Tag Archives | blue eyes

Some new research into blue eyes and KIT mutations

BlueNAmber1
A tobiano pony with partially blue and partially gold eyes (cause unknown)

A year ago I posted expressing my doubts about the theory that blue-eyed horses with KIT mutations (tobiano, sabino and the white spotting patterns) must carry an additional mutation to account for their eye color. At the time, I did not think that there was enough evidence to merit the absolute terms that were often used regarding this theory. It is also true that, having tracked so many instances of blue eyes in the patterns in question, I thought the weight of probability favored the theory that the blue eyes were part of at least some of these patterns and not always a separate mutation. At the time I wrote:

The common theory in horses is that these W-series horses must have a splash mutation as well. And they may. Certainly there are far more mutations for white patterning than previous expected. I have long thought that the numbers of blue eyes on the dominant whites, particularly among the founder horses (ie., the horse that carried the initial mutation) were just too high for them all to happen to have a splash mutation as well. I did not have an exact number, though – just a sense that it was high. But the new sort gave me a number – six of twenty.

The argument that KIT mutations are “incapable” of producing blue eyes is based on information about how pigment is formed in the eyes of mice. I found the theory difficult to evaluate because the passages referenced by its supporters do not deal directly with eye color in horses, and I simply do not have the deep level of understanding of eye structure necessary to extrapolate beyond the specific subject, which was not the absence of blue eyes but the presence of dark eyes in a particular mouse color.

But perhaps more importantly, I just wasn’t sure that mice and horses were the same in this regard. In fact, I just wasn’t sure that eye color in various mammals might not be different in significant ways, just as other aspects of coloration vary between species. In that same post, I included a picture of what was then a newly-identified KIT mutation in dogs – the panda pattern in German Shepherds. I found the blue eyes on the founding dog particularly compelling because in dogs blue eyes are not generally associated with the common forms of white patterning, despite the fact that the mutations for most of those patterns have been found on MITF, which those of us more familiar with horses think of as the “splashed white” gene.

It seemed to me that these blue-eyed (KIT) German Shepherds, and the more common dark-eyed “irish marked” (MITF) dogs were a pretty good argument that when it came to eye color, there was probably some variation from the mouse model.

BlueNAmber4
The same pony, with one predominantly blue and one
predominantly gold eye (and an occluding spot over his blaze)

Some time after that post was made, I ran across a paper on Blue-Eyed White (BEW) alpacas. In that paper, the blue-eyed white phenotype was linked to the presence of two mutations to KIT. The alpacas, which were uniformly blue-eyed, were compound heterozygotes for two different white patterns (bew1 and bew2), both located at KIT. Since that time I have been able to confirm with one of the researchers that there were no mutations at the sites associated with splashed white in horses that could be correlated with the blue eyes on these alpacas.

Then just this month a paper was published linking the blue-eyed white phenotype in cats – called Dominant White (W) – to a mutation on KIT. In that study, the authors were quite clear about the connection between KIT and blue eyes.

In the population sample, we were also able to examine the correlation between genotype at the W locus and iris color. An individual that is homozygous W is much more likely to have blue iris, exhibiting odds 77.25 times larger than the odds of having blue irises of a genotype other than W/W (p < 0.0001).  An individual that is heterozygous (W/w+) also demonstrates increased odds of having blue iris (OR=4.667), four times larger than the odds of having blue irises of a genotype other than W/w+ (p=0.046) The odds of having blue irises of a wild type individual is 0.

With those two studies seeming to cast considerable doubt on the “Never From KIT” theory, I decided to contact one of the corresponding authors with some questions in hopes of getting a better understanding of this topic and of eye color in general. What I was told was that eye color is most likely a polygenic trait, and that it really does depend on the species, as well as the specifics of each particular mutation to KIT. That could explain why some KIT mutations are more prone blue eyes than others, as well as why there appears to be a higher incidence of blue eyes in homozygotes of some patterns.

BlueNAmber2
On this eye, the blue and gold portions are interspersed in such a way that the colors appear softly blended

BlueNAmber3
On this eye, there are fewer flecks of blue, as well as irregular patches of dark brown

So why does it matter if KIT mutations can produce blue eyes alone, or if they need a splashed white mutation? What purpose does this kind of information serve? The fact is that knowing the cause can help breeders more reliably get the outcome they desire, whether they wish to breed for or select against blue eyes. Likewise, breeders seeking to produce – or avoid – the splashed white phenotype need to know if blue eyes are always significant. If it is possible for some of the other patterns to produce blue eyes independent of a splashed white patterns, then assembling a herd of blue-eyed tobianos in hopes of developing a line of splashed whites is going to ultimately prove frustrating.

It is also true that quite a few blue-eyed horses have come back negative for the known splashed white patterns. It is likely that some of them have as-yet-unidentified splashed patterns. However, if some have blue eyes that are just a less common aspect of a pattern that is already identified, then knowing this could spare their owners time and money spent looking for something that is not really there. So while the subject is quite technical, it really does have a very practical aspect.

