We have shown that when two genes are on separate chromosomes or are on the same chromosome and located 50 or more recombination units apart, the testcross AaBb x aabb ratio will be ¼AaBb:¼aabb:¼aaBB:¼AAbb.
Linkage distorts the above ratio in the progeny of the testcross. To detect linkage in the testcross we need to compare the expected ratio for unlinked genes to the ratio that is actually observed. Too large a discrepancy between the expected ratio for unlinked genes and the observed ratio can be detected by the X2 test. If the X2 test is significant, the results could be explained by poor viability for individual gene pairs, or explained by linkage. To distinguish the cause of the distorted genetic ratios, we must test for the expected segregation for each locus and then test for linkage.