Questions: Which of these is a testcross? A? x aa A ? × A a

The answer is the first one (or A): A? x aa

Explanation for each option:

1. A? x aa: This is a testcross. A testcross involves crossing an individual with an unknown genotype (A?) with a homozygous recessive individual (aa). The purpose of a testcross is to determine the genotype of the unknown individual based on the phenotypes of the offspring.

2. $a a \times a a$: This is not a testcross. This is a cross between two homozygous recessive individuals, which will only produce homozygous recessive offspring.

3. $A$ ? $\times A A$: This is not a testcross. This is a cross between an individual with an unknown genotype (A?) and a homozygous dominant individual (AA). The offspring will not provide clear information about the unknown genotype.

4. $A ? \times A a$: This is not a testcross. This is a cross between an individual with an unknown genotype (A?) and a heterozygous individual (Aa). The offspring will have a mix of genotypes, but it will not clearly reveal the genotype of the unknown individual.

5. A? $\times$ A?: This is not a testcross. This is a cross between two individuals with unknown genotypes. The offspring will have a mix of genotypes, but it will not clearly reveal the genotype of either parent.

Summary: The correct answer is the first one (or A): A? x aa. This is a testcross used to determine the genotype of an individual with an unknown genotype.