Morning glories can have blue, purple, or red flowers. flower color is controled by two genes. blue color is produced when at least one dominant allele is present at both genes, a and b. purple flowers result when a dominant allele is present at only one of the two gene loci, a or b. flowers are red when the plant is homozygous for two recessive alleles at each gene (i. e., aabb). two pure-breeding purple strains are crossed, and all the f1 plants have blue flowers.