Threshold analysis of genetic effects for secondary sex ratio in domestic sheep using Bayesian approach via Gibbs sampling

- Genetic parameters were estimated for secondary sex ratio (SSR) in domestic sheep.
- Estimates of direct and maternal heritabilities for SSR were low in all sheep breeds.
- Improvement of SSR in sheep seems possible in selection programs.
- Genetic variation in SSR can take benefit of sexual dimorphism for traits such as growth rate.

This study was conducted to estimate variance components, genetic parameters and genetic trends for secondary sex ratio (SSR) in three domestic sheep breeds of Iran (Guilan, Moghani and Shall) using threshold animal models. The data set and pedigree information of Guilan, Moghani and Shall sheep comprised 21,736 lambs born during 1994 to 2014, 35,076 lambs born during 1994 to 2012, and 64,964 lambs born during 1982 to 2012, respectively. Threshold animal models included direct and maternal genetic effects with covariance between them and maternal permanent environmental effects were implemented by Gibbs sampling methodology. Posterior mean estimates of direct heritabilities for SSR in Guilan, Moghani and Shall sheep were 0.042, 0.051, and 0.153, respectively. Also, posterior mean estimates of maternal heritabilities for SSR in Guilan, Moghani and Shall breeds were 0.050, 0.052, and 0.083, respectively. Estimates of correlations between direct and maternal genetic effects of SSR in Guilan, Moghani and Shall sheep breeds were negatively high and equal to −0.61, −0.64 and −0.90, respectively. Also, the posterior mean estimates of permanent environmental effect due to the dam were 0.038, 0.114 and 0.008 in Guilan, Moghani and Shall sheep breeds, respectively. Although the results of current study indicated low heritability estimates for SSR, but improvement of SSR in sheep seems possible in selection programs and applicable genetic variation in SSR can take advantage of sexual dimorphism for economically important traits such as growth related characters, which may, in turn, make possible greater intensity of selection and therefore greater genetic change, as well as decreasing the costs related to animal replacement.