In one experiment, Morgan created fruit flies having (in Mendel's notation, with modern terminology) genotypes WwYy, where W = red (normal) eyes, w = white (mutant) eyes, Y = grey (normal) wings, and y = yellow (mutant) wings. Note that geneticists use the term "wild-type" instead of normal. To get these hybrid flies, Morgan crossed normal flies (WWYY) with double mutant (wwyy) flies.
He then backcrossed these hybrid flies to the double mutant parental type, so that he could infer the constitution of the offspring immediately (why?). Following Mendel, we would expect the gametes produced by such flies to have constitution WY, Wy, wY, and wy with equal probability 1/4. Instead Morgan found the following:
Apparently the grandparental contributions were strongly linked in the gametes. (In fact both these loci are on the fly's X chromosome, which makes the traits sex-linked.)
In 1911 Morgan postulated that this deviation from independent segregation (linkage) was due to exchanges between homologous chromosomes during meiosis, a possibility that had been foreshadowed by De Vries. With this, we turn to meiosis.