An Aa by Aa Punnett square gives the exact genotype ratio.
A Punnett square enumerates gamete pairs
For Aa crossed with Aa, put one parent's gametes across the top and the other's down the side. The body cells are the offspring genotypes.
Aa×Aa
The genotype ratio is one to two to one
Counting the table gives genotype ratio 1:2:1 for AA, Aa, and aa.