Incomplete Dominance

Incomplete dominance is the first, and simplest, of several genuine exceptions to Mendel's Law of Dominance that later geneticists discovered once they extended the study of inheritance beyond Mendel's original seven pea characters to other organisms and other genes. In incomplete dominance, NEITHER of the two alleles at a gene locus is able to completely mask or dominate the other in a heterozygous individual, so instead of resembling one parent exclusively (as a Mendelian dominant character would), the heterozygote displays a distinct, intermediate phenotype that blends visible features of both homozygous parental types. The textbook example, one of the clearest and most frequently cited cases of incomplete dominance in all of introductory genetics, is the inheritance of flower colour in the four-o'clock plant, Mirabilis jalapa (so called because its flowers characteristically open in the late afternoon). When a true-breeding red-flowered Mirabilis plant, genotype RRRR, is crossed with a true-breeding white-flowered plant, genotype RWRW, EVERY plant of the F1 generation produces flowers that are neither red nor white but a distinct, intermediate PINK — a colour that appears in neither parent, produced because the heterozygous genotype (RRRW) manufactures an intermediate quantity of the red floral pigment, roughly half of what the homozygous red parent produces, giving a visibly diluted, pink appearance rather than full red.

When these pink F1 plants are then self-pollinated, the resulting F2 generation segregates into three distinct classes in the ratio of one red-flowered plant, to two pink-flowered plants, to one white-flowered plant (1 red : 2 pink : 1 white). What is particularly instructive about this result, and worth stating explicitly, is that in this F2 generation the PHENOTYPIC ratio (1:2:1) is numerically identical to the underlying GENOTYPIC ratio (1 RRRR : 2 RRRW : 1 RWRW) — a situation that never arises in a straightforward Mendelian monohybrid cross exhibiting complete dominance, where the two different genotypes TT and Tt are phenotypically indistinguishable from each other and therefore collapse into a single "dominant" phenotypic class, yielding the familiar 3:1 phenotypic ratio rather than a 1:2:1 one. In incomplete dominance, by contrast, the heterozygote's own visibly distinct, intermediate phenotype means every genotype can be identified directly just by observing the plant, with no need for a test cross to distinguish a heterozygote from either homozygote. …