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

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/c J in order to produce a F-2 generation with 948 female animals; selection o f trait groups with extreme high ((13 offspring) and extreme low litter siz e (5 offspring)); typing of 56 microsatellites with an average distance of 32 cM; detection of different chromosome regions with associations to heter osis in litter size. Chromosome 19 was associated to heterosis in litter si ze. Additional animals with extreme high and low litter sizes were then typ ed for four DNA markers on chromosome 19 and used for QTL mapping. A QTL wa s identified for litter size in segment D19Mit28 - D19Mit99 with a maximum at 15 cM (p less than or equal to 0.05). The QTL explains about 11 % of the phenotypic variance in the F-2 generation. With a degree of dominance of 4 .09 the QTL shows that superdominance can explain heterosis in litter size.