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More albino dogs

WhiteDobe
Casper, a “white” Doberman owned and photographed by Kira Vance.

In the previous post, I mentioned that one form of albinism in dogs had been formally identified in just the last few months. In October of last year, a group from Michigan State University discovered the mutation responsible for white (also called albino) Dobermans. Researchers looked at the four genes known to be involved in Oculocutaneous Albinism in human beings – OCA2, TYR, TRYP1 and SLC45A2. (“Oculocutaneous” means that both the skin and the eyes are involved.) The mutation was found on SLC45A2. Those familiar with the molecular end of horse color genetics might recognize that gene, which used to be called MATP. That is the gene where Cream was found. To someone familiar with horse color, these dogs would be the equivalent to cremellos.

There are differences, of course. In horses, Cream is quite infamously an incomplete dominant. That fact is so well known that the color is often the most effective way to explain the concept of incomplete dominance to a horseman. White in Dobermans is recessive, and so is invisible in the carriers. It might be more accurate to say this is like Pearl in horses, which is another SLC45A2 mutation. Pearl is recessive, but horses of that color typically have dark eyes.

In appearance, however, homozygous Cream and “Doberman White” are a lot alike. Both have faint residual color that is more visible where the animal would have been black. If you look at Casper, the white Doberman pictured above, you can see the faint outline of the black-and-tan pattern that a fully-pigmented individual would have. Double-diluted Cream horses are likewise not truly white, as can be seen in contrast between the white mane and tail and the pale cream of the body on the stallion below. Although not clearly visible in these photos, the dogs have pale blue eyes. Homozygous cream horse are, of course, known as blue-eyed creams (BEC) in many countries. 

cremello_3_by_sooty_bunnieSm
Cremello Saddlebred stallion, Denmarks Platinum Playboy, photographed by Estelle Low.

So is the horse above an albino? Are the horses below a carrier (palomino, to the left) and an affected (cremello, to the right)?

CremandPalo

The position of many within the horse community has been that cremellos, perlinos and smoky creams are not albinos because they do not have pink or red eyes, and because they do have some pigment. It is the position of the Doberman Club of America that because the mutation was assumed to have occurred in the same gene as Oculocutaneous Albinism in human beings, the dogs were by definition albinos. In fact, while referring to a cremello horse as in albino will probably engender a certain amount of derision in the horse community, arguing that these Dobermans are anything but albinos would get the same reaction among their breeders. So which is it? As I said in the last post, I suppose it all depends on whether you define albinism strictly or loosely.

It should be understood, too, that there are reasons behind both positions. There have been horsemen campaigning for some time to change the belief that double-diluted creams were defective. Most do say that the horses are somewhat sensitive to the sun, much like fair-skinned people are, but otherwise they contend that the horses are healthy. They would certainly not agree with the Doberman Pinscher Club of America that these horses are carrying a “deleterious mutation which affects the whole body” that is considered “a genetic defect in all creatures”, which is how that organization describes albinism. The desire not to have these horses, or other pink-skinned white horses, called “albino” is about avoiding that kind of stigma. Meanwhile, the insistence that these dogs be called albinos, and not white, is very much about placing that stigma on the dogs. Most breeders of Dobermans would like to have these dogs removed from the stud books. That is the official position of the breed club, though the structure of stud books in the dog world means that only the American Kennel Club (AKC) has that power. The AKC has a long history of inaction when it comes to denying papers based on color. Because the breed club lacks the power to ban the dogs, what is left is convincing breeders not to perpetuate the color, and buyers not to purchase the puppies. Statements made about the color have to be understood in that light.

This also ties back to the discussion about founders and their colors. However one defines albinism, and whatever one believes about the health of the white Dobermans, this statement about the source for the color is misleading.

In November 1976, a mutation occurred with the whelping of a cream colored Doberman.

The white color in Dobermans is recessive. As I mentioned in the previous post on Splashed White founders, an individual with a recessive color that appears unexpectedly is the not the founder. The initial mutation could not have first occurred in Padula’s Queen Sheba, the cream-colored female referenced in this statement, because she carried two copies of the same mutation. To be the founder, both copies of her SLC45A2 gene would have to have mutated at exactly the same time, in exactly the same way.

A 4,081 base pair deletion was identified between chr4:77,062,968-77,067,051. The deletion start site lies within the last exon of SLC45A2 and results in a loss of the last 50 amino acids of the normal protein, as well as the stop codon, and causes the addition of 191 new amino acids before a new stop codon occurs.

That is the precise change that was identified as causing the color. That exact error would need to happen twice, to the same dog, for Sheba to be the founder. That exact error would need to happen twice, to two different dogs, for her parents to be the original founders. Sheba may well have been the first ever occurrence of the color in the breed, but it did not start with her, nor with her parents. If she was purebred, and an investigation conducted by the AKC when she came to light concluded that she was, then the color came from somewhere on both sides of her extended pedigree. 

