Single- versus Multi-locus trait distributions

    A trait determined by a single locus with two alternative alleles (A and a) occurs in three (31) genotypic classes (AA, Aa, and aa). These correspond to three distinct phenotypes, if A is co- or semi-dominant with a. As the number of contributing loci increases, e.g., from five loci (A1 A5) with 35 = 243 possible phenotype classes or n loci with  3n, the character distribution becomes essentially continuous, and follows the normal distribution.

    Note that a multi-locus gene model satisfies the three requirements for a normal distribution: (1) many factors that (2) contribute independently to the feature, with (3) equal effect.

Figure ©2002 by Griffiths et al.; all text material ©2014 by Steven M. Carr