Final answer:
In a testcross involving an organism of genotype Aa, the genotype of the other individual is always aa, which helps determine the genetic composition of the Aa individual.
Step-by-step explanation:
If an organism of genotype Aa is used for a testcross, the genotype of the other individual used in the cross is aa, which is a homozygous recessive individual. This approach is used to determine the genotype of an organism expressing a dominant trait, as the testcross will reveal whether the organism is a homozygote or a heterozygote for that trait.
When performing a testcross in this manner, all offspring will display the recessive phenotype if the tested individual is a homozygote dominant (AA), whereas a 1:1 ratio of dominant and recessive phenotypes will appear if the tested individual is a heterozygote (Aa).