Individual genes affecting growth in body weight and morphology will be identified in mice, mapped onto chromosomes, and their effects quantified. Recent advances in both molecular and quantitative genetics have made it possible for the first time to identify genetic variants with small, quantitative effects on morphological features. Genetic variants of small effect are considered to be the most important variants to developmental and evolutionary change. However, the chromosomal position, pattern of effect, and molecular basis of variation in such genes is virtually unknown. Instead geneticists have had to rely on untested assumptions concerning these factors in developing their models. It is now possible to directly evaluate assumptions about the direct and pleiotropic (or correlated) effects of genes on morphological characters. Specifically, individual genes with quantitative effects on body weight, growth and on a series of morphological features which differ in their timing of growth will be discovered. Based on our previous work, we predict that there will be single genes with either independent or opposite effects on early and late growth. The existence of such genes places important constraints on the evolutionary possibilities open to mammalian development. These genes will be identified using an intercross experiment between the Large (LG/J) and Small (SM/J) inbred strains of mice in which these strains will be crossed to produce a heterogeneous offspring generation which will then be bred to produce a grand- offspring generation in which genetic variants from the LG/J and SM/J strains segregate independently. Regression and interval mapping techniques will be used first to identify molecular markers linked to genes affecting growth and morphology and then to map these genes among the markers and measure their quantitative effect on somatic growth and morphology.
