A suggested mode of inheritance for wool shedding in sheep.

The ability of a sheep to shed its own wool has an attraction in scenarios where the costs of harvesting wool outweigh its value. Certain breeds and composites have the ability to shed their wool in the spring, and these are investigated in this work in an attempt to outline the genetics of wool shedding. One flock from a breeding group in Southern England (UK) containing sheep with wool-shedding characteristics provided shedding scores (1 to 5 scale; no shedding to complete shedding) that were used in a range of genetic analyses. The particular nature of wool shedding suggested that there may be a major gene segregating in these populations that facilitates wool shedding. In addition, there was clearly variation among wool shedders in the speed and extent of shedding, so a polygenic trait was also investigated. The breeding group used a range of shedding breeds and composites in a regular program to introduce wool-shedding genes into their flocks. This allowed the testing of Mendelian ratios for shedders:nonshedders in both first-cross and first-backcross animals. Four modes of inheritance were tested: autosomal recessive, sex-linked recessive, autosomal dominant, and sex-linked dominant. The most likely mode of inheritance was autosomal dominant (P < 0.05), with a low level of incomplete penetrance. In first back-cross animals, this mode of inheritance was confirmed but with complete penetrance. Approximately 11% of shedders did not exhibit the trait as lambs. Mixed-model analyses of shedding scores allowed an investigation of factors that affected wool shedding and also the extent of any genetic and permanent animal variance. Shedding score was found to have a heritability of 0.54 ± 0.07 in lambs and 0.26 ± 0.06 in animals of all ages in one flock using Easycare, Wiltshire Horn, Katahdin, and Dorper shedding animals. Shedding score as a lamb had a genetic correlation of 0.94 ± 0.08 with shedding score as a 2 yr old, but at the phenotypic level this correlation was only 0.39 ± 0.05. No permanent animal effect was found for shedding score. Breeding for increased wool-shedding ability is possible, but improvement of the trait needs to be considered in 2 stages. First, the dominant gene needs to be introduced into the population, and then selection between animals can proceed by using EBV for the polygenic trait (speed or extent of shedding) as the basis for selection.