A plant breeder is trying to develop a deep purple violet. She knows the violets come in deep purple, light purple, and white. She crosses two violets hoping to produce only the deep color. Which parent plants should she use?

The exposed example might be a case of incomplete dominance. Incomplete dominance is a condition where neither of the alleles completely dominates over the other one. Dominant alleles cannot completely cover up the recessive alleles. Descendents possess an intermediate phenotype between the two parental phenotypes and not the dominant one, which would appear if this would be the case of complete dominance.

In the example, deep-purple violet (Genotype PP) and white violet (Genotype pp) might be the homozygote plants, while light-purple violet (Genotype Pp) might be the heterozygote plant -the intermediate phenotype-.

If the plant breeder wants to produce only deep-purple violets, she needs to cross deep-purple flowers to get 100% homozygote plants for that color. This is:

If she crosses deep-purple (PP) and light-purple plants (Pp), the descendants will be 2/4 deep-purpled (PP) and 2/4 light-purpled plants (Pp).

If she crosses deep-purple (PP) and white plants (pp), all the descendants will be light-purpled (Pp).

If she crosses light-purpled plants (Pp) with white plants (pp), the progeny will be 2/4 light-purpled plants (Pp) and 2/4 white plants (pp).

If she crosses two white plants (pp) she will get the whole progeny white (pp).

If she crosses two light-purple plants (Pp) she will get 1/4 deep-purpled plant (PP), 2/4 light-purpled plant (Pp), and 1/4 white plants (pp).

And finally, if she crosses two homozygotes for deep-purple plants (PP), 100% of the progeny will be deep-purpled plants.