Consider a gene that affects flower color. this gene has two alleles, a and a. if aa flowers are red, aa flowers are purple, and aa flowers are blue, how would we describe the effect of these alleles

The allele would be called codominant

When a dominant and recessive allele exist in an organism, only the dominant allele will be expressed. Dominant allele always mask the recessive because the gene code a protein while recessive gene doesn't. When both gene code a protein, they will be codominant and both gene will be expressed. The result could be a mixed effect. In this case, the purple is mixed color of red and blue.