Over 20 years ago, D. S. Falconer and others launched an important avenue of research into the
quantitative of body size growth in mice. This study continues in that tradition by locating
quantitative trait loci (QTLs) responsible for murine growth, such as age-specific weights and
growth periods, and examining the genetic architecture for body weight. We identified a large
number of potential QTLs in an earlier F2 intercross (Intercross I) of the SM/J and LG/J inbred
mouse strains. Many of these QTLs are replicated in a second F2 intercross (Intercross II) between
the same two strains. These replicated regions provide candidate regions for future fine-mapping
studies. We also examined body size and growth QTLs using the combined data set from these
two intercrosses, resulting in 96 microsatellite markers being scored for 1045 individuals. An
examination of the genetic architecture for age-specific weight and growth periods resulted in
locating 20 separate QTLs, which were mainly additive in nature, although dominance was found
to affect early growth and body size. QTLs affecting early and late growth were generally distinct,
mapping to separate chromosome locations. This QTL pattern indicates largely separate genetic
and physiological systems for early and later murine growth, as Falconer suggested. We also found
sex-specific QTLs for body size with implications for the evolution of sexual dimorphism.