A new strategy of heterosis research in mice – approach and results on chromosome 19

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.