A genetic analysis of the mandible and maxilla in the rat.

A quantitative genetic analysis of eight osteometric traits from the oral region together with femur length in the rat are described. The relevant questions being examined are (1) the heritability of each trait; (2) the relative contribution of genetic, maternal environmental, and residual environmental effects to the correlation between the oral traits, and (3) genetic and nongenetic components of the correlation between the oral traits and overall body size. Some quantitative genetic aspects of covarying traits is briefly reviewed with special emphasis on allometric variation. Phenotypic regression coefficients from log-transformed data (= allometry coefficients) of oral traits onto femur length are partitioned into components due to genetic, maternal environmental, residual environmental, and total environmental causes. All phenotypic regression coefficients and all but one based on an environmentally determined covariance component are significantly different from zero, suggesting a substantial body size effect in the oral region resulting from nonheritable causes. However, three genetic coefficients from regressions of oral traits to femur length are not different from zero, indicating a genetic correlation with body size in five traits but not in three others. A principal components analysis was carried out on the phenotypic, genetic, and environmental correlations of the eight oral traits to provide a multivariate depiction of the components of covariation in the maxilla and mandible of the rat. General multivariate effects due to body size together with group or special effects are demonstrated for each component of morphogenesis.