This is the second post in our three part series looking at what it takes to get the elusive blue roan Brabant horse that everyone is seeking. In part one, we looked at the genes that control color in the horse and which traits are dominant or recessive. Today, we'll consider the color possibilities for different crosses.
When a stallion and a mare are bred the offspring receives genetic material from both parents. Each parent has two copies of each gene- called alleles- one inherited from each of their parents. One of the two alleles will be passed along to the offspring for every trait, and the combination of a single allele from the sire and a single allele from the dam will produce the foal's genetic code, which controls their color. A horse has a 50% chance of passing on a given allele for each trait to the foal that they produce.
In order to consider the potential color outcomes for a foal we will explore a few different color pairings using horses here at All the King's Horses. To keep things simple and allow for a consistent comparison of probabilities we will consider all crosses out of our imported blue roan stallion, Ash. Ash has been color tested and is genetically EE (black based, will never throw the red allele), aa (no chance of throwing the Agouti/bay allele), Rnrn (can produce roan or solid colored foals). The desired blue roan foal will be black based, not carry Agouti, and have at least one copy of the Roan gene. What happens when we cross Ash to different colored mares?
Blue roan x Bay roan
The majority of our mares are bay roan so this is a logical place to start our analysis. We know that Ash is always going to throw the black gene, which is dominant, so that takes care of getting a black based foal. The bay coloration is impossible to predict without knowing the mare's genetics, specifically if she carries one or two copies of the dominant A allele that leads to bay coloration. The chances of getting a solid black (no bay) foal will be either 50% (if she carries only one copy of the A allele) or 0% (if she has two copies of the A allele and is therefore guaranteed to pass it to her foals). Similarly, with Roan the exact odds depend on which alleles the mare carries. If she carries two copies of the dominant Rn allele then the foal is guaranteed to be roan (bay or black based) while if she carries only one copy of the Rn gene the odds of crossing two single Rn carriers results in a 75% chance of a roan foal. Taking the odds of solid black AND roan together, we find that the chance of getting a blue roan when crossing Ash to a bay roan mare could be 0% (0*1.00 or 0*.75), 37.5% (.5*.75) or 50% (.5*1.00).
Blue roan x Solid bay
This calculation is similar to the bay roan in regards to producing a solid black foal- depending on the mare's genes the odds are either 50% or 0%. However, in this case the mare will never throw the dominant Rn allele so the roan coloring can only come from Ash, an allele he will throw 50% of the time. This means that when we cross Ash to a solid bay mare the chance of getting a blue roan foal is either 0% (0*.5) or 25% (.5*.5).
Blue roan x Blue roan
This combination has the highest chances of producing a blue roan foal. Because Ash carries two copies of the black allele we know that his offspring will always be black based. When both parents are blue roan we can infer that neither carries the dominant Agouti gene, so there is no chance of the offspring being bay. In this case, the only real variable is the Roan gene. We know that Ash will throw roan 50% of the time. The dam may carry one or two copies of the roan gene which means that the foal has either a 75% or a 100% chance of being a blue roan. Pretty good odds here!
Blue roan x Sorrel
A sorrel mare carries two copies of the recessive e allele and will always throw the red gene, but this color will be masked by Ash's dominant E allele and produce a black based foal. Where things get tricky is with the Agouti gene. Because the mare does not carry any coding for black color she has the possibility of carrying the dominant A allele without expressing it! This means that, depending on her genetics, the odds of producing a black (non-bay) foal is 0% (if she is AA), 50% (if she is Aa), or 100% (if she is aa). As a solid colored mare she will never throw the dominant Rn gene but Ash will provide roan coloring 50% of the time. In the end, the chance of getting a blue roan foal from this pairing can be 0% (0*.5), 25% (.5*.5), or 50% (1.00*.5).
These are examples from our own herd but obviously a stallion with different genetics would result in different probabilities. If, for example, Ash carried one copy of the recessive e allele in addition to the dominant E allele he would still present as a blue roan but rather than producing all black based foals he would pass on the red coloration 50% of the time, further reducing the odds of getting a blue roan offspring unless bred to a mare with two copies of the E allele. A bay or bay roan stallion complicates matters further as they carry the A allele which is dominant and produces a bay foal rather than a solid black/blue roan. Similarly, if someone is dead set on getting a filly all of the probabilities would have to be cut in half to account for the fact that half of the blue roan offspring will be colts instead.
I know this summary has a lot of 'math-ing' but it's important to realize that the color of the foal is dependent on many separate alleles and if just one is off the color can change completely. Crossing blue roans to blue roans can improve the odds of getting the desired offspring, but requires finding the difficult-to-locate colors to begin with! Kudos to you if you made it this far... next time we will take a look at the practical side of things and how to deal with getting the right foal for your needs.
Our sorrel mare, Chestnut, is an example of the hidden bay gene- she produces bay foals despite not looking like a bay herself due to her having no coding for black coloration
