There isn't a specific set of letters used to represent heterozygous. Instead, it's represented by using two different letters to indicate the alleles.

Here's how it works:

* Alleles: Different versions of a gene.

* Heterozygous: Having two different alleles for a specific trait.

Example:

* Eye color: Let's say the allele for brown eyes is "B" and the allele for blue eyes is "b".

* Heterozygous for eye color: A person with one "B" allele and one "b" allele would be represented as Bb.

Important: The specific letters used depend on the trait and the alleles involved.