A suggested mode of inheritance for wool shedding in sheep1

Abstract

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.

Estimating heritability of wool shedding in a cross‐bred ewe population

“…Repeatability of WS was also moderate with mean 0.338 (SD 0.028). This contrasts the findings of Pollott () and Matika et al . () who reported negligible estimates of permanent environmental effects of zero and 0.05, respectively.…”

“…Heritability for the liability of WS was moderate, with mean 0.204 (SD 0.031). That estimate was consistent with that obtained by Pollott () in a mix of shedding breeds when WS was considered Gaussian, and a repeatability model was fitted. That author reported heritabilities ranging from 0.16 ± 0.04 to 0.23 ± 0.03.…”

“…Pollott () also reported a large effect of age on shedding scores ( p < 0.001) in mix of shedding breeds, with lambs shedding less than older animals. Rathie et al .…”

“…). Pollott () and Matika et al . () reported that wool shedding was moderately to highly heritable, and therefore clearly under genetic control.…”

“…Because of the discrete nature and skewness of WS, normality cannot be assumed. Through the use of link functions (logit, probit), generalized linear mixed models are often used in the analysis of this trait (Pollott ; Matika et al . ).…”