DL, I agree. And when we think about how many matings there have been since the Angus cow was bred to the Simmi bull then the heifer calf was bred to a Maine bull, etc etc, I think that while there might be slight the chance, the probability of it is low enough it really isn't a concern. Especially when like you said, most of those carriers of AM, NH and CA weren't what we were looking for in the clubby world.
Sue, I see it as the same way in most senses. While yes, there was black influence in a LOT of our Shorthorns, a lot of it came before the NH mutation came about. I am not sure I can say the same for the other too but breeding one up since then from a Shorthorn x Angus cross animal, the probability that they are carriers of any of these is so low it is probably not a risk.
Just my two sense, thought it kind of applied to the topic.
Idaho, I've been thinking a lot about your question. I'm the kind of person that if you don't know the full genetic background of your females, don't take the chance, test them or use clean bulls. To me it's not worth the slim chance you're best cow will be a PHA carrier and have an affected calf, losing her and the calf....