Incorporation of Z-linked effect into random regression model to evaluate the body weight in chickens.

1. The present study investigated the extent and impact of a Z-linked genetic effect on sexual size dimorphism. It estimated genetic parameters for the body weights of layer chickens during the period of 7 to 84 d, using random regression model.2. Body weights of 5,296 birds were measured at four different ages. Two random regression models that described the weight of individual birds, with and without a Z-chromosome relationship matrix, were compared. The fixed effects covariate included the coefficients of genetic group and inbreeding (both quadratic effects). Heterogeneous residual variances were modelled by grouping the test period into seven subgroups. A sixth-order polynomial was used to model the mean weight of the population.3. Covariance components were estimated using the AIREML method. Models were compared using Akaike information criterion, Bayesian information criterion and residual values. The models including quadratic and cubic polynomials for additive direct and permanent environmental effects, respectively, and linear polynomials for Z-linked genetic effects were the best.4. According to the best model, autosome heritability was estimated in the range of 0.19-0.38 with a decrease from 7 to 20 d of age and a subsequent fluctuation around 0.22 to the end of the trajectory. The Z-linked heritability increased from 0.14 at 7 d to 0.50 at 56 d and then declined moderately to 0.40 at 84 d.5. Permanent environmental variance increased with age. However, the maximum proportion emerged at 16 d of age. The estimate of Z-linked genetic correlations between weeks was high at over 0.82.6. The results illustrated the potential of a Z-linked relationship matrix for genetic evaluations of body weight in poultry, using random regression models.