The foxglove, Digitalis purpurea, is a common plant in many parts of the world. Fig. 5.1 shows a foxglove. [Figure 5.1] Flower colour in foxgloves is controlled by two genes that interact with each other. • Dominant allele M codes for an enzyme involved in the production of a purple pigment. • Recessive allele m codes for a non-functioning enzyme so no purple pigment is produced, resulting in a white colour. • Dominant allele D interacts with allele M to produce dark purple flowers. • Recessive allele d does not interact with allele M. • Neither allele D nor allele d interact with allele m. A double homozygous foxglove with dark purple flowers was crossed with a double homozygous recessive foxglove with white flowers. All the offspring had dark purple flowers.