WhitePedigree

That means the mutation could be found in other lines, or outside the Doberman breed completely if it predated the formation of the breed. Sheba’s breeder claimed there was another white puppy born to this same pair prior to Sheba, but it did not survive. That would be completely expected. In fact, if this breeding were repeated it would be surprising not to have any other white puppies. Sheba was homozygous for a recessive mutation, or else she would not have been white herself. If she was homozygous, both parents were carriers. Bred together, two carriers would be expected to produce an average of 25% white puppies. After Sheba came to light, it was found that seven other unrelated dogs were registered as “white”. (Sheba’s registration attracted attention because her owner attempted to register her as “albino”, which was not an option.) Dr. Jim Edwards, then an official with the AKC, wrote to the following statement:

This investigation has uncovered an additional seven (7) Dobermans registered as “white” and not included in Shebah’s pedigree. These occur sporadically, with ages varying from two years to fifteen years. Only two of these have produced litters, and only three litters have been registered. We are continuing our investigation into these cases and will provide appropriate updates to the DPCA when our analysis is complete. I am encouraged that this number is small, remembering that color assignment is sometimes difficult for the novice breeder.

For someone interested in founders, those dogs would be of particular interest. The breed club believes that each case was one of a misidentified fawn; that is, a dog that has both the blue and the brown dilution. Fawns are an accepted, if not an especially popular, color in the Doberman breed. I am not sure of the basis for this determination, so I cannot comment on the ultimate color of those dogs. I can say that entries like that are where I look when I am searching for clues about color founders, and about the spread of color mutations in general. I would also add that when trying to track something down, confusion about color assignment works both ways; if registered whites may be in fact fawns, then registered fawns may in fact be white. I have often regretted that fact, because it would be a lot less work tracking down colors if this was not the case! But my interest in color research is purely academic, and motivated out of curiosity. Even if they were fawn – and it is quite possible they were – it would not change the fact that there were likely carriers outside of the one known line, at least at some point. Whether they still exist is an open question. Because this issue, in this breed, has been emotionally charged for some time, chances are that strict culling has been practiced in any cases of white puppies. This far down that path, it is unlikely that if the mutation was in other lines, it is anything but rare now. But the truth is that strong stigmas, while they do promote culling, also promote secrecy. When the stakes become so high, and the reaction to an unexpected white puppy less rational, even the most ethical breeder is forced to weigh the costs of total honesty. When doing my own research, I have learned to include this kind of situation in how I weigh the reliability of information.

With the discovery of the actual mutation, it should be possible to develop a test to identify the white mutation in Dobermans. Hopefully the breed club will support its development and encourage its use by all breeders. If the color is not wanted, then testing and avoiding crosses between carriers is the real answer to the problem. Widespread testing could also provide answers about where the color began, along with the facts about where it can still be found.

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Another minimal classic splash

HairicaneLilBitRowdy1
Hairicane Lil Bit Rowdy, Miniature Horse mare

Here is another interesting splash white that is homozygous for SW1, sent in by Robin Cole. Unlike the Gotland in the previous post, this mare does have blue eyes, but the white on her face is quite minimal. Like the Gotland, she has four white feet, but all but one leg are very minimally marked. Her pattern looks even more minimal when seen from her other side, probably because the skew in her blaze keeps most of the white away from this side of her face. 

HairicaneLilBitRowdy2

Although she comes from a line that carries frame (Oh Cisco), she tested negative for that pattern. And while she might look tobiano, especially to readers familiar with how tobiano often skews in Miniatures and Shetlands, neither parent is a tobiano. They do look like heterozygous splashes, and the family is known to produce splash patterns. In fact, this is her full sister.

LilBitRowdyFulSis

So while homozygous SW1 horses are pretty consistent in appearance, minimized patterns can be found. That seems especially true among the breeds – like the Gotlands, Icelandics, Shetlands and Miniatures – that are already known to minimize the tobiano pattern. Here are links to a few interesting examples of minimal homozygous SW1 from those breeds:

Bládís vom Moarschusterhof, an Icelandic mare with what appears to be one dark front leg
Hnísa vom Römerberg, an Icelandic mare, also with a seeming dark foreleg
Unun frá Efri-Úlfsstöðum, an Icelandic mare with an irregular blaze and minimal white
Glæta frá Ártúnum, an Icelandic mare with a large star, large snip and what may be a dark eye
(with each of these links, you can click on the first image to pull up additional images)

Opp, a Gotland mare with a face marking much like the one on the Miniature in this post
She does have a more extensive body pattern, though.

And finally, while it is not a minimized pattern, this Gotland mare Nanna does have an unusual skew to her pattern. Along with minimizing the white, skewing patterns seems to be another (relatively) common aspect in this group of breeds.

I had intended to do a post on blue eyes and negative splash tests, but I think it makes more sense to shift that post to after the ones about white markings since those two things are closely related. I am also going to take a little detour into blue eyes of a different kind, though, before tackling the markings. Watch for that in the next day or two. And if you have not done so already, you can get notifications of new posts to the blog via email by filling out the subscription form in the right hand sidebar. I apologize to long-time readers that had a subscription before and now have to re-subscribe, but that was the one thing that did not transfer with the blog when it was moved to a self-hosted site.

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