No confounding between dominance and additive genetic variances for composite reproduction traits of Zandi sheep

The aim of the present investigate was to study the existence of confounding between dominance and additive genetic variance for two composite reproductive traits included total litter weight at birth per ewe exposed (TLWB1/EE) and total litter weight at weaning per ewe exposed (TLWW1/EE) at first lambing using data collected from 1991 to 2008 in the breeding station of Zandi sheep. Two animal models including model with additive genetic effects and model with additive and dominance genetic effects were considered for the estimation of variance components of the considered traits. For both TLWB1/EE and TLWW1/EE the differences between estimated additive genetic variance under both considered model were negligible; implying no confounding between additive and dominance genetic variance. The estimated values of dominance genetic variances for both TLWB1/EE and TLWW1/EE were higher than the estimated values of additive genetic variances. Rank correlations between the estimated breeding values under both tested models and between total genetic and estimated breeding values under model including additive and dominance genetic effects were low and medium in magnitude, respectively. The obtained results revealed that in spite of no confounding between additive and dominance genetic effects for the studied traits animals were ranked differently under the both tested models. Goodness of fit measures revealed that taking dominance genetic effects into account should be considered in the models used for increasing accuracy of genetic evaluation for these traits.