In this study a genetic evaluation, based on a mixed inheritance model, was developed for birth traits (calving ease, gestation length and birth weight) in dual-purpose Belgian Blue (dpBB), a separated type inside Belgian Blue Herd-book. About 80% of dpBB animals have a single or a double copy of the muscular hypertrophy gene. This heterogeneity is the reason of a great variability in birth performance traits like calving ease or birth weight. The muscular hypertrophy gene substitution and dominance effects for calf genotype had a significant impact both on birth weight and calving ease, in accordance with partially recessive expression of the muscular hypertrophy gene. Observed high heritability estimates of direct calving ease (0.334) and birth weight (0.260) suggested that a large genetic variability for birth traits was present in dpBB, and that genetic improvement was possible through selection. This variability has allowed dpBB breeders to apply mass selection successfully in the past. However analysis of breeding values showed that a sire selection for calving ease within genotype was progressively applied by breeders, the selection intensity being more important for calving ease in double muscled lines. This study illustrated the possible confusion that can appear by the use of a major gene in selection, and the importance to use appropriated models combining polygenic and monogenic information, like mixed inheritance models.