THHNBasics.pdf

HORSE COLOR

Lesli Kathman, 4700 Lone Tree Ct., Charlotte, NC 28269 <leslikathman@earthlink.net>

Covering the Basics

Several weeks ago some hobbyists and I were discussing horse

color inheritance, and one of the participants asked why anyone

involved in model horses would ever need to understand something

as technical as genetics. After all, she commented, all a model horse

shower needed to know was what colors were possible in the various

breeds. So before I start this column, I think it would be appropriate to

explain why having a basic understanding of coat color genetics is

important to hobbyists.

It is true that unless you are involved in pedigree assignment you

will not need any more information than my friend indicated; what

color does this or that breed come in. But that is a far more

complicated task than it might seem at first. It might not matter

whether my Quarter Horse with a tan body and dark points is a

buckskin or a dun, because both colors are found in the breed. But it

might matter a great deal if I want to paint my new Connemara model

grulla and I have found a picture of a tan, dark-pointed Connemara. If

that picture represents a dun Connemara, and not a buckskin, then

my grulla model is possible. But I have to be able tell the difference

between the two similar colors to be able to determine this. And

contacting the breed registry will not help me, because the

Connemara registry uses the term "dun" for a color that is actually

buckskin.

A similar example of this problem is found in the Arabian breed.

Their registry will allow horses to be registered as "roan". But

Arabians don't come in true roan. Two patterns are found in the breed

that look a little like roan (sabino and rabicano), but they are not the

same as roan. The roan gene will give an even mix of dark and white

hairs on the body, with a dark head, legs, mane and tail. Sabinos that

look like roans will have white ticking throughout the coat, and

extensive white markings. Rabicanos have white ticking primarily

along the flanks, and white tail tops. But if you are not familiar with

color genetics and don't know that these are three unrelated patterns,

you could find a picture of a rabicano Arabian, and decide that the

picture proves there are "roan" Arabians (after all the registry says

"roans" can be registered). Then you could use a lovely reference

picture of a true, dark-headed roan Quarter Horse as a guide for

repainting your Classic Arabian Mare. But Arabians that color don't

really exist!

Because model horse hobbyists tend to have an interest in

horses that crosses breed lines (we often like, and own, models of

many different breeds), we are particularly vulnerable to these

inconsistencies in color names in the different breeds. Adopting the

terms used by geneticists for the different colors can help to create a

more "universal" language of color that can be used for all breeds.

Once these terms are understood, it is possible to speak more

precisely and accurately about horse color. These terms are not

based on a given registry's designation of color, but rather on the

different genes thought to be responsible for those colors.

It is my intention to focus on a different color (gene) in each issue,

with an explanation of how the gene is inherited and the effect it has

on the horse's color. I will also try to make it easier to identify the color

when you see it, how to tell it apart from similar colors, and details to

remember when painting the color. In these future installments of the

column, I will avoid being overly technical, but in this first one, I will

have to establish some basics involving genetics and horse color to

avoid confusion later.

Skin and Hair Pigment

Normally horses have black pigmented skin. This is visible in the

areas where the hair is particularly fine--the muzzle, the genitals, and

the inside of the ears. The black skin is why a grey horse that has

turned white with age will seem to have a dark grey muzzle. The black

pigmented skin shows through the sparse hair around the mouth and

nostrils.

When a horse has white markings, the skin underneath will be

pink. This is because white isn't really a pigment, but rather the lack of

pigment, and the skin appears pinkish from the blood vessels

underneath. A common mistake of beginning repainters is to use grey

to shade a blaze, not realizing that there is white skin around the

muzzle that gives it a pinkish, and not a greyish, tint. The same is true

of hooves. If there is white on the leg touching the hoof, the hoof will

be unpigmented (shell-colored), and black-skinned legs will have dark

hooves. On grey horses that have turned white, the leg markings may

no longer be visible, but the color of each hoof will show which legs

once had white markings. Stripes on hooves are usually caused by

skin that has white and black spots along the coronet band.

The above descriptions involve skin pigment. The pigment of the

hair is a different matter entirely, and in this column, use of the term

pigment (such as "black pigment") will refer to hair pigment unless

otherwise noted. Pigmented hair is always either red or black

(remember white is not a pigment, but rather the lack of pigment).

