Final answer:
A dihybrid test cross between two genes, such as in pea plants, often yields a 9:3:3:1 phenotypic ratio if the genes are not linked. A Punnett square can be used to visualize the expected offspring's genotypes. If the offspring show a 1:1 ratio, it indicates a heterozygous parent, while a deviation from the 9:3:3:1 ratio suggests linked genes.
Step-by-step explanation:
A dihybrid test cross in genetics refers to a cross between a homozygous recessive individual for two traits and an individual with unknown genotype for those traits. If we consider a dihybrid cross such as between pea plant genotypes AaBb (heterozygous for each gene) and another AaBb, we use a Punnett square to determine the offspring's genotypes. As Mendel demonstrated, this typically results in a 9:3:3:1 phenotypic ratio, given that the traits are not linked and assort independently.
If a dihybrid test cross produces offspring in a 1:1 ratio, it indicates that the parent carrying the unknown genotype is heterozygous for both traits. In other scenarios where genes are linked, the genotypic ratio would deviate from the expected 9:3:3:1 ratio based on independent assortment. For instance, a test cross between EEWW flies (homozygous dominant for both traits) and eeww flies (homozygous recessive for both traits) would result in heterozygous EeWw offspring if the genes are not linked.