Autosomal and sex-linked genetic parameters for body weight gain and its relationship with efficiency-related traits in sheep

The aim was to estimate autosomal and sex-linked genetic parameters for pre- and post-weaning body weight gain (BWG1, BWG2), pre- and post-weaning Kleiber ratio (KR1, KR2) and pre- and post-weaning efficiency of growth (EF1, EF2) in Zandi sheep. To this end, each trait was analyzed with a series of 8 animal models which were identical for fixed and autosomal additive genetic effects but differed for combinations of X chromosome additive genetic and maternal effects. The Bayesian Information Criterion (BIC) was used to rank models. (Co) variances between traits were estimated using bivariate analyses. For BWG1, BWG2 and KR1, models containing the sex-linked genetic effects fitted data substantially better as evidenced by lower BIC values. By including sexlinked genetic effects in the model, additive genetic variance decreased by 23%, 19% and 10% for BWG1, BWG2 and KR1, respectively. Estimates of sex-linked heritability (h2s) was in a range between 0.04 (KR1) to 0.08 (BWG1), smaller than corresponding estimates of autosomal heritability (h2a) which ranged from 0.09 (EF1) to 0.15 (EF2). Comparing additive breeding values obtained by models with and without sex-linked genetic effects for top 10 and top 100 animals indicated that by ignoring sex-linked effects, estimates of additive breeding values changed and, on average, 50% and 35% of the top 10 and top 100 animals went out from their groups, respectively. BWG1 and EF1 were affected by maternal genetic (h2m=0.05) and maternal permanent environmental effects (h2c=0.04), respectively. Sex-linked additive genetic correlation (rs) between traits were -0.935 (BWG1-BWG2), - 0.995(BWG2-KR1) and 0.931 (BWG1-KR1). Autosomal additive genetic correlation (ra) was in a range from -0.540 (KR2-EF1) and 0.999 (BWG1-KR1). Since presence of sex-linked effects in the model increased the model power to fit the data, prevented overestimation of heritability and increased the accuracy of genetic evaluation, inclusion of sex-linked effects in the model for genetic evaluation of Zandi lambs is recommended.