More about Muleys: The Genetics of Horns

Here's a recycled article from a couple of years ago.

Some time ago I wrote an article about horns, and I have meant for a while to revisit the subject. What interests me is muleys, or hornless bulls, like Major Payne, of whom I am very fond. But this article totally got away with me and took me into the realm of the genetics of horns in cattle. So be forewarned, it gets a little crazy here.

Those of us who concern ourselves with bull riding know that most bucking bulls have horns. Now and then you see a polled bull - that's a bull with no horns, also called a muley - come out of the chute, but it's uncommon. So when I tell you that the gene for hornlessness is dominant, you might be surprised. I was surprised too. But even though it is dominant, it didn’t end up being that simple. So let’s proceed with caution. And forgive me in advance if the only thing about this that you care about is, well, none of it.

The gene for the trait of hornlessness or polled (P), is dominant to the gene for horns (p). It can work out this way:

• If an animal has two polled genes (PP) it is homozygous for polled and it will not have horns.

• If an animal has one polled and one horned gene (Pp), it is heterozygous for polled and it will not have horns.

• If an animal has two horned genes (pp), it is homozygous for horns, and it will have them.

You can see this illustrated in the diagram up top, where the polled gene is shown as white, the horned gene is shown as black, circles are cows, and squares are bulls. Both the sire and the dam are heterozygous polled (Pp). Neither have horns, but both carry a horned gene. This is why one of their calves, the heifer second from right, was able to inherit two horned genes (pp), one from each parent. She is homozygous horned, and thus has horns. Four of the other calves are polled but, like their parents, each of them has one polled gene and one horned gene (Pp). The last of the calves, second from left, inherited two polled genes (PP), so he is the only homozygous polled calf in the bunch. The four heterozygous polled calves have a chance of having a horned calf if they are bred with other heterozygous polled cattle. The homozygous polled bull will have only polled calves, even if he mates with a horned cow.

But this is where the complication comes in. If the animal is a descendant of Zebu cattle, it may or may not, carry another gene (called the African horned gene, or Af) that dominates the gene for hornlessness. Bucking bulls are almost all crossbred cattle. They have all got a little of this and little of that and most of them have bloodlines that aren’t documented far enough back to know who’s got what. It's nearly impossible to tell whether or not they carry the Af gene, unless they are muleys, in which case they do not carry the Af gene. If they did, that gene would have trumped the polled gene and they would not be muleys. Further complicating matters is the fact that many bucking bulls have Brahman blood, and Brahmans are almost certainly descendants of African Zebu cattle.

It is thought that Af is is a sex-linked gene, behaving differently in cows than it does in bulls. In bulls, the presence of Af trumps the polled gene. So if a bull has the polled gene, but also has an Af gene, he will have horns. But the Af gene is recessive in females. A cow with polled genes may or may not carry an Af gene as well. If she carries two Af genes, they will dominate any polled genes she has, and she will have horns. A homozygous polled bull may produce horned bull calves if he is bred to a cow that does have the Af gene.

So where does that leave us? Besides confused? It leaves us pretty much where we started off: with mostly horned bucking bulls. And with the muley Major Payne, who does not have the Af gene, even though he is from a genetic line that traces back to a Brahman bull (possibly a Zebu descendant) crossed with red Angus cows. That’s about as far as I can go without losing my mind. And as for you, if you’ve made it this far, well then, I salute you.