How would you write an allele that is heterozygous?

Representing heterozygote allele characteristics depends on the nature of the heterozygote.

If one of the alleles is dominant, and the other recessive, the genotype is written as Aa - where the capital letter (A) represents the dominant allele, and the lower case letter (a) represents the recessive allele. The dominant (upper case) allele is always written first.

However, the genotype cannot be written in this way if both alleles are codominant (neither one is dominant or recessive). In this case, the convention is to choose a letter to refer to the flower-color-locus as a whole. You would then add different superscript letters to denote the different alleles.

For example, you could choose the letter A to refer to the flower-color-locus, and then choose C and D as the two co-dominant alleles. This genotype would be written as ACAD.