Genetic Analysis of Autosomal and Sexual Chromosomes of Pre-Weaning Traits in Mehraban Sheep

Introduction Mehraban sheep is one of the important Iranian breeds. This breed mainly distributed in the western province of Hamadan, Iran. This breed is fat-tailed and has carpet wool with light brown, cream or grey colour. There are approximately 3 million heads of Mehraban sheep and primarily used for meat production. Accurate estimation of genetic parameters and in particular genetic correlations requires large data sets and effective methods for genetic analyses with considering all potential effects. In livestock, the animal mixed model using REML procedure, which allowed to estimate variance components such as maternal heritability, permanent environmental effects and the correlation between direct and maternal genetic effects for investigated traits, is used extensively for data analysis. Many researchers reported that ignoring maternal effects leads to over-estimation of direct heritability and consequently, reduced accuracy of prediction. Therefore, in order to obtain an optimal genetic gain in a selection scheme, including maternal effects will decrease the bias of predicted responses to selection. In genetic evaluation of animals, it is generally assumed that only autosomal chromosomes are involved and the effect of sex chromosomes ignored.  In Markhoz goat, it has been shown that sex-linked genes may have less effect at the early ages and more noticeable effects were indicated on body weights at older ages. The aim of this study were to estimate the genetic parameters of autosomal and sex-Linked additive genetics effects for pre-weaning traits in Mehraban sheep using multi-trait analyses and to estimate genetic correlations between traits.
Material and method The data set used in this study was collected between 1994 and 2010 from Mehraban sheep breeding station in Hamadan province, Iran. The analyzed traits were birth weight (BW), weaning weigh (WW), average daily gain (ADG) from birth to weaning and Kleiber Ratio from birth to weaning. The GLM procedure of SAS software was used to identify which the fixed effects that are required to be considered in the animal model. The model accounting for fixed effects included herd-year (combined as herd–year effect), sex (male or female), birth type (single or twin), age of dam (2-6 years old) and the age of weaning (in days) was used as a covariate for WW (P<0.001). Also, random effects included direct additive genetic effects in autosomal chromosomes, direct additive genetic effects in sex-linked, maternal genetic effects, maternal permanent environmental effects and the residual effects. (Co) variance components and genetic parameters were estimated using a multi-trait analysis via the Restricted Maximum Likelihood (REML) method with WOMBAT software. Convergence criterion was assumed 10–8.
Result and discussion The interaction between herd and year of lambing was significant for all traits. Significant influence of herd and year of lambing could be explained by difference in management, climate conditions and feeding. Also, all traits were significantly affected by sex, birth type and age of dam. The significance of these effects is mainly due to endocrine system between two genders, limited uterine space especially in young ewes, competition for milk between the twins or maternal ability of dam. For birth weight, weaning weight, average daily weight from birth to weaning weight (ADG) and Kleiber Ratio (KR), estimated direct autosomal heritabilities were 0.21, 0.18, 0.34 and 0.55, estimated direct sex-linked heritabilities were 0.03, 0.11, 0.10 and 0.07, estimated maternal heritabilities were 0.01, 0.15, 0.11 and 0.09, and ratio of maternal permanent environmental variance to phenotypic variance were 0.04, 0.0, 0.0 and 0.06, respectively. According to these results, growth rate and Kleiber ratio in Mehraban sheep are categorized as moderate and high heritable traits, therefore a favorable genetic gain would be expected through selection programs. Estimated direct sex-linked heritabilities were close to the estimates of maternal heritabilitiese. This suggests that sex-linked effects need to be considered in selecting for growth traits in Mehraban sheep. The autosomal, sex-chromosome, maternal genetic, maternal environmental effects and phenotypic correlations were ranged from -0.31 to 0.91, 0.33 to 0.99, -0.26 to 0.99 and -0.28 to 0.94, respectively.
Conclusion The results of this study indicated that the direct variance of sex chromosomes has an affect similar maternal genetic variance, and it could contribute to a more accurate estimation of the direct autosomal heritability. Also, positive genetic correlations between sex chromosomes could increase the response to selection of each sexes.