Incomplete Dominance

Incomplete dominance is an allelic interaction in which one allele is not completely dominant over the other, so the heterozygote shows a phenotype intermediate between the two homozygous parents - genuinely different from either parent, not identical to one of them. German botanist Carl Correns demonstrated this in 1905 using the four o'clock plant (Mirabilis jalapa): crossing a pure-breeding red-flowered parent (R1R1) with a pure-breeding white-flowered parent (R2R2) produced an F1 that was entirely pink (R1R2), an intermediate colour matching neither parent. Selfing these pink F1 plants gave an F2 in which phenotypic and genotypic ratios were identical - 1 red : 2 pink : 1 white, or genotypically 1 R1R1 : 2 R1R2 : 1 R2R2 - the reappearance of the original pure red and white phenotypes in F2 is the key proof that the alleles themselves stayed discrete and unaltered rather than blending permanently, upholding Mendel's Law of Segregation even though the F1 heterozygote looked like a blend. At the molecular level, the R1 allele encodes a functional enzyme that synthesises red pigment; the R2 allele encodes a defective version of that enzym …