Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Another exception to Mendel's laws is a phenomenon called codominance . For example, our blood type shows codominance. Do you know what your blood type is? Are you A? O? AB? Those letters actually represent alleles. Unlike other traits, your blood type has three alleles, instead of two!. Answer: alleles
A:
What do the letters in our blood types represent?