A purple flowering plant can be obtained by crossing pure red and blue plants which does not obey the law of dominance of Mendel and obeys the law of incomplete dominance.
Suppose the genotype of a red plant is RR and that of the blue plant is BB. When these plants are crossed the offspring produced would be purple with genotype RB. both the R and B genes expresses its characters partially and the purple flower is formed by the law of incomplete dominance.
