The dominant N gene in New Zealand Romney sheep

Abstract

The existence of the dominant N gene, derived from two different sources, has been established by monogenic ratios. The gene is variable in expression in the heterozygote, and is pleiotropic. Horns are a sex-influenced expression of this gene, being dominant in males and recessive in females, with exceptions of the order of one in 10 in both sexes. There are hints that the exceptions in rams have a genetic basis, and strong evidence that this is so in heterozygous ewes, a dominant gene, F, variable in expression, being postulated. In the birthcoat of heterozygotes the expression of the N gene varies over a range as wide as is possible. There is extreme variation in halo-coverage over the body in animals of N-abundance on the back, and all halo-grades, from borderline-N to no-halo, are found in the one-sixth of heterozygotes less than N-grade. There is evidence that some part in causing the heterozygote to be less than N-grade is played by genes which reduce the abundance of halo-hairs on the main area of the body in non-N lambs. Possibly one dominigene, not linked with N, is powerful in the heterozygote. Heterozygous dominant N-grade lambs, with very few exceptions, can be distinguished from homozygotes in having halo-hair abundance reduced below that of N-grade at the anterior end of the body, at least in a small area behind the shoulder which is called the shoulder patch. The absence of the shoulder patch in heterozygotes has a genetic basis. Very occasionally there appears to be overlap between the two genotypes in the opposite direction. Recently a very small number of lambs thought to be homozygotes have been a little short of full halo-hair abundance on the shoulder patch on either one or both sides of the body. It is also of some help in distinguishing the genotypes that homozygous ram lambs have lumps in the horn positions, while heterozygous ram lambs frequently have no lumps at birth. On the average homozygous fleeces are more coarsely hairy than heterozygous, and contain more secondary kemp. In setting out the facts from which the above circle of conclusions is drawn it is sometimes convenient to make use at an earlier stage of something which is not proved until later. This applies especially to distinguishing homozygote and heterozygote.