Passage of information from parents to offspring — practice question

The foxglove, \textit{Digitalis purpurea}, is a plant commonly found in many regions of the world. Fig. 5.1 shows a foxglove. In foxgloves, flower colour is determined by two genes that interact with one another. 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, giving 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 of the offspring had dark purple flowers.