As you may have seen briefly addressed on the ram page, here I will discuss my ongoing investigation into the mystery of the strange horn inheritance that I have observed in my ram, Freyr. This research is by no means complete, and conclusions will likely not be drawn for years as I take careful notes of my breedings to try to solve this.
Freyr is a horned ram, with fairly average sized, nicely symmetrical horns that by all appearances are just standard homozygous horns. (For an explanation of horn inheritance, see “Horn Inheritance”` in the Library.) There is a detailed article explaining Icelandic sheep horn inheritance in the Vol. 5, No. 4 issue of the ISBONA newsletter, based on years of research and careful measurements of ram horns in Iceland. This article, written by a Ms. Eythorsdottir, says of the largest type of heterozygous horns, “The horns are lacking the ridge of full horns, they are smooth and round on 3-4 month old male lambs and usually smaller, more narrow and less curled than full horns on mature males.” She also says of homozygous (p’p’) horns, “Full horns are not uniform and there is a lot of variation within this phenotype as to the size and shape of horns – they can be thick or thin, long or short, sweep back or to the side, etc. What is common, is a sharp ridge on the underside of the horn, especially on males, so if you grab the horn it’s never smooth and round but more triangular. These animals are homozygous pp – recessive at the polled locus.” So by this definition, Freyr should have homozygous horns.
Note in the first picture the distinct ridge. His horns are definitely triangular, not rounded at all. Also take note of the heavy texture on his horns. Heterozygous horns are said to be more smooth. The second picture shows the nice symmetry of Freyr’s horns. While this article doesn’t specifically mention it, my observations have been that heterozygous horns often lack symmetry. Heterozygous horns are also said to lack much curl. While Freyr’s horns don’t have as massive of a curl as some rams I’ve seen, I have also seen rams with less curl who are defnitively homozygous horned. In fact, in my search through the photos and pedigrees of rams whose ancestry has left no possibility for the presence of a polled gene, many of their horns at Freyr’s age were about the same size, some even slightly smaller.
The point is, by my observations as well as by current research, Freyr’s horns should be homozygous.
The trouble with only knowing phenotype, or what is observed, is that the genotype is not always fully expressed within the phenotype. In this case, one of Freyr’s lambs, a little ewe lamb named Solfi, is still polled at over two months old, and will likely stay that way.
That is not possible if Freyr is homozygous horned, because the mother of this lamb, Tryna, does not have any polled ancestry, so could not have contributed anything but a horned gene. Freyr had a heterozygous polled mother, and polling in his ancestry on the father’s side.
Update June 30, 2012: In my searchings for new breeding stock, I came across a Raftur son out of a homozygous polled mother. Raftur was homozygous horned. So this son should be pretty certain to carry one polled gene (P) and one horned gene (p’). The horns that he developed are about half the size of normal horns, and very narrow and close to the face. But the thing I found notable about them is that they are distinctly ridge and a distinctly triangular if a cross section were to be taken. This suggests that despite what the article claims, that a heterozygous horned (Pp’) ram can have ridged, and not smooth round, horns. These horns are shaped the same as a p’p’ ram, just smaller in diameter and stature.