Sex differences in birth weight contribute to within-litter variability, which itself is connected to piglet survival. Therefore, we studied whether the sex difference in piglet birth weight is a genetically variable sex dimorphism. For that purpose a linear mixed model including sex-specific additive genetic effects was set up. A hypothesis testing problem was defined to detect whether these genetic effects significantly differ between sexes. In a second step, the effect of sex-linked genes was studied explicitly by partitioning the additive genetic effects into autosomal and gonosomal effects. Furthermore, a definition of heritability for the sex difference of a randomly chosen pair of littermates with opposite sex was given. The proposed models were applied separately to a Landrace and Large White data set. Significant genetic variability for the sex dimorphism was found in Landrace but not in Large White. Heritability estimates were at 3 to 5% depending on the model. The X-chromosomal genetic variation was not significant at all, whereas the Y-chromosome significantly contributed to the genetic variation in Landrace with a corresponding SD of 34 g. It can be concluded that the sex dimorphism of piglet birth weight is genetically variable and a potential target of genetic improvement.<\/p>\n