Red and black pigment - sekelsta/horse-colors GitHub Wiki

Mammals (such as horses) can make two types of pigment. One is a coppery reddish brown, or more yellow-gold in some animals, called pheomelanin. The other is darker, black or chocolate in color, and called eumelanin. In horses, they are simply called red and black pigment.

Pigments

Red	Black
Range of color of red pigment (inexact simulation) This chestnut horse's fur has red pigment.	Range of color of black pigment (inexact simulation) This black horse is colored by black pigment.

To get a sense of what red pigment looks like, we can look at the fur of a chestnut/sorrel horse. And to get a sense of what black pigment looks like, we can look at a black horse.

Each pigment cell is capable of producing either red pigment or black pigment. Certain signals tell it which type to make. Sometimes mutations affect how the decision is made, and they change where red and black pigment are placed on the animal.

A bay horse, for example, produces red and black pigment in different areas.

Red is produced on the body, while black is produced on the mane, tail, lower legs, and skin. Often there is also a little black on the body. Here's some illustrations to show this better:

Bay is pretty close to the color horses were before they were domesticated (bay dun), and you can think of bay horses as having all the biological signalling systems working properly. Mutations to the red/black type signalling are responsible for chestnut and black horses.

In a chestnut (or sorrel) horse, the fur everywhere is red, though the skin is still black.

The chestnut color comes from a variant of the extension gene. You can think of this variant as taking all the black from the horse's fur, getting rid of it, and replacing it with red so you're left with chestnut. (See the biology section below for more details.)

A black horse makes black pigment everywhere, in the fur all over the body as well as the skin.

Black comes from a variant of the agouti gene. You can think of this variant as taking the black parts of a bay horse and spreading that color across the whole body in place of the red, making the horse fully black. (See the biology section below for more details.)

The two genes extension and agouti control whether a horse is bay, chestnut, or black.

First, extension separates chestnut (ee) from "not chestnut" (EE or Ee). Then, if not chestnut, agouti separates bay (AA or Aa) from black (aa).

Pigment cells can switch between making red pigment or making black pigment. The genes extension and agouti are part of the signalling system that decides which color to make.

The extension gene codes instructions to make a receptor. This receptor is a molecule that sits on the cell membrane, the boundary between the inside and the outside of the cell. Its job is to listen for a message from the outside of the cell saying to make red pigment or to make black pigment, and pass that message along to the inside of the cell. Think of it like a switch that flips between RED and BLACK.

That's what the normal version of the receptor, made by the E allele at extension, does. Bay horses and black horses both have the working E version. However, a mutation to extension, the e allele, has different instructions. These instructions make a broken receptor that is stuck on RED no matter what signals it gets from outside. Chestnut horses have these broken receptors, e, that can only tell the cell to make red pigment, which is why their fur is red everywhere.

The agouti gene codes instructions to make a signalling molecule. The agouti molecule's job is to pass a message from outside of the cell to extension's receptor, which should then pass the message along to the inside of the cell. Specifically, agouti's molecule says to make red pigment.

But wait, if agouti's signal says to make red, where does the signal to make black come from? That is actually another molecule called MSH. MSH binds to extension's receptor and switches it to the ON mode. Agouti's signalling molecule works by blocking the receptor so that MSH can't bind to it, leaving it OFF.

Agouti's signalling molecule is produced on the horse's body, which tells it to be red. But it's not produced on the mane, tail, or lower legs, leaving them black.

The functional version of agouti's molecule is produced by the A allele of the agouti gene. That's the one that interacts with the receptors as above, and it's what bay horses have. A mutation to agouti, the a allele, has different instructions and the molecule they make doesn't work right. It can't tell extension's receptor to make red pigment, so only the signal to make black pigment is there. This can make horses that are black everywhere.

Here's some examples of chestnut/sorrel horses:

• Bright chestnut (ignore any white markings)
• Reddish chestnut (ignore the white brand)
• Reddish chestnut
• Flaxen chestnut

Here's some examples of black horses:

• Black
• Black
• Sun-faded black

• An energy-conserving hair reflectance model - The diagrams of eumelanin and pheomelanin color variation are based on the one here

More than you ever wanted to know about melanin can be found from these articles:

• Signaling Pathways in Melanogenesis - Nice overview of elanocytes and melanin production. Includes diagrams.
• Chemistry of mixed melanogenesis--pivotal roles of dopaquinone - Lots of details about the chemistry for producing eumelanin and pheomelanin. Includes kinetic rate constants.
• Impairment of mixed melanin-based pigmentation in parrots - Parrots do it differently, they have a different, brighter, pigment for red and don't bother with pheomelanin at all.