Beyond simple dominance

Not every pair of alleles follows simple dominance.

Incomplete dominance. In the snapdragon (dog flower), crossing red (RR) with white (rr) gives pink (Rr) F₁ flowers: neither allele fully dominates, so the heterozygote is in between. In the F₂, the ratio is 1 red : 2 pink : 1 white. Here the phenotype ratio is the same as the genotype ratio, because every genotype looks different.

Why are some alleles dominant at all? A gene carries the code for a product, often an enzyme. A changed (mutated) allele may make a less efficient enzyme, a non-functional one, or none. In a heterozygote, the normal allele still makes the working enzyme, so the plant looks normal. The changed allele is then recessive.

Co-dominance. Here both alleles show fully in the heterozygote. The ABO blood groups are the classic case. One gene, I, decides which sugars sit on the surface of red blood cells, and it has three alleles:

• Iᴬ makes sugar A; Iᴮ makes sugar B; i makes no sugar.
• Iᴬ and Iᴮ are each dominant over i.
• But Iᴬ and Iᴮ are co-dominant: a person with IᴬIᴮ has both sugars, giving blood group AB.

Since a person carries two of the three alleles, there are six genotypes and four blood groups: IᴬIᴬ and Iᴬi give A; IᴮIᴮ and Iᴮi give B; IᴬIᴮ gives AB; ii gives O. With three alleles for one gene, the ABO system is also an example of multiple alleles. (Any one person can carry only two of them; multiple alleles show up only in a population.)

One gene can even show different kinds of dominance depending on how you look. The pea gene for seed shape controls a starch-making enzyme: BB seeds have large starch grains and are round; bb seeds have little starch and are wrinkled; Bb seeds are round. So for seed shape, B is fully dominant. But the size of the starch grains in Bb seeds is intermediate: for that, the alleles show incomplete dominance.