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HORSE GENETICS MADE SIMPLE (I hope!)
by Pat Elder
The guide for everyone who wants to understand genetics without studying it.
How did I get a black when I bred a bay to a palomono? What's the deal with all these flaxen-maned sorrel Miniature Horses? And where did that overo pinto foal come from when I have no overos in my breeding herd? The answers to all of these questions --- and thousands more --- lie in the complicated world of equine color genetics.
Genetics is a science that requires time and work to understand. It is my intention to try to simplify it enough that everyone can make sense of it --- even those of you who don't care to pick up a genetics text and study it. Let me know if I am successful, will you?
Horses inherit their color (and all their other attributes, as well, but we won't get into that here) by genes that are passed on from their parents. And they have dozens of gene pairs that affect color alone. For the sake of simplicity, we will deal in this article with only 2 genes at a time. We will start with the "T" gene that produces the tobiano pinto pattern. (What is the tobiano pinto pattern? See our article on tobiano and overo.) If "T" means tobiano, then "t" means non-tobiano; in other words, any color or pattern besides tobiano. Most tobiano pintos have one "T" gene and one "t" gene, written this way: "Tt". Non-tobiano horses have no tobiano genes; they are, therefore, "tt". If a "Tt" tobiano is bred to a "tt" non-tobiano, they can be expected to produce 50% "Tt" (tobianos) and 50% "tt" (non-tobianos). How did I figure that? Use the following method and try it yourself, keeping in mind that each parent passes only one of its two genes to each of its offspring.
So, out of 4 offspring, the likelihood is that 2 will be tobiano, 2 non-tobiano.
Okay, so what if two "Tt" tobianos are bred together?
There is a 50% chance (2 out of 4) of producing a Tt tobiano, a 25% chance (1 out of 4) of producing a tt non-tobiano, and a 25% chance of producing a TT. What is a TT? It is a tobiano that carries only tobiano genes. This horse cannot produce a non-tobiano no matter what it is bred to. It is "homozygous" (ho-mo-zi-gus) for tobiano --- a 100% tobiano color producer. Look at the following example of a homozygous tobiano crossed on a non-tobiano.
The result? All offspring are Tt, "heterozygous" (het-ero-zi-gus) tobianos who carry one tobiano gene and one non-tobiano gene.
Is this beginning to make sense? I hope so. Now you should know that the gene we've been studying (T) is a "dominant" gene. All that means is that if a horse has one T (dominant) and one t (recessive), the dominant gene will override the recessive and the horse will be tobiano. If a horse has even one tobiano gene, it will have the tobiano pinto coat pattern.
Hence, two non-tobianos (tt) bred together can never produce a tobiano, because neither of them can contribute the necessary T gene. It makes no difference if they both had tobiano parents. If they didn't receive the tobiano gene (and therefore the tobiano coat pattern) then they don't have the gene to pass on to their offspring.
Not all genes, of course, are dominant. Some of the ones that are include: G (the gene that produces true gray, the kind that fades with age, not silver dapple) and Rn (the gene that produces roan).
Roan bred to non-roan should produce 50% roans. The gray gene works the same way. In order to be roan or gray, a horse must have inherited at least one roan or gray gene from a roan or gray parent.
The gene that produces sorrel or chestnut horses is the most familiar of the recessive genes. Note: this is true sorrel or chestnut, not the flaxen- or silver-maned, smoky-legged Miniatures who carry a silver dapple gene. To designate the sorrel or chestnut gene, geneticists use "ee." "EE" is the term for pure for black horses --- horses who carry no red (sorrel) gene, while "Ee" is for black, brown, or bay horses who carry only one black gene.
Notice how a true sorrel or chestnut horse bred to another true sorrel or chestnut horse can only produce true sorrels or chestnuts.
From here, color genetics get more and more complicated. Most genes, such as the gene for black mentioned above, are partially dominant. A black horse bred to a sorrel horse can produce blacks, browns, bays, and sorrels, and possibly a surprise or two, depending on what "hidden" genes the black parent supplies. For example, a black that has a palomino or cremello parent may harbor, and pass on, a palomino gene. But some genes cannot be hidden, such as the tobiano gene. Remember: if a horse carries even one tobiano gene, it will show a tobiano pinto coat pattern.
Other genes that cannot be hidden are gray, roan, and silver dapple. If a horse carries one of these, he will be gray, or roan, or silver dapple (or one of the "silvers," such as silver bay, which is what most of the flaxen-maned "sorrel" Miniature Horses are).
If you aren't tearing your hair out yet, good for you! If, in fact, you're intrigued and want to learn more, there are several great books on the subject, as well as some great articles available over the Internet. Do some research on your own. It only gets more interesting from here, folks!
