Sorry, I do not have a single photo of a pearl–just haven’t been able to find one. Sponenberg doesn’t, either. You’ll have to go with the descriptions.
Last week we discussed palomino as if there were just two forms of the gene associated with palomino color: cream and non-cream. The whole story is a little more complex, but I’ll have to introduce some genetic terminology to explain it, even though I’ve used the terminology, without explaining it properly, several times already.
The three new words we’ll be using are locus, allele and wild–type.
Locus means place in Latin, and it originally meant a place on a chromosome. Since genes code for proteins and are now known to be a little more complex than just the place on the chromosome, it now means the particular protein coded for.
There can be slightly different forms of a protein having the same function, and the different stretches of DNA (genes) that code for these slightly different forms are called alleles. Using this terminology, every horse has two alleles, one from each parent, at each locus (plural loci.) Last week we discussed two alleles, cream and non-cream, at the cream (C) locus.
The complications come from the fact that there are in fact three rather than just two alleles at the cream locus. Each individual horse can have any two of these three alleles. To avoid confusion, I am also going to introduce the term wild-type for the gene assumed to be the “normal” allele at a locus in the wild ancestor of a domesticated animal. What we called “non-cream” last week is in fact the wild-type gene that gives normal full color.
(Note that the wild ancestor of the horse is not the “wild” horse of the West—these are in fact feral, descended from domesticated stock. The only true wild horse alive today is Przewalski’s horse in Asia. The Tarpan in Europe was also wild, but became extinct in the 19th century.)
Using our new terminology, the cream locus has three alleles: wild-type, cream, and pearl. Pearl was recognized quite recently, and it has a very low frequency except in a few Spanish and Portugese breeds and their derivatives. It could be considered a weaker allele than cream, as it has less diluting effect on the coat.
A horse with one wild-type allele and one pearl allele will look very much like a wild-type horse—chestnut, bay or black depending on what genes are present at other loci. A very close look will show skin slightly lighter than normal, or with small pale spots.
A horse with two pearl alleles will have the red pigment diluted only slightly more than would be expected for a horse with one cream allele and one wild-type allele. Black pigment, however, will be diluted much more than is the usual case for a horse with one cream and one wild-type allele. Thus a bay with two pearl alleles at the cream locus dilutes to tan or gold on the body with chocolate mane, tail and lower legs. A chestnut becomes virtually identical to a pumpkin-skinned palomino (technically gold champagne.) And a black becomes a grayish tan with chocolate mane, tail and lower legs. All of these colors appear very similar to those produced by a single dose of the champagne gene, which is a completely different gene at a different locus, but give very different breeding results. Luckily there is a DNA test for pearl.
If a horse has one pearl allele and one cream allele, the resulting color will be cream, usually slightly darker than the cream resulting from two cream alleles (cremillo, perlino or smoky cream.) In particular the eyes are generally blue or amber, and darker than those of cream horses with two cream alleles.
As I mentioned before, there are a number of different ways of diluting horse color, and when two or more at different loci are combined some very odd colors can result and it may not even be possible to tell what genes are present—or what colors can be produced—without DNA testing.
Next week I’ll consider linebacked dun—one of the few horse genes where the wild-type allele is rare today in many breeds.
(If anyone has photos I could use to illustrate some of these horse coat colors, I would really appreciate them.)