Types of modification.

This model illustrates how alleles of modifier and modified genes affect penetrance, dominance modification, expressivity and pleiotropy, and modulate the phenotypes of mutant organisms. Throughout this diagram, the solid curves and lines show the distribution of trait values without the modifier, the dashed curves and lines show the values with the modifier and the vertical line shows the phenotypic threshold. (A) Penetrance: modifiers move the threshold for expressing the trait relative to the distribution of trait values. By moving the threshold, a greater (or smaller) proportion of the mutant homozygotes are affected, thereby increasing (or reducing) penetrance. (B) Dominance modification: modifiers move the threshold for expressing the trait relative to the distribution of trait values. By moving the threshold into (or out of) the range of the heterozygotes, some mutant heterozygotes are (or are no longer) affected, thereby converting a trait with recessive effects into one with semi-dominant effects (or the converse depending on the direction in which the threshold moves). (C) Expressivity: the location of the distribution of trait values for mutant homozygotes or mutant heterozygotes, or both, but not for the wild-type homozygotes is shifted relative to the phenotypic threshold.