When writing a key, consider how many dominant alleles there are. An allele is considered dominant if it shows up in the offspring. In both examples above, both alleles show up in the offspring. They could stay separate as with co-dominance or they could blend together as with incomplete dominance. In both cases, there are two dominant alleles so there will be two letters in the key. For the fish example, you will use 'R' for red and 'B' for blue. The offspring is 'RB' for red and blue. In the flower example, you use 'R' for red, 'W' for white, and 'RW' for pink.