Viability matters, too.

PureFrame
This horse has one copy of the frame (LWOS) mutation. Horses with two copies are not viable.

In the previous post I talked about how the physical location of a mutation can limit the possible pattern combination. There is another potential limitation, which is viability of the organism.

Those of us that like horse colors, particularly the white patterns, are accustomed to thinking of colors as something that is added to what would otherwise be a horse of ordinary coloring. So the horse above has white markings on his body in addition to his chestnut coloring. That is certainly how a lot of artists would approach painting such a horse.

But from a genetic standpoint, that’s not what has happened. Generally speaking, white patterns result when one of the genes involved in pigmentation is impaired. Something prevents the normal function of the gene, and as a result pigment is not distributed in the normal fashion.  That is what we see most clearly, because changes to coloration are really obvious. But those same genes do not just regulate color, and those other functions may be effected as well. Hampering coloration is largely cosmetic, but altering the function of the gene can have more serious implications.

That’s why horses with two copies of the frame mutation are not viable. With just one impaired gene, the horse is not completely pigmented (ie., it has white patches) but is still functional. The horse still has one non-mutated copy of EDNRB, the gene involved with the frame pattern. It can “pick up the slack” for the necessary functions that gene performs. When the horse inherits two copies of the mutation, there is no backup and the gene cannot perform its function in the development of the embryo. In this particular case, no pigmentation occurs, which is why the resulting foals are white, but more importantly the colon is incomplete which means the foal cannot survive.

Lethal White Syndrome is probably one of the best known problems with color because it involves the heartbreak of a live birth of a foal that must be humanely euthanized. Other colors, most notably the various forms of Dominant White, are also thought to be lethal when homozygous. Like the frame mutation, two copies impair the function of the gene to the point that the embryo is no longer viable. The difference between Dominant White and Frame Overo is that the embryo is lost early enough that no foal is born. This may explain why programs centered around breeding white-born horses in the seventeenth and eighteenth century were often plagued by infertility issues.

800px-BayRoan
At one time, roan was also thought to be a homozygous lethal. (Photo from Wikimedia Commons.)

In the past, before tests were available, lethal conditions like this were determined by analyzing production numbers. If the ratio of mutated to non-mutated offspring was off, and if true-breeding individuals could not be found, the trait was suspected of being lethal when homozygous. That was why roan was assumed to be a homozygous lethal for so long. Initial studies of production records showed that the ratios of mutated offspring were like those of a homozygous lethal, rather than a simple dominant. Proven homozygous roan stallions have since been identified, so it is clear that two roan genes are not always lethal, at the very least.

So what does this have to do with the KIT mutations? In the comments section, there was speculation of the last post about whether or not mutations could crossover, resulting in a single gene with two separate mutations, rather than two separate genes with one mutation on each. Not asked, but an equally valid question, is whether or not a gene that already contained a known mutation could mutate again. If either were to happen, the next question would be: could the situation result in a viable embryo? Would the added layer of impairment change the coloring, or would it damage or even destroy the organism? Have we not yet seen a horse with three copies of some of these patterns because the statistical chances are infinitesimally small, or because the function of some gene is too compromised to result in a viable embryo? For those involved in hypothetical situations of this sort – like artists or simulation game developers – that is another aspect of color genetics that has to be considered.

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6 Responses to Viability matters, too.

  1. Mary Jo Nerly November 3, 2011 at 10:20 pm #

    I have a question that is completely unrelated. I have heard that to get grey you have to have grey. The foundation quarter horse mare was brown. I have never seen a brown Quarter Horse since and the stallion was Palomino with lots of dark hair in the mane (sooty palomino) the foal was born sorrel and when the foal hair shed the colt was a gray roan like the Thoroughbred Grays. If these two individuals were bred what would be the color possibilities?

    Mary Jo Nerly

    • The Equine Tapestry November 4, 2011 at 6:47 am #

      Yes, greys have one grey parent. Non-greys can have white hairs, though, and chestnuts in particular tend to have them. What kind of markings did the parents have?

      • Mary Jo Nerly November 5, 2011 at 8:01 pm #

        Both the mare and stallion had minimal markings. What colors would be possible if a brown is crossed on a sooty palomino.

  2. Pauline H November 4, 2011 at 6:50 pm #

    I have yet another a bit unrelated question. Or well, it concerns Roan, so it might be a bit related…
    I’ve heard that you can’t test a horse for roan zygosity if it carries sabino, since the sabino mutation might “interfere” with the marker test. In this case it’s not a SB1 horse that was referred to, but one showing quite normal markings with some sabino characteristics. I’ve emailed UC Davis but haven’t got a reply, so that’s why I’m tryng to search for answers elsewhere from people that might know. It sounds a bit unlogical to me, since surely some of the horses that tested positive for roan must have had some phenotype sabino markings?

    • The Equine Tapestry November 4, 2011 at 10:18 pm #

      I had not heard that, but then I don’t know if they have tested a horse to carry both Roan and Sabino1. I know there are historical cases of Walking Horses that produced both Sabino1 and true roan, but I am not sure how well I would trust the records. (There are reports of rather hinky stuff with papers from the era in question.)

      I will say that roan as a rule seems rather resistant to markings, which others have noted in the post. You can see roans with flashy marking in some breeds like the Welsh Pony, but those tend to be of the more even-edged than the sort of soft roan outline common on Sabino1 horses. So in those cases, you probably have Roan + some other form of sabino. (Most things that are phenotypically sabino don’t actually test to carry Sabino1 anyway.)

      Have you tried calling UC Davis? I have had good luck with asking questions directly by phone there. They should be able to tell you about any conflicts, and may even be able to see if they have had any horses test as having both genes.

      • Pauline H November 5, 2011 at 8:25 am #

        Ah, ok. Thank you so much for the information. I’m not that good at the details about the exact genetics behind the different colors and patterns, so I wasn’t sure about how they would possibly affect each other.
        The problem is that I live in Sweden, so it’s both cheaper and easier to email them since there’s some difference in the time of the day as well;) But I might end up doing that in the end anyway I beleve, since I really want to get an answer to that question.