Inheritance of Polledness, Horns and Scurs in Beef Cattle

B. C. Allison, Department of Animal Science, North

In beef cattle of European ancestry the trait of being polled or having horns is determined by one pair of genes. One gene in the pair is inherited from the dam and the other from the sire.

The polled gene (P) is dominant to the horned gene (p). If an animal has two polled genes(PP), homozygous, or one polled and one horned gene (Pp), heterozygous, it will be polled. However, if it is heterozygous polled (Pp) it may pass either the polled or horned gene on to its' offspring.

The only situation when an animal will be horned is when it possesses two recessive horned genes (pp), homozygous horned. Table 1 illustrates the expression of polledness or horns and what genes and traits can be expected to be passed to the offspring from the various matings.

If an animal of European breeding, not of Zebu ancestry, has horns then you can determine from visual observation that it is homozygous for the recessive horned gene. However, if the animal is smooth polled or scurred it is impossible to determine from visual observation if it is genetically homozygous polled (PP) or heterozygous polled (Pp). The homozygous polled bull with two polled genes will sire only polled calves. Only through the offspring produced can the number of polled genes be determined.

The best test of a bull for bred to horned cows is to mate the polled bull in question to horned cows. A polled bull bred to horned cows that produce one or more horned calves is heterozygous polled (one recessive gene for horns), regardless of how many polled calves are produced.

Table 2 gives the probability of a polled bull being homozygous polled if no horned calves are produced from matings with horned cows.

There are additional genes that affect hornlike growth, scurs, on an animal's head. Scurs are incompletely developed horns which are generally loose and movable beneath the skin, not attached to the skull. They range in size from small scab-like growths to occasionally almost as large as horns. Because the gene for scurs is transmitted separately it has no effect on the presence or absence of horns. Not all horned cattle carry the gene for scurs and not all polled cattle lack scur gene.

The gene for scurs is expressed differently from the gene for polledness/horns. The way the gene for scurs is expressed depends on the sex of the animal. In males the scur gene is dominant, meaning that if only one of the two genes is for scurs the bull will be scurred. Therefore, it is easy to detect the scur gene in the bull and eliminate it from the herd. In females the scur gene is recessive, meaning that she must posses both genes for scurs in order for the cow to be scurred. If the cow possesses only one scur gene she will not have scurs herself but has a 50 percent chance of passing the scur gene on to her calf. The smooth polled cow may have the recessive scur gene, resulting in much more difficulty in identifying/eliminating the scur gene from the herd.

Table 3 depicts the scurred inheritance patterns. The presence of the scur gene is indicated by Sc and the absence of the scur gene by Sn. These gene patterns are for polled animals, as the horn growth will cover up any scurred condition if it exists.

Another factor that complicates the inheritance of polledness/horns is that in cattle with Zebu ancestry, like Brahman, Santa Gertrudis and others there is an additional gene that affects the inheritance of horns. Inheritance of horns in Zebu-type cattle is different from that observed in the British breeds. The polled gene (P), and the scur gene (Sc) can both be present in American cattle with Zebu ancestry.

However, another gene, the African horn gene (Af) also affects inheritance of horns in these animals. The absence of this gene is expressed by the symbol (An). Geneticists are reasonably certain that the way the Af gene is express is dependent on the sex of the animal, much like the way scurs are expressed. In males the Af gene is dominant to the polled gene, An. This means that a single Af gene will result in a bull being horned, even if he is heterozygous or homozygous polled. In females the Af gene is recessive to the polled gene An. In heterozygous polled females two of the Af genes must be present for the animal to have horns. In animals possessing the Af gene in addition to the polled gene (homozygous or heterozygous) the inheritance patters shown in Table 4 can be expected.

Like scurs, the presence of the African horn gene is easy to detect in males since the presence of only one Af gene results in the male having horns. Therefore, progeny testing for the Af gene in malesis unnecessary. If the bull is polled he does not possess the Af gene. If he is horned when his genetic ancestry shows that he should be polled, the reason may be that he possesses an Af gene.

Cattle producers should also keep in mind that a proven homozygous bull will produce some horned calves if he is bred to horned or polled cows that carry the African horn gene. As you can see there are ultimately three pair of genes that may determine if cattle have horn-like tissue on their head in the form of horns or scurs.