This applies to the hair on the body and the points - mane, tail, lower

legs. So horses are either red with red points (chestnut), or black with

black points (black) or red with black points (bay). Chestnut, black and

bay are the basic colors found in horses. All other colors and patterns

are the results of modifiers working on these three colors.

Modifiers

No matter how exotic a horse's color may appear, he will always

be basically chestnut, black or bay. The variety of colors and patterns

found in horses are the result of genes known as modifiers, which are

all inherited separately from the basic color. Grey, palomino, roan, dun

are all caused by modifying genes. But these genes work

independently from the basic color. It is not a matter of "pinto is

dominant to chestnut - the pinto gene has nothing to do with whether

or not the horse inherits the chestnut color! It is a modifier to that

chestnut color.

For example, if you breed a chestnut mare to a bay pinto stallion,

it is not as if the foal can be either chestnut or bay pinto. The foal

could be either bay or chestnut and either pinto or non-pinto. In this

way, modifier genes are like on/off switches. The bay pinto stallion in

the example is carrying a gene for spotting ("on"), and the chestnut

mare has the gene for non-spotting ("off'). The resulting foal may have

the pinto gene either "on" or "off'. Of course, most pinto breeders will

hope it is on!

Now imagine all the possible modifiers a horse can have (pinto

patterns, greying, roaning, dilutions, appaloosa patterns - everything!)

as a long row of switches, one switch for each possible modifier. Any

combination of these switches can be on or off, depending on what

combination of genes the horse inherited. The gene can be roan or

notroan, or tobiano or not-tobiano, etc. Any of those switches that are

"on" will modify the horse's basic color.

Please note that this is all very simplified, and that the actual

genetics involved are far more complicated. Many genes and their

effects are not fully understood, even by experts. But the concepts

presented here will work for a basic understanding. The important

thing to remember is that color inheritance is not as random as it

might seem. It is complicated, and at times convoluted, but not

random. It really can be understood, even if you don't like things that

are "too technical"!

Next issue we will start with the first of the modifying genes.

Ways That Modifiers Work

Modifiers, like the name suggests, alter the appearance of the

basic coat color of the horse. Any given modifier will work differently.

Some modifiers are specific to one kind hair pigment, either red or

black. A modifier that was specific to red hair would change any red

hairs on a horse, but wouldn't affect any black hairs. Other modifiers

are location specific. These modifiers will only affect the body color, or

only the point color. Some modifiers are not specific at all, and will

affect both red and black pigment and all locations (body and points).

Knowing how each modifier works is essential to understanding color.

A Word About Dominance

Probably the term that most often leads to confusion about color

inheritance is "dominance." What exactly is meant when a gene is

dominant?

Going back to the very basics, all horses will inherit two genes

for any given trait, one from each parent. Colors or patterns that are

visible even when the horse only inherits the gene from one parent

are said to be dominant. If the horse must inherit the same gene from

both parents for the color or pattern to be visible, it is said to be

recessive. Colors that look one way when the horse inherits a gene

from one parent, and look different still when the color is inherited

from both parents are said to be "incompletely dominant." Palomino is

a good example of incomplete dominance: if one gene is inherited, the

horse will be palomino, but if two are inherited he will be cremello.

Dominance does not refer to the frequency with which the color will be

inherited. No matter how dominant a gene might be, the horse never

has more than a 50/50 chance of inheriting any one gene! It also has

nothing to do with how common a particular gene is in the general

horse population; there are many dominant genes that are extremely

rare!

Dominance refers to how the gene interacts with its opposite. For

example, the ability to form black pigment (black) is dominant to the

inability to form black pigment (chestnut). If a horse carries both

genes (black and chestnut) he will be black and his offspring have an

equal chance of inheriting either black or chestnut from him. The

concept of dominance comes in when that one gene he gives is

paired with the other parent's one gene. If he gives his foal the ability

to form black pigment (black), that foal will be black, because the

ability to form black is stronger ("dominant") to the inability to form

black pigment. By the same token, if he gives his foal the inability to

form black pigment (chestnut), it will not affect the color unless the

other parent gives the same inability to form black (chestnut), because

this trait is weaker ("recessive") than the ability to form black. So

dominance doesn't make the gene more or less likely to be passed on

to offspring, but rather determines which of the two genes inherited

will be visible when you look at the horse.

