2. Partial or Incomplete Dominance. This is a type of dominance in which the heterozygote exhibits a character that is intermediate of the alternative characters carried by the two alleles making up the heterozygous genotype. In this case both alleles equally but partially contribute to the phenotype of the heterozygote.
With reference to the parents of a cross which possess homozygous genotypes (s1s1 and s2s2; p1p1 and p2p2), it is the dominance relation in which the heterozygote (s1s2 and p1p2) exhibits a phenotype that is halfway or a 50-50 mixture (intermediate) of the two parental characters.
As to our hypothetical heterozygote (s1s2), it will exhibit a medium character or halfway between the stem lengths of those exhibited by the s1s1 and s2s2 genotypes while p1p2 will be a yellow-green pod color that is halfway between green and yellow.
The genotypes s1s1, s1s2, and s2s2 will be expressed differently and so with p1p1, p1p2, and p2p2. Therefore, the F2 progeny will consist of three distinct phenotypes with a ratio that is identical to the genotypic ratio, that is, 1:2:1. These phenotypes consist of two parental types (dominant and recessive types) and an additional intermediate phenotype. In contrast, there are only two F2 phenotypes under complete dominance, both of which are parental types.
The effects of incomplete dominance on the phenotypes of the F1 and F2 progeny are further demonstrated below with reference to Mendel’s experiments.
• F1 genotype: s1s2; p1p2
• F1 phenotype: medium height / stem length; yellow-green pod
• F2 genotypes and ratio: 1:2:1 for both stem length and pod color (1s1s1:2s1s2:1s2s2; 1p1p1:2p1p2:1p2p2)
• F2 phenotypes and ratio: 1:2:1 short, medium, and tall (1s1s1:2s1s2:1s2s2); 1:2:1 green, yellow-green, and yellow (1p1p1:2p1p2:1p2p2)
Incomplete dominance is responsible for the pre-Mendelian popular belief that heredity is a “blending” process so that an offspring has features which are a blend of parental characters. It therefore results to the formation of a new phenotype that is distinct from the parental types.
Much later, however, it was realized that the blending process of heredity did not apply in all cases. Thus Mendel (1865) made the following remarks:
Experiments which in previous years were made with ornamental plants have already afforded evidence that the hybrids, as a rule, are not exactly intermediate between the parental species. With some of the more striking characters, those, for instance, which relate to the form and size of the leaves, the pubescence of the several parts, &c, the intermediate, indeed, is nearly always to be seen; in other cases, however, one of the two parental characters is so preponderant that it is difficult, or quite impossible, to detect the other in the hybrid.