When referring to height in terms of genotypes, is there a reason why v and v are used to commonly represent the alleles?

Explanation:

Geneticists use letters to represent alleles of different characters.

(Alleles: one of the two or more alleles  of a character.)

A single letter is used to represent both the alleles of a particular character.

A capital letter represents a dominant allele, while a small letter represents a recessive one.

The letters do not just have one standard representation. same letter can be used to represent different characters. However, the letters may relate to the allele. For example: the letter 'B' can be used to represent Brown eyes.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.