There must be an event to unlink these genes. How to confirm crossing over: creighton and mcclintock: combining cell bio and genetics prove morgan"s theory. Looked at the chromosomes and observed the actual switching. They were able to visually distinguish different parts of the chromosome (knob, extra length) and they knew where particular alleles were located. They were able to follow the exchange of chromosomes during meiosis and correlate it with the displayed traits. Identify genetic distance between the two genes on a chromosome. Recombinant 23, 21 (undergo crossing over events) Unit definition: 1% recombinant progeny=1map unit=1centimorgan (cm) Multiple events separated by distance of recombination. No, it doesn"t make a difference to homozygotes. For flies, crossing over only occurs in females. 3 x-linked mutant genes figuring out gene order from 3-point mapping experiments: perform the cross and count the f2s. These classes represent dcos (double crossover events) E. g. yw+ec is oen of the most rare gene class.