The recessive N gene, nr, is demonstrated by monogenic ratios. For the most part nr/nr characters are intermediate between those of N/N and N/+. About a sixth of nr/nr lambs have no shoulder patch, and in certain fibre type details, and in hairiness, nr/nr animals tend to be intermediate between the two dominant-N genotypes. On the other hand, all nr/nr ewes have been hornless; a few have had horn-lumps, one scum By contrast, about one N/+ ewe in 10 grows horns. All the nr/nr rams reared to 1 year, except one with scurs, have grown horns. Carrier (+/nr) sheep have birthcoats which we do not know how to distinguish from those of ordinary Romneys, though there is evidence that one dose of nr increases halo-abundance on the back slightly. A few +/nr lambs have been grade VI. Just a few +/nr sheep have fleeces as hairy as the average N/+. A quarter or more of the +/nr rams have had horns, mostly small, at 4 months, and nearly all the others have had scurs. The genes N and nr are not allelic, but may be linked. Because the sheep has 26 pairs of autosomes free assortment seems more probable. In general the characterization of sheep deemed N/+.+/nr is intermediate between that of N/N.+/+ and N/+.+/+. The noticeable difference between N/+.+/nr and +/+ nr/nr is the more powerful growth of horns in the double heterozygotes. An analysis of breeding results from N/+.+/nr sheep indicates that the great majority of N/+.nr/nr animals of both sexes have the horn and halocoverage characters of homozygous dominant-N's. In N/+.+/nr, some ewes have horns, and the shoulder patch is absent about as often as in nr/nr. The frequencies of these characters in N/+.+/nr are applied in showing that the genetic basis of horns in ewes, and the genetic basis of full halo-coverage, in heterozygotes of the dominant-N stock are different; and that it is not the gene nr which makes horns grow in the dominant-N heterozygous ewes; and one piece of evidence suggests that it is not the nr gene that determines absence of shoulder patch in the birthcoats of the dominant-N stock. It is concluded that if the dominant-N stock is not completely free from nr, that gene has only the same sort of frequency as in the Romney breed. Matings made as +/+.nr/nr X +/+.+/+, the latter no-halo ewes, from flocks outside the College, or if bred at the College unrelated to any N-type sheep, have given a small proportion of N-grade lambs. It now appears that at least most of these lambs, called 'dominoes', have received one dose of a dominant gene for N-grade, probably the gene N, from the no-halo parent. One no-halo ewe from an outside source had an N-grade son, proved nr/nr, by a domino ram, the dam thus being shown to carry the gene nr.
Characterization of recombination features and the genetic basis in multiple cattle breeds