Which type of cross is used to determine if two genes are linked?

The testcross is a method used to determine whether an organism exhibiting a dominant phenotype is homozygous or heterozygous for a particular trait. This is accomplished by crossing the individual in question with a homozygous recessive individual for that trait. If the genes in question are located on the same chromosome and are linked, the offspring will show a greater proportion of parental phenotype combinations than recombinant types. In contrast, if two genes are assorting independently (not linked), the expected phenotypic ratio among the offspring will reflect this independence.

A backcross typically involves crossing an F1 individual with one of its parents or with an individual having the same genotype as one of the parents. This method focuses more on identifying traits in subsequent generations rather than explicitly determining linkage.

A monohybrid cross focuses on the inheritance of a single trait and does not provide information about linkage between two genes. Meanwhile, a dihybrid cross examines the inheritance of two traits but, like the monohybrid cross, does not specifically reveal information about the linkage of those traits unless analyzed within the context of a testcross.

Thus, the testcross is the appropriate method to unveil the linkage between two genes through the phenotypic ratios observed in the offspring.