These two horses are genetically the same color - bay - but look very different

due to the presence of different modifying genes. The top horse has the dun

and frame overo modifiers, while the pony on the bottom has the cream

(buckskin) and silver modifiers.

color that originally appeared in The Hobby Horse News, a magazine for the model

horse collecting community that is no longer published. The articles may be copied for

personal, non-commercial use. Individuals may also offer the files for downloading

provided they are properly attributed, but inclusion in commercial publications

(magazines, newsletters, books) is forbidden unless written permission is obtained from

the author.

To further explain the "'switches" analogy, take "Fred", an

imaginary horse of a color typical of models: a grulla-going-grey

tobiano.

His genetic "code" could be: GgEEaaDdTOto

Now think of these letters, which represent genes, as

switches which come in pairs. Capital letters are

switches

genes (no black hair at all) then his ‘A’ genes - whatever they were

- wouldn't matter because he wouldn't have any black to restrict.

But the switch is still there. It is just like if you turned the breaker

box off - it doesn't mean your light switch cannot be in an on or off

position, it just means that it doesn't have an visible effect when

you flip it.

By this same token, our Fred could have had the ‘Cr’ gene

(palomino), but we'd not have known it without examining his

parents or his offspring. This is because even if the dominant Cr

gene was

switches (recessive). It

only takes one uppercase letter for the overall effect to be “on”.

Breaking Fred down gene by gene, we have the ‘G’ gene,

which is the greying gene. For Fred the grey switches are

off

and

(’Crcr’), we would never see it because ‘Cr’ lightens

red hair, but not black. And all poor Fred has is black hair. So his

‘Cr’ switch could be on all this time, and we'd be none the wiser.

But we can see that Fred is grulla, because he carries the

dun (’D’) gene. Fred's dun is

for grey, Fred is going

to prematurely grey. And if we breed Fred, half his offspring will get

‘G’ (grey

- ‘Gg’ - and because it only takes one

) and half will get ‘g’ (grey

off

). Now if our Fred had

switches (’GG’) instead of ‘Gg’, all his offspring would be

grey, because it only takes one ‘G’ (dominant) to make a horse

grey, and Fred would be giving one ‘G’ to all his foals.

Now for the next two pairs of genes, which decide whether

Fred is chestnut, bay or black. Because genes cannot be

anything more than

, because he is ‘Dd’. If Fred were

dd he would be not-dun, like the vast majority of horses. ‘D’ is the

dun gene (on) and ‘d’ is the not-dun gene (off). The ‘D’ gene

dilutes the body color, but leaves the points undiluted. It does not

matter how Fred's other switches are positioned, it is just going to

take the results and lighten the body. So because Fred ended up

black, it's going to lighten his body (making it mousy colored) and

leave his legs, mane and tail black. If Fred had ended up bay

instead, this gene would have made him yellow dun.

Finally, Fred also has one Tobiano gene

, we cannot actually have a "chestnut

gene" or a "bay gene." What we have are two pairs of switches

that end up giving us those three results. In Fred's case, these

switches give us black as a base color.

Of these two genes, the first is ‘E’, which means the horse

can form black pigmented hair. Because Fred is ‘EE’, his offspring

will all be able to have black pigmented hair. For Fred, the black

switches are both

off

(’t’). So he is tobiano spotted to some extent, and just like his grey

gene, half his offspring will get this on switch as well.

What geneticists don't know is why Fred might end up a

tobiano with a lot of white, or just a little hint of the tobiano pattern.

And this is true of all the patterns - what is confusing is what

genes, or combination of genes, cause the patterns or markings to

look different. And some patterns that may seem like "one switch"

may in truth he a set of switches that often go together. But the

basic idea of on/off switches still applies.

(’T’) and one

. If both switches were off, Fred would be

chestnut (’ee’).

The other gene is ‘A’. The A gene says whether the horse's

black pigment is restricted to the points (legs, mane and tail) or

not. If this restriction is

, so his

black hair is not restricted at all. He is all black. If Fred had two ‘ee’

off

(dominant). Lower case letters are

off

two

, the horse can only have bIack hair on

the points, and is bay. But Fred's switches are both

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