Inheritance of Polledness and Scurring - Part 1
T. A. OLSON 

In spite of the fact that inheritance of polledness and scurrying has been studied for over 70 years, certain aspects remain unclear. It is quite clear that a major gene, P, is common in cattle and dominant to its normal allele p⁺ such that Pp⁺ animals are polled. Angus are homozygous or nearly so for the P gene. The p⁺ (horned) gene appears to mutate to P at a fairly high frequency, as indicated by the periodic production of polled animals from horned parents. One recent example was the polled Simmental bull, Polaris, whose parents were both horned, fullblood Simmental. The polled mutant has also occurred in Herefords, resulting in the Polled Hereford breed, and in the Shorthorn, Holstein, and Brahman breeds. The polled gene also has been incorporated into breeds by selecting for it in successive generations of upgrading programs to horned breeds where polled cows were used as foundation dams.

The inheritance of scurs, small growths of horn-like material on the heads of polled animals where horns would have developed, has been difficult to clarify. The traditional explanation, that the expression of scurs is sex-influenced, was based on the fact that scurs tend to be more common on bulls than cows. For example, the cross of Brown Swiss and Angus results in scurred bulls and smooth polled (non-scurred) cows. In theory, polled bulls heterozygous for the scurring gene Sc and its allele sc⁺ would be scurred, whereas heifers of the same genotype would be smooth polled. Evidence from recent years, however, suggests that a bull must carry p⁺, in addition to Sc, to express scurs. Heifers are thought to have to be homozygous for the gene for scurs, ScSc, to express scurs. For the breeder interested in producing polled, non-scurred cattle, the simplest approach would be to never use scurred heifers and to only use scurred bulls if no acceptable non-scurred bulls are available. While bulls of thegenotype PPSc_ are supposed to be scurred, it would be safest to assume that any scurred bull is a likely carrier of the horned gene, p⁺.

Breeders of polled cattle are often interested in determining whether or not a polled bull is homozygous for the polled gene. Polled bulls whose sire and dam were both polled are sometimes referred to as "double-polled." This is unfortunate terminology as it may lead to the assumption that such bulls are homozygous for P when, in fact, if both the bull's sire and his dam were heterozygous polled, a polled bull would have only a 33% probability of being homozygous. There is a relatively easy procedure for testing a polled bull for the presence of p⁺. If a polled bull bred to horned cows produces seven or more polled progeny without a single horned calf, he can be assumed to be homozygous for P with less than a 1% probability of error (see table 2.5). Such a testing procedure would be recommended for all scurred bulls and bulls with one or both parents that were known to be heterozygous for p⁺

One additional complication is that occasionally bulls that are PP will produce a horned calf, particularly from cows with Zebu breeding. An explanation is that the presence of the "African horn gene" from the dam of the calf is epistatic to P, resulting in horned progeny from a PP sire. Support for the existence of the African horn gene was given in an Australian study of inter se mated crossbred populations of Brahman x Polled Shorthorn and Africander x Polled Shorthorn origin (Frisch et al., 1980). In both crossbred populations, horned bulls bred to horned cows produced polled calves; in fact, 23% of heifer calves from such matings in the Africander crossbred population were polled. This indicates that many of the horned parents carried the African horn gene. Thus, a smooth-polled bull that has sired 10 or more polled calves from horned Zebu cows should not be discarded as a carrier of p⁺ simply because he produced a single horned calf. To try to avoid complicating the testing procedure, Brahman crossbred and cows of dairy breeds ( which also have been reported to carry the African horn gene) should not be used as tester animals.