RFLPs for somatotropic genes identify quantitative trait loci for growth in mice

Restriction fragment length polymorphisms for somatotropic genes were tested for associations with body weight and postweaning growth rate in mice. Polymorphisms for growth hormone (GH) and insulin-like growth factor 2 (IGF-2) genes were identified in stock population lines which had been subjected to long-term selection for high 42-day body weight (H lines) or randomly mated (FP and C lines). Two F2 populations of mice (5F2 and MF2) were generated from crosses between a single H line of mice and two unselected control lines and subsequently, two divergently weight selected sublines were generated from each F2 population. The GHh allele which had originally been fixed in three of four H lines and absent from all FP and C lines was found to have a significant (P less than 0.01) effect on 42-day weight and postweaning growth rate in the F2 populations. However, GHh was associated with lower 42-day weight in the F2 populations, suggesting that the positive association between GHh and weight in the stock population was unique to the high weight selected genetic background of those lines. In agreement with this, the frequency of GHh increased in sublines selected for high 42-day weight and decreased in sublines selected for low 42-day weight. The IGF-2H5 allele was associated with higher weights in a sex-dependent manner in 5F2. In the high selected subline derived from 5F2, a significant increase in the frequency of IGF-2H5 was observed. Therefore this allele, in contrast to GHh, appears to be a positive indicator of growth irrespective of genetic background.