We must always use a single letter to represent a single gene, and different forms of that letter to represent any different versions (alleles) of that gene. The convention for representing alleles is that, if the allele is a dominant one, then the capital form of the gene symbol should be used whereas, if the allele is a recessive form, then the lower case form of the gene letter should be used. Therefore, for example, two alleles of a gene should never be referred to as B and D.
