POLLED

“The Polled Allele and Shetland Sheep”
By Juliann Budde, edited by Michelle McMillen

The poll gene has been present in North American Shetlands as long as North America has had Shetland sheep. It was originally introduced through the Dailley importation. Since then, at least two of the more recent importations of semen from the UK have contributed new poll genetics. Efforts are underway to better understand this commonly maligned but completely natural genetic trait, and secure a place for the polled Shetland alongside the horned Shetland in North America. The poll gene in Shetland sheep was brought in with the original Dailley importation, probably through one of two white ewes, either Canada 70 Z0970 or Canada 81 Z0981. The gene has been inherited by subsequent generations, passed down by both rams and ewes. Usually, if a ram lamb developed short, stunted horns, he was quietly culled as having "bad horns". But the poll gene went undetected in some aberrant-horned rams, so they passed on the poll gene to some of their offspring, as did ewes which unknowingly carried a poll gene. These ewes threw normal-horned rams, along with the occasional scurred one. Polled rams also cropped up, which some shepherds retained for use as breeding stock.
Eventually, the occurrence of these polled rams attracted the attention of a handful of Shetland breeders, and piqued their curiosity. How is the poll gene inherited? Could the poll gene be selected for and successfully propagated? These Shetland breeders formed an on-line discussion to learn everything possible about how horn genetics are expressed. Textbooks and scientific articles were sought out, poured over and debated. Shetland sheep suspected or known to carry at least one poll gene have been collected, evaluated and test bred. Breeders then share their collective experiences to learn even more. Although the study and breeding of polled Shetlands is still in its infancy, some knowledge of the polled gene, its inheritance, and its effect on the typically horned Shetland ram has been obtained, and is being tested further. Following is a brief summary of horned/polled genetics, and how the genes are passed down.
	A model of horn type inheritance was developed by S. Dolling (1961) and has a wide applicability to other sheep breeds (Kinsman 2001). The presence of horns is controlled by the autosomal horn locus (HO), with three alleles. The three alleles are listed as follows, in order of dominance: P (HO*P) – This is the poll allele, and is incompletely dominant. Over time it is possible to completely eliminate the horn gene from a flock and produce consistently polled rams, as demonstrated in commercial meat flocks. p (HOhl) – This is the allele for sex-limited expression of horns. This allele expresses itself in some breeds, including most Shetlands, with substantial horn growth in rams, and no visible horn growth in ewes, although most ewes do have small bone knobs palpable on the poll beneath the skin. Ewes that are (pp) are most accurately labeled "hornless" because they lack obvious horn growth themselves but will produced male offspring with normal horns. They are not considered polled unless they carry at least one copy of the poll gene. p' (HO+) – This is the allele for horns in both sexes, also known as the "horned ewe allele." This produces substantial horn growth in both rams and ewes; large, heavy, spiraling horns on the rams and goat-like horns on the ewes after the first year. Breeds such as the Icelandic, Scottish Blackface, and Jacob are typically (p'p'). As in color genetics, each parent randomly passes down one of its two horn locus genes to its offspring, resulting in a total of two alleles at the HO locus. Since the majority of North American Shetland sheep carry two genes for horns (pp) at the horn locus (HO), the majority of resulting lambs will also be (pp).
A sheep inheriting one horn gene and one poll gene is a half-poll. (Pp) rams can exhibit a vast range of horn material, from very small scurs that break off at a touch to very long, almost-normal looking growth called "aberrant horns. Aberrant horns can be nearly indistinguishable from regular (pp) horn growth.
A sheep inheriting two P genes is a full poll (PP). (PP) rams are smooth-headed or exhibit very minimal scur growth.
Which allele will be passed down with each breeding is a flip of the coin. Some breeders may question the use of a scurred ram in a polled breeding program due to the unattractiveness of some scurs. But these scurred (Pp) rams carry a poll gene, and 50% of their offspring will inherit that poll gene, increasing the number of (Pp) sheep for further breeding. If two (Pp) sheep are bred together, they have a 25% chance of producing (PP) Shetland sheep – and a 25% chance of producing (pp) Shetland sheep. It will take some time for breeders interested in poll genetics to be able to consistently produce smooth polled Shetland rams. Scurs and aberrant horns in ram lamb offspring are to be expected from (Pp) rams, until the horn gene is completely bred out of polled flocks. A scurred ram lamb should be evaluated like any other ram that may potentially be used for breeding, considering general health, conformation, and fleece type. Ram lambs should be as free as possible from internal parasites, as parasites can stunt the growth of normal horns (Clutton-Brock/ Pemberton 2004). The polled Shetland is another expression of the wonderful gift of diversity the breed has to offer. Although many people love their horned Shetlands, there are shepherds and potential shepherds who are uncomfortable around horned livestock for various reasons. By preserving lines of polled Shetland sheep, these individuals may choose Shetlands instead of investing in another breed of polled sheep, which will benefit all North American breeders. Sources: "Breeding Merinos" (1970) by Scott Dolling "Mendalian Inheritance in Sheep" (1996) Dolling, Montgomery. "Soay Sheep- Dynamics and Selection in an Island Population " (2004) edited by Clutton-Brock and Pemberton."