Winter White Genetics
"Also, the imperial gene is homozygous lethal. No defective babies are actually born, though. Any babies which would have received two imperial genes (one from each parent or about 25% of the litter) will die in the womb and not be born. This is not generally considered a problem since it is believed that this may actually aid the mother both by decreasing the litter size and by giving her added protein and nutrition during pregnancy. Update: The imperial winter white gene mutation is now believed extinct."