Beyond simple dominance, and the dihybrid cross

Not every trait follows simple dominance. Sometimes the F1 looks like neither parent.

• Incomplete dominance: the heterozygote is in between the two parents. In the four o'clock plant (Mirabilis jalapa), red (RR) × white (rr) gives pink (Rr). Selfing the pink plants gives red, pink and white in 1:2:1, so here the phenotype ratio is the same as the genotype ratio.
• Codominance: both alleles show fully, side by side. Red (RR) × white (WW) cattle give roan (RW) calves, with red and white hairs mixed. In the human MN blood group, people with the genotype LᴹLᴺ carry both the M and N antigens: blood group MN.

Mendel also followed two traits at once: a dihybrid cross. Round yellow seeds (RRYY) × wrinkled green (rryy) gave an F1 that was all round yellow, so round and yellow are the dominant forms.

In the F2 he found four kinds of seeds in the ratio 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green. Two of these (wrinkled yellow and round green) were new combinations, seen in neither original parent.

This showed the law of independent assortment: the alleles of different genes are sorted into gametes independently of each other. So an RrYy plant makes four kinds of gametes (RY, Ry, rY, ry) in equal numbers.