Abstract. In order to demonstrate the potential power of maternal effects on litter size and growth in pigs a reciprocal cross of Berlin Miniature Pigs and Durocs was carried out. The average body weight of the Duroc dams (151 kg) was more than three times higher than the weight of the Miniature Pig dams (48 kg). A considerable maternal effect on fertility, estimated from reciprocal crossbred differences in the litter size of 3.5 piglets was found. The least Squares means of birth weight were 1.18 kg and 0.75 kg for Fl animals bom from Duroc dams and Miniature Pig dams, respectively. This maternal caused disadvantage in body weight of crossbred offspring from Miniature Pig dams is compensated in the interval from weaning to the 13th week of age. The end weight estimated by means of Gompertz and Bertalanffy growth functions showed only minor differences between the reciprocal crossed groups in favour of Fl born from Duroc dams. Our data suggest that the uterine capacity of the dam could play an important role in the fertility of extreme breed crosses. Concerning growth performance the milk yield of the sow and the influence of the birthweight of the piglets were discussed.

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Abstract. The following steps were performed to analyse heterosis and QTL effects in litter size of mice: intercross of mouse inbred strains C57BL/6J and Balb/cJ in order to produce a F2 generation with 948 female animals; selection of trait groups with extreme high ((13 offspring) and extreme low litter size (5 offspring)); typing of 56 microsatellites with an average distance of 32 cM; detection of different chromosome regions with associations to heterosis in litter size. Chromosome 19 was associated to heterosis in litter size. Additional animals with extreme high and low litter sizes were then typed for four DNA markers on chromosome 19 and used for QTL mapping. A QTL was identified for litter size in segment D19Mit28 &ndash: D19Mit99 with a maximum at 15 cM (p ≤ 0.05). The QTL explains about 11% of the phenotypic variance in the F2 generation. With a degree of dominance of 4.09 the QTL shows that superdominance can explain heterosis in litter size.

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