Genetic and environmental influences on serum levels of human C4

In a family study, levels of the C4 component of complement measured in 195 individuals show a continuous normal distribution. There is a slight but significant age effect, especially in females. Heritability is estimated at 62%. The effect of environment, however, remains substantial, and it is suggested that at some time during development the mechanism controlling the C4 levels establishes a general steady-state level whose effects remain after the divergence of sib environments with the approach of adulthood.