For a dihybrid cross, how do you find out what proportion of offspring are expected to have a certain genotype?

The first step in tackling this is drawing a genetic diagram (which I would draw on the whiteboard). Say we are looking at a cross between two parents with genotype AaBb, so the cross would look like AaBb x AaBb. Simply write, under each parent’s genotype, all the possible genotypes of gametes that each parent can produce. In this case, those would be AB, aB, Ab, ab. Then, drawing a Punnet square (which I would also draw) is a nice way to visualise and answer this question. This is a table of all possible combinations of gametes, taking one from each parent. Simply write all the gamete genotypes from parent 1 on one side of the table and the ones from parent 2 on the other side. In this case, this would lead to a 4x4 table, which you can fill cell by cell by combining the two gamete genotypes on the sides that correspond to each cell. Then, you can simply count the number of times your desired genotype occurs in the table and divide that by the total number of cells in the table. For example, for AaBb, the number of times it occurs is 4, and the total number of cells in the table is 16, so the expected proportion is 4/16 = 1/4 = 25%. aabb only occurs once, so the expected proportion is 1/16 = 6.25%