Effects of alleles at the agouti locus on odontometric traits in the C57BL-6 strain of house mice.

UANTITATIVE geneticists have long been concerned with the nature of the Q genes controlling metric characters. Some years ago a fundamental distinction was made between these “minor” genes and the “majoryy genes affecting qualitative characters (MATHER 1943). Although this distinction was convenient at first, it has broken down in recent years. It now appears likely that minor genes (polygenes) are normal alleles of the major genes ( WADDINGTON 1943; LERNER 1954; GRUNEBERG 1955; FALCONER 1960). Although the effects of minor genes on quantitative characters generally are difficult to detect individually, a number of studies have attempted to show major gene effects on quantitative characters. Unfortunately, the experimental data accumulated are somewhat rare and scattered (often being simply observed in investigations with other objectives) so that the extent of effects of major genes on a variety of metric traits is not known. Even among those studies which have been performed, the data are not always conclusive. This is chiefly owing to the requirements for proof of such effects, particularly the availability of a uniform genetic background. Some of the earliest studies concerning the effect of major genes on quantitative characters were those involving various coat color alleles in rodents (FELDMAN 1935; GREEN 1935; CASTLE et al. 1936; CASTLE 1941 ; MACARTHUR 1949; BUTLER 1954). Although different color genotypes exhibited differences in body weight, in most instances it was not clear whether this was due to the pleiotropic effect of these genes o r to the effects of other genes linked to these latter (HEDRICK and COMSTOCK 1968). In an extensive series of studies, GRUNEBERG (1963 for a review) found effects for a number of mutants in the mouse on numerous quasi continuous characters (phenotypically qualitative but with an underlying quantitative basis). The basic objective of the investigation reported in this paper was to detect differences (if any) in molar widths among six classes of house mice each of which differed solely in the particular combination of alleles possessed at the agouti locus. Molar widths were chosen as especially favorable quantitative characters since they are essentially age independent, and because of the extensive collateral genetic information on them (BADER 1965a; BADER and LEHMANN 1965 ; LEAMY and BADER 1968 j WALLACE 1968).