The plant needs to have two white-causing alleles to get white flowers. If any one of several alleles gives rise to a certain phenotype regardless of which allele is present on the other chromosome, then these alleles are said to be co-dominant. Someone who is AB expresses both the A and the B antigens on the surface of their red blood cells. Sometimes an intermediate phenotype results from crossing two organisms that are homozygotes but with different alleles at a locus. The book discusses crossing white and red snapdragons and getting pink ones in the first generation, then red, white, and pink in the second generation.
Pink are heterozygotes.
In this example, the gene encodes for a protein which is an enzyme necessary to generate the red pigment in the flowers. One allele, giving rise to the white flowers, either fails to make the protein or makes a defective protein, so no red pigment is made. The other allele makes a functional enzyme. Sometimes an allele gives rise to several different character traits. The word pleiotropic comes from pleio meaning more and tropic from a word meaning turn.