Genetic and Environmental Variation in the Growth of Mice

Abstract

This chapter discusses the assessment of the relative importance of genetic and environmental factors as determinants of the body weights of mice at different ages. The relative importance of heredity and environment as determinants of any continuously varying character, such as body weight, can be evaluated by partitioning the total phenotypic variance into genetic and environmental components. The total genetic variance cannot be estimated without the use of special stocks, but part of the genetic variance — the additive portion — can be estimated from the degree of resemblance between relatives. The heritability, which is the additive variance as a proportion of the total phenotypic variance, provides a lower limit to the degree of genetic determination. An analysis of the variance applied to half-sib and full-sib families of mice provided estimates of three main components of variance of body weight at weekly intervals from birth to 8 weeks. The components were the additive genetic variance, the environmental variance common to litter-mates, and the environmental variance within litters. The two environmental components increased from birth to weaning and then declined as a result of compensatory growth. The genetic component increased continuously to 8 weeks. The heritability was 10% or lower in the early weeks, but after weaning it increased in consequence of the reduced environmental variance and reached a value of about 40% at 8 weeks.