This question is poorly asked...
When talking about genes, traits, and stuff like that, people use letters to describe characteristics. If I were to say that I was describing people with eye colors, I would use the letter of the dominant color over the recessive color. (i.e. Brown would be "B" and Blue would be "b"." The dominant trait would be capitalized, and the recessive trait would be the letter of the dominant trait, but it would be lowercased.