Incomplete dominance, co-dominance and multiple alleles

In incomplete dominance the heterozygote looks intermediate. In the dog flower (snapdragon, Antirrhinum sp.), red (RR) × white (rr) gives pink F₁ (Rr), and selfing F₁ gives F₂ of 1 red : 2 pink : 1 white.

In incomplete dominance the phenotypic and genotypic F₂ ratios become the same (1:2:1), because every genotype has its own look.

Dominance can be explained at the level of gene products: a normal allele usually makes a working enzyme, while a modified allele may make a normal enzyme, a less efficient one, a non-functional one, or none. Where the modified allele gives no working product, it behaves as recessive.

Dominance is not an absolute property of an allele; it depends on the gene product and on which phenotype we choose to look at. In pea seeds, one gene controls starch synthesis: BB seeds have large starch grains and are round, bb seeds have smaller grains and are wrinkled, and Bb seeds are round but with intermediate grain size. Judged by seed shape, B is dominant; judged by grain size, the alleles show incomplete dominance.

In co-dominance the F₁ shows both parental characters fully. Human ABO blood groups are the NCERT example.

The ABO gene I has three alleles, Iᴬ, Iᴮ and i. Iᴬ and Iᴮ make slightly different forms of the sugar polymer on the red blood cell surface, while i makes none. Iᴬ and Iᴮ are fully dominant over i but co-dominant with each other.

With three alleles there are six genotypes and four blood groups: A (IᴬIᴬ, Iᴬi), B (IᴮIᴮ, Iᴮi), AB (IᴬIᴮ) and O (ii).

ABO is also an example of multiple alleles, where more than two alleles govern one character. Since one person carries only two alleles, multiple alleles can be detected only by studying a population.