Final answer:
One-fourth of the F1 generation from the testcross Aa Bb × aa bb are expected to be homozygous recessive (aa bb) for both the A and B alleles.
Step-by-step explanation:
In the testcross Aa Bb × aa bb, to determine the proportion of individuals in the F1 generation that are homozygous for both genes, we must consider each gene separately. The Aa parent can provide either the A allele or the a allele, and the Bb parent can provide either the B allele or the b allele. Given that the testcross is with a aa bb homozygous recessive individual, the offspring can only receive a and b alleles from this parent. The fractions of homozygous recessive offspring for each gene would be ½ for A (aa) and ½ for B (bb). Therefore, using the product rule to combine these probabilities, we get ½ × ½ = ¼. So, the final answer is 1/4 of the F1 generation are expected to be homozygous recessive (aa bb) for both A and B alleles.