The summer squash plant, Cucurbita pepo, produces edible fruits that vary in shape. [Figure 2.1] shows the fruits of three different varieties of squash plants. Fruit shape in squashes is controlled by two genes, A/a and B/b, that are located on different chromosomes. A disc-shaped fruit is produced when both dominant alleles, A and B, are present. A spherical fruit is produced when either allele A or allele B is present, but not if both A and B are present. A long fruit is produced when both allele A and allele B are absent.