wild-type crossed with black, vestigial-winged flies produce heterozygous F1 dihybrids, all of which appear wild-type.

A testcross of F1 females with black, vestigial-winged males produced 2,300 F2 offspring.

  1. If the 2 genes were on different chromosomes, independent assortment should yield equal numbers of the 4 types of F2.

  2. If the 2 genes were on the same chromosome, each allele combination (B+ vg+ and b vg) should stay together and only yield parental phenotypes in the F2.

  3. The high percentage of parental phenotypes in the F2, with some non-parental (recombinant) phenotypes imply the 2 genes are physically close to each other on the same chromosome.