's essentially recessive to... anything else? Literally any other coat base and/or modifier? Is that right? And based on cave paintings and Przewalski's, it doesn't seem like the original, primitive, wild horse breeds would've been chestnut. They had more bay or dun coloration, primitive markings, etc. So do u think part of the reason chestnut is so common is bc, at some point in human history, it WASN'T considered this basic plain jane color, it was actually like "OOOOH, this horse is RED!! It's JUST RED!!" and people tried to breed for that and it became more common bc it WAS considered special at some point? And then ofc some horses were used to establish breeds where types and colors were restricted and bam, proliferation, but my point is, it feels like chestnuts get overlooked a lot as rly plain, but do u think at one point they were considered special and sought after? idk im emotional i saw that post abt how pigeons were once loved and cared for and now they're treated like rats with wings and idk why i started thinking abt chestnut horses but do u ever think something similar happened??? im drunk and sorry

you sound like me when i'm manic i can't give an honest answer cause i am not a genetics nerd and haven't looked into the history but i mean they could have been like "hell yeah dude it's red, lets name it ruby.. and that one too" thus creating the never ending cycle of red mares named ruby