The Science of Horse Racing: Coat of Many Colors

Racing fans often express their affection for a particular color of horse over another, especially if their favorite horses happen to share that coat color. You might gravitate toward the grays or bet mostly on the bays, a particular shade always grabbing your attention first. Where do those colors come from though? How does a horse with a bay sire and a roan dam come to have a distinctive coat color like Knicks Go? That is a byproduct of their genetics, the essential building blocks of organisms inherited from their parents.

For this month’s Science of Horse Racing, let us explore how genes determine coat color and even how to predict whether a foal might be bay or gray.

Building Blocks

A horse’s coat color is an inherited trait, transmitted on the chromosomes they receive from their sire (father) and dam (mother). Those chromosomes are made of deoxyribonucleic acid, or DNA, which is stored in nearly every cell in the body and tells the cell how to behave. DNA gives those instructions in a gene, and, since a horse comes from a male and a female horse, then they get one copy of each gene from each parent.

Genes come in variations called alleles, where the variations or mutations that express the differences between one horse and another are located. While each horse will get genes for coat color from their sire and dam, the allele is the type of gene that the horse receives. Because of that, it is possible that one mare can produce a foal that is bay and one that is gray, depending on the alleles inherited from the sire.

The color in hair and skin comes from melanin, the pigment that determines the color for all mammals, from horses to humans. Melanin comes in two forms: pheomelanin, a red to yellow color, and eumelanin, which is brown to black. Since melanin itself is not a protein, there is no one genetic component that determines its concentration; instead, genes that affect melanocytes, the cells that produce melanin, determine whether melanin is produced and where.

All horses start with either pheomelanin or eumelanin as their base color. Those with pheomelanin will have a red base color to their skin and then their genes will determine what color the hair itself will be; this is the base skin color for most horses that are not bay or black or a variation of those (like grays). Horses with eumelanin will be black or bay, again depending on the alleles that they inherit from their sire and dam.

Horses with eumelanin skin have two main genes that control their coat color: extension (or E) and agouti (or A). E determines how much black the coat will contain while A will tell where the black will be on the body. Bay horses will have black on their points, or legs, ears, mane, and tail, depending on how the agouti gene is expressed in their DNA.

Genes and Shades

The basic horse coat colors are black, bay, and chestnut. Variation, known as shades, among the three coat colors exists, hence why you have a reddish chestnut versus a golden chestnut. Gray is not a color so much as an absence of color in a coat; gray horses tend to start out as one of the basic three colors at birth and then eventually the gene for gray – or the absence of color – will kick in and they will shed that original coat.

Even though a foal will inherit the two copies of the same gene from their parents, those genes may or may not have the same alleles for color. So, it is possible for a horse to inherit the instructions for two different colors, like gray and chestnut for example, but the foal’s coat color depends on which alleles are dominant and which are recessive.

When we discuss genetics, dominant alleles are represented by a capital letter, as in G for gray, W for white, or E (for extension) for black. Recessive alleles are represented with a lowercase letter, like e for black or g for gray. The simplest way to understand the genetics of coat color is to consider what alleles a horse inherits from their sire or dam. So, a black horse can have two dominant alleles for that color, or EE, or one dominant and one recessive (Ee) while a chestnut will have two recessive alleles for black, ee.

For a gray horse, the genes for hair color will have a dominant G allele. For a non-gray horse descended from a gray, they will have a recessive g allele. So, if you paired two horses with G alleles, then their foal will be gray. If you paired a stallion with a dominant G allele with a mare with a recessive g allele, then you will still get a gray horse, with the Gg alleles. Grays are born with a darker coat, but, as they grow, that G allele will tell their cells to stop producing melanin in their hair and their coat will grow progressively lighter though the skin underneath will remain the same color.

All of the Possibilities

To help us determine how dominant and recessive traits might be distributed in an offspring, genetics uses a Punnett square, a square diagram that allows users to determine the probability of a particular outcome in genetics. When it comes to determining what color a foal will be, a Punnett square is a good tool for predicting possible coat colors based on the known dominant and recessive traits. If you have sire that is black with Ee alleles, he has one dominant allele and one recessive allele. Matching him with a mare that is chestnut, or ee, with two recessive alleles, will yield these possible outcomes:

That Punnett square tells us that the foal in question has a 50% chance of being black and a 50% chance of being chestnut because of two recessive alleles. If you were to pair a gray stallion (GG) with two dominant G alleles and a chestnut mare (gg) with two recessive g alleles, then the Punnett square would look like this:

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Given that the stallion possesses two dominant alleles, the foal has 100% chance of being gray since the mare has only recessive genes for gray. The challenge with this type of assessment, though, is that we know that other coat colors, like roan and its variants, exist, injecting more complexity into the discussion. While the genetics of coat color depends on more variables than we can cover here, this should give the basics for understanding how gray stallions like Tapit can produce bay horses like Flightline, a completely different color than their sire.

Blazes and Socks

I know what you might be thinking: what about horses with white markings, like blazes or socks? Where does that come from? While the predisposition for such markings on a horse is genetic, the location of those instructions in a horse’s genes still eludes researchers. The ability to predict where those markings will occur and how large or small they will be is not quite within our reach – yet. However, the study of genetics does allow breeders to understand how coat colors work and how to get a particular color. It also gives racing fans insight into how horses like Arrogate can produce a chestnut Secret Oath, a foal that might resemble her sire more in how she runs than how she looks.