What pair of letters should appear in Box 1 of a punnet square?

In a Punnett square, the pair of letters that should appear in Box 1 typically represents one parent's genotype for a specific trait. For example, if one parent is homozygous dominant for a trait, you would use two uppercase letters (e.g., AA). If one parent is heterozygous, you might use one uppercase and one lowercase letter (e.g., Aa). The specific letters depend on the alleles being studied.