Single- versus Multi-locus trait distributions

A
trait
determined by a single locus with two alternative alleles (A and a) occurs in three (3^{1}) *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 (A_{1} A_{5}) with 3^{5} = 243
possible phenotype classes or n loci with 3^{n},
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.

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