Final answer:
In a dihybrid test cross, if one parent has the genotype RrYy, the genotype of the other parent would be rryy to determine the unknown genotype of the first parent through the resulting offspring's phenotypes.
Step-by-step explanation:
If one parent in a dihybrid test cross has the genotype RrYy, the genotype of the other parent for a test cross would be rryy. A test cross involves crossing an individual with a dominant phenotype but unknown genotype with an individual that is homozygous recessive for the traits of interest. This helps to determine the genotype of the unknown parent. In the case of a dihybrid, which involves two traits, the test cross would be conducted with a parent that is homozygous recessive for both traits (rryy). This is because we want to see whether the offspring display the recessive traits, which would only be possible if the unknown parent is carrying the recessive alleles.
When discussing Mendelian genetics and dihybrid crosses, the progeny of the F2 generation exhibit a phenotype ratio of approximately 9:3:3:1 for the two traits when both parents are heterozygous for both traits (RrYy x RrYy). However, when one parent is RrYy and the other parent is rryy (as in a test cross), the progeny ratio will be simplified to 1:1:1:1, due to the homozygous recessive nature of the second parent.