The first conversation is almost predictable at this point... "Hi! I'm interested in a Brabant horse and wondered if you could help me?" "Sure, they are great horses! What exactly are you looking for?" "Well I want a blue roan filly, 100% Brabant." "You and everyone else ;)" It's crazy how many times I've had this conversation. And I totally understand the beauty of the majestic, blue roan Brabant horse shown in many photos. But the reality of genetics means that producing that blue roan can be a challenge. In this short 3 part series we'll explore what it takes to make that dream Brabant and the importance of keeping an open mind when tracking down your dream horse. We will begin by looking at the science of genetic inheritance, then take a look at what that means for the probability of producing a blue roan foal (doing a little math along the way) and finally we'll consider what this means for someone shopping for their dream horse. So hang with us through the start and I promise we'll get to the good stuff (or just wait for part 3).
To begin, we need to understand how color inheritance works in horses. The following summary is adapted from an article that I wrote for the American Brabant Association's Trace Chains newsletter.
A horse’s color is controlled by specific genes. A horse has two copies of each gene (called alleles), one inherited from the sire and one from the dam. An allele is either dominant- meaning that if the horse carries it they will express the correlating trait- or recessive- meaning that it will be overruled by the dominant trait if the horse carries a dominant allele. A horse will only show the recessive trait if they inherit the recessive allele from both parents. There are 3 genes responsible for the primary colors we see in Brabant horses, which are black, red/sorrel or bay all of which can be solid or roan. (The Grey gene/color is also possible in Brabants but is a relatively rare trait and does not affect the probability that the horse will be a blue roan so we're going to set that one aside for the purposes of this series).
Extension Gene (base color)
The base color for a horse is always either red or black and is controlled by the Extension gene. Black is the dominant color, denoted E, while red is recessive and is denoted e. If a horse shows any black (whether it is full body or just their points) they must carry the E allele. We cannot infer whether a black based horse carries e, as the red allele is recessive and is masked by the dominant black allele. A horse with no black must carry two copies of the e allele.
Agouti Gene (bay)
The bay coloration can be confusing as the horse’s body is red but they have black on their legs and points (ears, muzzle, mane/tail). This modification is due to the Agouti gene. Technically a bay horse is black based, but the dominant allele, A, restricts a horse’s black coloration to their points (resulting in a bay). The recessive allele, a, leaves the black coloration over the horse’s whole body (resulting in a non-bay horse). Similar to the Extension gene, a bay horse must carry at least one copy of the dominant A allele and may or may not carry the recessive a allele. A black bodied horse has two copies of the a allele. It is worth noting that a red based horse (ee) may be a carrier of the A allele, but it will show up as a sorrel rather than a bay because the Extension genes do not code for any black coloration.
Roan Gene
Any color can also show the roan pattern. The roan gene results in white hairs sprinkled through the colored hairs. The roan pattern is dominant, Rn, while solid coloration is recessive, rn. A roan horse may have one or two copies of the dominant Rn allele and a solid colored horse carries two copies of the recessive rn allele. From this we see that a roan foal must have at least one roan parent, as the dominant allele responsible for the pattern is always expressed.
Due to the effects of dominant/recessive alleles, we often can’t tell what alleles a horse carries simply by looking at them. However, there are genetic tests available that will tell what colors and patterns a horse carries by testing a hair sample. In the next post in this series we will explore how the genetics of the parents translate into different color possibilities for their offspring.
A colorful team! Left to right we have solid bay, blue roan, and solid red (sorrel)
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