Explain how a testcross can provide evidence for or against linkage.

Testcrosses clarify linkage because each phenotypic class of progeny corresponds to each gamete type produced by the dihybrid parent.

Explanation:

A test cross involves the crossing of an individual with another phenotypically recessive individual so as to determine the zygosity of the former by analyses of the proportions of offspring phenotypes.In order to determine linkage, the test cross shows that if the parentals are more than the recombinants, we can say that the two genes such as b and c  are genetically linked and therefore, they must be on the same chromosome.Also, the test-crosses help to find out which alleles came from which parent.By setting up testcrosses in which one parent is homozygous for the recessive alleles of both genes,we can analyze the gene combinations received in the gametes from the other, doubly heterozygous parent.