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7. WHITE, ALBINO, AND LETHAL WHITE GENES
There are lots of horses that *appear* white but few are truly white (white throughout life, with pink skin).
1) Aged grey horses that have completely greyed out can look completely white. However, they were born with some other color. Even when white they still have black skin.
One of the earlier posters on horse color referred to Arabs as having a lot of white horses. Actually Arabs are NEVER white. All grey Arabians will eventually turn white with age, some faster than others. Although there are a few instances of Arabians being born "white", these horses would still be registered as grey because they have the black skin. Therefore, although there are many white-looking Arabians, these are in actuality grey horses that have turned white with age.
2) Cremellos are distinctly off-white rather than true white. A double dilution of chestnut.
3) Perlinos are also off-white, with rusty points. A double dilution of bay.
4) True white horses can occur in Paint horses (tobiano, overo, tovero, or sabino) that happen to be born with a lot of white and hardly any color. All-white overo foals almost always die. It is unclear if this is inherent to the overo pattern, or whether it is a specific lethal gene that some but not all overos carry.
5) Another type of true white is "dominant white". This is an all-white horse, white from birth, with pink skin and brown, hazel, or blue eyes. These horses are called American White Horses and have a registry. (They used to be called American Albinos, but the name has changed.) This is caused by a lethal dominant gene. Heterozygous horses survive and are white, but homozygous white foals die in utero.
No true albino gene has ever been discovered in horses. True albino means the absence of *all* color, even in the eyes. The "true whites" known in horses -- all-white paints and dominant whites -- still have dark or blue eyes instead of the albino pink eye. (I have been told that paint breeders call the all-white overo foals "true albinos" but they're not.) This is strange as albinos have been found in almost every other species.
LETHAL WHITE #1 -- All-white overos
In the Paint world, there is a genetic disorder of all-white foals called "lethal white". These foals cannot absorb water, for some reason, and die within a few days of birth. So the last thing a Paint breeder wants to see is an all-white foal.
More information on lethal overo white, from Tracy:
There are actually several "lethal white" genes which everyone may or may not be aware of. The lethal white that has been discussed extensively on the net is a situation that occurs in paints, particularly in Overo paints. It is actually not due to a specific lethal gene, but rather to the overo pattern itself [this is controversial -- see below for another view]. For instance, unlike Tobianos, you cannot select for how much white or color you get when breeding Overos. Overos range from almost solid colored to nearly white or white. It is these white foals that suffer the intestinal problems that lead to miscarriage or death shortly after birth. A nearly solid Overo bred to a nearly solid Overo can have a white or nearly white foal; conversely a largely (though not totally) white Overo bred to a largely white Overo can have a solid horse whose only indication that it is paint is high white on the legs and a lot of facial white. The key factor in how much white is present in Overo babies appears to be womb temperature. Also, the gene responsible for Overo coloring is recessive, meaning that their must be one contributed from each parent to make an Overo baby. Now things get complicated, because Tobiano coloring and Sabino coloring are dominant genes and it only takes one of those to produce their color. Many medicine hat paints are Sabinos. Sabino and Tobiano nearly or totally white babies do not die at birth unlike Overo babies. However, because breeders have crossed all gene types together, a horse that looks like a Tobiano or Sabino may carry an Overo gene and when bred to another horse that carries an Overo gene may produce an Overo baby, and can rarely produce an all or mostly white Overo that will die soon after. By the way, the other way to produce the medicine hat pattern is with a horse that is both Tobiano and Overo, so called Toveros.
More information on the lethal overo white, from Sara White:
Just thought I'd add what I could find about the lethal white foal syndrome that occurs in foals of overo parents. I am getting my information from The Horse by Evans, Second edition, 1990.
It appears that there is one locus (physical location on a chromosome where a gene is located) that primarily controls the overo color pattern (this is completely different from the tobiano locus). At this locus there are 3 possible alleles, or different forms of the gene, which are O, o, and oe (this should be o superscript e). Every foal inherits from its parents two alleles, one from each parent. These may be the same (for example OO) or they may be different (for example Oo). The O allele is dominant, and the o and oe are recessive. If the O allele is present, the foal will not show overo markings.
SO--A normal overo horse has a genotype of oo or ooe. However, if a foal has a genotype of oeoe, it has lethal white foal syndrome and will die. Any horse that has the oe gene (whether it is solid colored and has a genotype of Ooe, or is a pinto and is ooe) is a carrier for the syndrome and, if bred to another carrier, may produce a lethal white foal.
What this means is that there is no set ratio for the number of lethal white foals produced in overo crosses. If neither parent is a carrier, then none of the foals will have the syndrome. If both parents are carriers, then 1/4 of the foals will have the syndrome, and another 1/2 will be carriers. If a carrier stallion is bred to several mares, some of whom are carriers and some of whom are not, the percent of lethal white foals produced will depend on the percent of the mares that were carriers; the more carrier mares, the more lethal white foals.
Sorry if this letter ended up sounding a little too much like a genetics lecture, but I hope the information is helpful.
LETHAL WHITE #2 -- Dominant White
Info from Tracy:
The Dominant White gene is a lethal white. It is a dominant gene that produces horses that are pure white with pink skin and brown eyes. It is NOT a true albino as pigment is present in the eyes, however, horses of this color are registered in the American Albino Registry, which is now called the American White Horse Registry because of the confusion over the genes involved. This gene is not associated with paint color and babies that are born white live and carry only one dominant white gene as the homozygous form die in utero. Only a Dominant White can produce a Dominant White and the ratio is approximately 2/3 of the babies will be white.
Another color gene which is lethal in the homozygous form is Roan. Yes, most of you don't know it, but there are only heterozygous roans out there. The homozygous form does not exist, it dies in utero, probably early in pregnancy.