There is a magic number of breedings, don;t recall what it is, that will give you the probability of th bull being homozygous. Works something like, if he's bred to 10 cows and all are polled the probability of him being homozygous is 75%, each 10 matings or so will polled calves out of horned cows increases the probability 5%. Those aren't the exact numbers but are close I think. If he throws one horned calf, no matter how matings to horned cows are made, he's heterozygous polled. If he's heterozygous, bred to horned cows the the chances of a calf being horned are 50:50, basicly the same as the chances of a heifer or bull being born from any mating. The difference is that polled/horned is controlled by a gene pair, gender is controlled by other factors but is still roughly 50:50.