Post by Snowfrost on Aug 13, 2011 5:24:43 GMT -5
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First and foremost, what you need to understand is that white horses must have a base. All white horses, therefore, are heterozygous. They cannot exist otherwise, as they die in the womb. This is called LWS, or OLWS (Lethal or Overo Lethal White Syndrome). This occurs whenever two dominant white horses, or two frame overos (matched with any other or no other paint genes) breed. Thankfully, there is only a 25% chance of OLWS or LWS occurring, and there is also only a 25% chance of a foal being born the combination of the dominant white horse's base colors.
Most foals who are dominant white are born white, or a duller shade (the base color) of white that brightens completely before the age of two. Agouti alleles do not affect the pigment of a dominant white horse. These are only used for when the foal happens to be solid (ww) -- no white in the genetics. White covers everything. Also, please understand: Dominant white is not the same as gray!
Dominant white horses are born with pink skin instead of dark skin, and usually brown eyes (unless there are underlying paint genes in the base, or some other circumstances which will be covered in a later thread). White horses do not go through a 'graying' stage, and white horses kill when homozygous where gray horses do not. The base of a dominant white can be any color, and any combination of patterns and possibilities, as well.
There are also darker spots (mostly of a faded version of the base coat) that occur sporadically on a white horse's coat sometimes. This is uncommon, and is not genetically explained. It is likely a mutation in the melanin, as whites themselves are mutations (not all mutations can be 100% correct, can they?). These markings also tend to fade more and more with age, much like how gray happens. Just remember, that White horses and gray horses are never the same. Also, remember that (so that you don't become confused) cremello horses have duller skin than their mane and tail, while white horses are completely white/pink (both body and hair).