This post has been updated, with more photographs.
A few horses are all white, with dark eyes and pink skin. These are not to be confused with aged greys, which may have a pure white coat but retain dark or at least mottled skin. This type of white can occur from the spotting genes we have discussed as producing pintos, especially if more than one type of spotting gene is present. There is also a type of dominant white which is lethal if two copies of the allele are present but which if one white and one wild-type allele are present produces a healthy white horse.
Remember also that many of the dilution genes we have discussed can produce a very pale cream color often mistaken for white, though most of these horses have light eyes.
All white marking genes on horses, from a conservative white star to a white horse with colored ears, seem to work by preventing the pigment-producing cells from getting to parts of the horse’s body. They do not affect or replace other genes for color. Thus no matter how extensive the white markings on a horse, it will still carry alleles at all of the color loci we have discussed. Further, it will pass those alleles on to its foals.
The white spotting genes grouped as “pinto,” “paint” or “parti-color” may occur in any combination consistent with the survival of the foal. (Two copies of the frame allele at the frame locus, for instance, results in white foal syndrome and early death regardless of what else is present.) A horse could easily have one frame allele, together with two each for sabino-1, tobiano and splash. Because the white areas from these alleles tend to affect different parts of the horse, the result could be a white horse. When bred to plain mates, the offspring would probably be spotted.
Horses with spotting due to a single locus can also be white or nearly white, if they are close to the extreme version of that pattern. Several of the spotting patterns converge to a “medicine hat” or “war bonnet” pattern with maximal white. This is probably most common in so-called tovero horses—those that combine tobiano spotting alleles with any of the non-tobiano alleles at other spotting loci.
There is one type of pink-skinned white with dark eyes that does not appear to produce spotting in the offspring. White to white breedings of this type, however, always produce some colored foals. Examination of the numbers of white and colored foals suggest that two white alleles at this locus are a prenatal lethal—the foal never develops or is aborted so early that the breeder assumes the mare has missed. This type of white is believed to be due to a dominant gene.
The white allele seems to have a surprisingly high mutation rate. Thus whites have been produced from colored parents in several breeds, and then reproduced as if they were dominant whites. I do not know whether DNA proof of parentage was available in these cases, however.
I do not believe a gene test has been developed for this type of white. Gene tests at other loci could be very useful, however, in determining what other color alleles the horse carries and could pass on to its foals.
White is a spectacular color and for that reason was popular in the age of horse power for flashy coach or cavalry horses. At least two western heroes–the Lone Ranger and Hopalong Cassidy–rode dominant whites, Silver and Topper. The downside? Keeping a white horse clean may be a problem, and the pink skin may be subject to sunburn.