I could be completely off base, but for some reason I think this might be slightly correct:
I believe that there is a lethal dilute gene... that is, some dilute colored horses (seen in smokey-coloreds, cremellos and palominos) carry the lethal gene - they will carry one normal gene and one lethal gene, so they themselves won't be affected but will carry the lethal gene. Horses with the lethal gene die well before reproductive age, so cannot reproduce.
Say X (capital X) =normal gene, and x (lowercase x)= lethal gene... dilute horses are either homozygous normal (XX) or heterozygous normal/lethal (Xx).
If two homozygous normal (XX) horses are bred together, there is a 100% chance the foal will be heterozygous normal as well, so it will not carry the lethal gene.
If a homozygous normal (XX) is bred with a heterozygous normal/lethal (Xx), there is a 50% chance the horse will be homozygous normal (XX), and a 50% chance the foal will be a carrier, that is a heterozugous normal/lethal foal(Xx). The foal has a 0% chance of having homozygous lethal gene (xx)
If two heterozygous horses are bred together (Xx), the foal has a 25% chance of being homozygous normal (XX), 50% chance of being a carrier (heterozygous normal/lethal, Xx), and 25% chance of being a homozygous lethal (xx) and will die.
Whew! Alright well that confirms it, I miss doing genetics in school!!
I could be completely wrong with this, but I think it's at least a little right?
It's part of the whole Lethal White Syndrome (LWS).. more here: http://en.wikipedia.org/wiki/Lethal_white_syndrome
Umm.. nevemind? Here's an excerpt, stating that cremellos, palominos and smoky-coloreds are not affected by LWS.. at least, that's how I read it?
"Healthy horses which appear to be entirely or mostly white are usually either cremellos, perlinos, smoky creams, ivory champagne, maximally-expressed sabinos, or the rarer white horse. None of these color genes are linked to the LWS gene."
GAH I don't know anymore. My Punnett Square stuff still stands, maybe it's right. I'm very curoious to hear the answer now!!