In diploid organisms, such as ourselves, the vast majority of genetic loci have two alleles, as discussed above. For example, the two loci in Figure 3.8 part (i) in the parent (left) have alleles A and a at the first locus, and B and b at the second, but there are also many loci at which the two alleles are identical. To use a real example studied by William Bateson, Edith Saunders and Reginald Punnett at the beginning of the 20th century (Bateson et al., 1905; Lobo and Shaw, 2008), let's for a moment look not at a human but at a sweet pea (Lathyrus odoratus), and more precisely at a locus that influences the flower's colour, with allele A resulting in purple flowers while allele a results in red flowers.

Dominance and recessiveness

But how can we talk about the effects of a single allele, such as A, in diploid organisms? We can look at homozygous individuals that happen to have two copies of the same allele at this locus and observe that AA individuals produce purple flowers while aa ones have red flowers. But what happens to heterozygous individuals that have different alleles at the same locus? In this particular case, the heterozygous Aa individuals (the order in which the alleles are written, Aa or aA, is irrelevant here) produce purple flowers, just like the homozygous AA individuals.