Mendel's peas. Historical artwork of the peas (Pisum sp.) used by Gregor Mendel (1822-1884) in his experiments into heredity. He cross-bred peas that produced yellow (A) and green (B) peas. This produces a generation in which the peas are yellow (C). This is because the colour of the peas is controlled by an allele in which yellow colour is dominant. An allele is a pair of genes that have different effects on the same characteristic, for instance controlling pea colour. By breeding the C generation peas together, Mendel found that the next generation had a mixture of pea colours (D). The ratio of yellow to green peas was 3:1. His work formed the basis of genetic theory.