After 40 generations of divergent selection for litter size in mice, the three high lines that had plateaued (Joakimsen & Baker, 1977; Vangen, 1990) were randomly mated for two generations to form the base population for 22 new generations of selection for high first parity litter size in different maternal environments. The lines H4, H8, H12 and HI were selected from litters standardized to 4, 8 and 12 pups at day one in the first three lines and from unstandardized litter size in the Hl-line. A control line HK was formed from the same base. The old control line K was also maintained (gen. 42 + 22 gen.) . In a line that had been selected for low litter size (L) and where realized heritability was zero from generations 20-40, selection was reversed and the selection criterion became the same as in the high lines H4, H8, H12 and HI. In the lines HK, K and L litter size was standardized to 8 pups at day one. After generation 16 standardization level was changed to 8 pups in all high lines as well.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XIII. Plenary lectures, molecular genetics and mapping, selection, prediction and estimation., , 341–343, 1990
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