GeneticsChapterII | salvatellastud

Genetics Chapter Two

Modifiers do what the name would suggest: They modify the base pigment. This can be in a subtle way, or it can be very extreme. Modifiers affect coat color differently for either base color. Modifiers are not just limited to the body, they can also affect the mane and tail.

Modifiers

Grey

Grey is probably one of my favorite colors because there is so much variety, and all of them are so spectacular…as well as beautiful in my opinion. Grey is dominant, and you should know what that means by now. Only one copy of the gene is needed for a horse to be affected. Grey is written with the letter “G”. In a genotype this could be written “Ee/Aa/GG, or Ee/Aa/Gg”. Additionally, grey is considered to be the strongest of all modifiers as it is dominant over every other color gene there is. Grey will always express itself, no matter what.

Very importantly, grey horses are NOT born grey. Think of it like an old person. When a person gets older, their hair starts to lose pigment. Some people only lose a little and get a “salt and pepper” look, while others will lose pigment until their hair is completely white. Horses are exactly the same. The grey gene causes a horse to gradually lose pigment in their coat over time. Some horses will grey faster than others, while some never completely grey out.

A horse who does not completely grey out is most commonly known as a flea-bitten grey. The “flea-bite” markings are merely just speckles of pigment not affected by the grey gene. Another example is mottling. This is caused by the depigmentation of the skin itself. As the skin loses its pigment, the horse might appear mottled until the skin is completely faded.

You might have heard the term “dapple grey”. This is NOT a color, merely an effect the grey gene causes. The pigment may fade in spots or smudges, instead of uniformly or smoothly, and can result in what we know as dapples.

Chestnut

Speaking of bay modifiers, it is hypothesized that chestnut horses who carry copies of the Agouti gene are affected by them. As this is only a hypothesis, I’ll only cover what color is thought each Agouti gene and/or modifier causes on a chestnut.

A+ is believed to cause light chestnut. This can be written A+A+, or A+a.
AA is thought to cause red chestnut. It can also be written Aa. As AA is a homozygous dominant Agouti, it is considered to cause the reddest color.
At is believed to cause a standard chestnut. It can be written AtAt, or Ata.
aa is thought to cause liver chestnut.

Knowing what we know about what each of these variations of the Agouti gene mean, it’s easy to see why those specific variations are thought to cause the different shades in chestnut.

Bay (Agouti)

Remember last chapter when I said we’d be going over the different bay modifiers? Well, tadah! You may have noticed that bays come in a large variety of shades and types. You might have heard the terms seal bay/brown or wild bay. These two variations are caused by certain alleles on top of the Agouti gene that affect the way it distributes the black pigment.

The modifying allele that causes seal bay specifically restricts the black pigment from the soft areas of the body. This includes the muzzle, flank, butt, and sometimes the chest. This is written as At. To simplify, you can also just write this as “At”. Seal bay is dominant, so only one copy of the gene is needed for it to be visible on a horse. To see this in an actual genotype, it could be written Ee/AtAt, Ee/AtA, or Ee/Ata.

The other bay modifier causes wild bay. It restricts black pigment to just the pasterns or up to the fetlocks and is written as A+. Again, to simplify we can just write it as “A+”. Wild bay is also dominant, and only one copy of the gene is needed for it to express itself. Wild bay can be written Ee/A+A+, Ee/A+A, or Ee/A+a. Remember, the extension gene (E) is completely separate and is not limited to the heterozygous form I used in the examples for seal bay or wild bay.

As a note, both seal bay and wild bay will still have a black mane and tail.

Flaxen is a modifier that affects only the mane and tail. It is written with the letter “F”. In a genotype, it could be written “ee/aa/FF, ee/aa/Ff, or ee/aa/ff”. It is recessive. This means that a horse will only be flaxen if it carries two copies of the recessive flaxen gene (ff). A horse with either “FF” or “Ff” are considered carriers of the flaxen gene. Remember, a carrier is a horse who does have the gene in its genotype, but does not express it visibly. A horse with homozygous dominant “FF” will never produce flaxen offspring because it can only pass a dominant copy of the gene to offspring.

Further, flaxen is a gene that only affects chestnut horses. Bay or black horses can be a carrier of the gene, or can even have two recessive copies of the gene, but it will not be expressed. Flaxen chestnuts are sometimes mistaken for palominos because of their lighter mane and tail, especially in the case of lighter chestnuts.

So to recap. Let’s play a little game. Can you name all the colors for the genotypes listed below?

EE/aa, Ee/AA, ee/aa, ee/Aa/ff, Ee/aa/Gg, Ee/Ata, Ee/A+A/STYsty, ee/aa/STYSTY, EE/AA/GG, and ee/aa/ff.

Bonus points if you can tell me why that genotype equals that color. (Remember your dominant vs. recessive genes).

Skylar Sovari

Flaxen

Sooty

Named for the way it looks like someone dumped a bucket of ash over a horse’s back, the sooty gene causes black hairs to be mixed in with the coat. The black hairs are generally most dense over the back, hips and shoulders. Occasionally they can also be over the neck, affect the mane and/or tail, as well as the ears and head.

Sooty is dominant; only one copy of the gene is needed to be expressed. Sooty is written “STY” or “sty”. In a genotype, it would be written “Ee/Aa/STYSTY, Ee/Aa/STYsty, or Ee/Aa/stysty”.

Sooty can express itself uniformly, or be sporadic and appear in smudges, patches, stripes, spots, or dapples. This can result in all that gorgeous dappling, or in rarer cases brindle or dun-like striping. As a note, dapples are NOT a color.

Sooty is interesting, because it can appear on any number of coats from chestnut, to bay, to palomino, to even appaloosa, tobiano, and roan. However, for the latter three, it will be under any present white hairs or markings. Sooty can also be present on black horses, but as expected, is a lot harder to see. Two copies of the sooty gene usually results in a stronger or more intense expression.