Horse Color Genetics - What About Bays? (Part 2) By Lydia K Kelly

If you read the first part of this article you will now be familiar with the basics of color genetics in horses. To quickly review things, there are two basic color genes, the black (B) and the red (b). All horses have a combination of these two genes. All other colors are created by special genes called modifiers.

Before we get started you need to understand some basic terms. The first thing you need to know is that dominant genes will always show their traits while recessive genes are silent when combined with a dominant gene. So a horse with one dominant color gene (B) and one recessive color gene (b) will always show the dominant trait which is black. Another important term to understand is homozygous. This means that the horse carries two copies of the same color gene. For example a chestnut horse (bb) must carry two copies of the red gene or it would not be chestnut. The complimentary term is heterozygous. This means that the horse carries one dominant copy of the gene and one recessive copy of the gene (Bb). The dominant gene will show, but the recessive gene can crop up when crossed to another horse who carries that recessive gene.

The purpose of a modifier is to modify, or change, the outcome of a basic color gene. For example, the most common modifier is the agouti gene. This special gene causes the black in a horse’s coat to lighten making it brown in color. On the other hand it does not affect the black on the points including the mane, tail and legs. If you have this pictured in your mind you will quickly see that the agouti gene is what makes horses bay in color.

So how does the agouti gene work? To make things simple let’s call the agouti gene (A). If a horse is black in base color (Bb) or (BB) and carries the agouti modifier it will be a bay (BbA) or (BBA). If it does not carry the agouti modifier it will remain black. Let’s look at this in a breeding scenario. Let’s take a bay stallion who is homozygous for the black gene (BB) but is heterozygous for agouti (A) and cross him with a black mare who is homozygous for the black gene (BB).

(BA)(B)
(B)(BBA)
bay(BB)
black
(B)(BBA)
bay(BB)
black

The stallion will only pass on his agouti gene half of the time, making half his foals from the black mare bays. The other half of the time the foal will not get the black gene and will turn out black.

Now the agouti gene is very common. This is why you do not see very many black horses. In addition to this, the agouti gene is also present in chestnut horses, but because they do not have any black it does not express itself. So you can have a chestnut mare who carries two agouti genes (homozygous) and breed her to a black stallion and wind up with only bay foals.

How do you know if your chestnut horse carries the agouti gene? This can be hard to tell without doing genetic testing. Really the only other way would be to breed her to a black stallion and see if she throws a bay foal. Even one bay foal from a black stallion means that she carries an agouti gene. Because the agouti gene is dominant all foals who carry the gene with the black gene will be bays.

Resource: http://www.isnare.com/?aid=299040&ca=Pets