HERITABILITY OF DENTAL CHARACTERS IN THE HOUSE MOUSE

Abstract

The currently popular synthetic theory of evolution has been forged from data and concepts contributed by both the neontologist and the paleontologist. In general, however, the organisms and characters studied by the former have little in the way of an historical record, while the groups and organ-systems of paleontological interest have rather infrequently (e.g., Olson and Miller, 1958; Berry, 1964) been subjected to modern microevolutionary analyses. Change in the molar teeth of mammals is one of our best historical evidences of evolution. Although dental morphology has received considerable attention from students of Recent and fossil mammals, knowledge of the hereditary and environmental components of dental variation remains meager in spite of the potential value for studies of evolutionary mechanisms. Study of the variation in the dentition of extant rodents would seem to offer a particularly attractive opportunity to subject to genetical analysis characters which do possess a reasonable fossil record. Many, if not most, of the traits utilized by the systematist are of a quantitative, i.e., continuously varying, nature. In recent years a fundamental approach to such characters has been to partition their phenotypic variation into a number of causal categories (Falconer, 1960). The primary step is, of course, the identification of the relative contributions of the heredity and the environment. These, in turn, may be divided into a number of subclasses of causation. In a recent study (Bader and Lehmann, 1965) the hereditary and environmental components were estimated for three dental traits in the house mouse by appropriate comparisons of the phenotypic variation in wild, randombred, inbred, and hybrid populations. The present study is concerned primarily with the heritability of these characters. The heritability is the proportion of the total phenotypic variance due to genetic factors acting in an additive fashion, or it may be regarded as the reliability of the phenotype as an indicator of the breeding value of the individual and the population (Falconer, 1960). The total hereditary, or genotypic, variance includes both the additive and non-additive genetic fractions. Estimation of the heritability of a trait depends essentially upon the measurement of the degree of phenotypic resemblance among animals of known genetic relationship. The most commonly used methods are those of regression of offspring on one or both parents and fullor half-sib correlation. The relative merits of the various analyses are examined in detail by Falconer (1960, 1963). The method employed in the present study is that of the paternal half-sib analysis utilizing the hierarchal, or nested, analysis of variance. This design enables one to obtain an estimate of the heritability relatively free of non-additive genetic variance, to obtain an estimate of the non-heritable maternal effect, if any, and to make all measurements on one generation. It is more efficient, in the statistical sense, than the regression analysis when the heritability is low (Robertson, 1959a).