Final answer:
The third step in creating a Punnett square is to pull the factors from the top parent into the boxes below, aligning paternal alleles with those from the maternal side to predict offspring genotypes.
Step-by-step explanation:
The third step in creating a Punnett square is to pull the factors from the top parent into the boxes directly below each factor. For example, if the first parent's factors are B and b placed at the top, you would bring down the B into the first box in each row and the b into the second box of each row. This establishes the paternal contribution to the potential offspring in your Punnett square analysis.
Once the top factors are pulled down, you align them with the factors from the second parent, which have been positioned on the side of the square. This allows you to determine all the possible combinations for the offspring's genotype, which are signified by the letters in the interior boxes of the Punnett square.