We already know the laws of inheritance, but what substance made up or carry on genes remained in the wind. Another scientist, Morgan who used to sceptical to the chromosome theory provided the first definitive evidence obtained from the results of the search on the fruit fly  to prove his suspicion.  He was reluctant to draw this conclusion that certain traits tend to be inherited together is the result that the genes governing these traits are on the same chromosome on which the genes are arranged in linear fashion. However,  recombination would happen according to the distance separating two genes on a chromosome, the farther apart the more likely such recombination will be.