Single- versus Multi-locus
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,
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