Very little is known about the genetic aspects of sexual
dimorphism of body weight in domestic sheep, and therefore this study was
conducted to quantify the genetic basis of sexual dimorphism for
early-growth-related traits in Afshari lambs. Traits evaluated included birth
weight (BW), weaning weight (WW), and growth rate (GR) in male and female
lambs. Male lambs were 6.6 % heavier at birth, had 14.4 % higher
preweaning growth rates and were 16.0 % heavier at weaning compared to
female lambs. Levels of sexual-size dimorphism (SSD), expressed as the ratio
of male to female means, for BW, WW and GR were 1.07, 1.14 and 1.15,
respectively, which indicated low levels of SSD in the traits studied. Fixed
effects of year of birth and type of birth interacted with sex effects, with
greater variability in birth and weaning weights among years and birth types
in male lambs, suggesting greater environmental sensitivity in the males.
Bivariate animal models and restricted maximum likelihood (REML) procedures were used to estimate phenotypic
variances and their genetic and non-genetic components in male and female
lambs. Estimates of the direct heritability (h2) and additive
coefficient of variation (CVA) for BW were higher in males. However, for
WW and GR, heritability estimates were higher in females. In contrast,
whereas the contribution of maternal permanent environmental effects
(c2) to variation of BW was higher in females, for WW and GR higher
estimates of c2 were observed in males. Respective genetic and
maternal permanent environmental correlations between records on males and
females were 0.986 and 0.723 for BW, 0.995 and 0.983 for WW, and 0.995 and
0.966 for GR, indicating possible sexual dimorphism only for maternal
effects on BW. Based on an approximate 95 % confidence interval, none of the
observed differences in variance components between sexes differed from zero
and none of the observed genetic or maternal correlations differed from 1.0,
indicating no need or opportunity for sex-specific selection strategies
