The chromosome theory of inheritance is the theory that shows genetics are carried by the chromosomes in the DNA of organisms. This theory was put forth by Walter Sutton and Theodor Boveri around 1902. The two men were working separately but at around the same time on the same material.
Despite these two men putting forth the theory, it was several years and many debates later before there was sufficient proof to make it accepted across the board. The tests performed by Thomas Hunt Morgan in 1915 involved fruit fly experiments with genetic linkage. Morgan mated a male white-eyed fly with a female red-eyed fly to try and track the inheritance pattern. The first offspring all had red eyes, as Morgan had predicted, based on the dominant recessive inheritance pattern.
When Morgan changed the base pairs to a white-eyed female and a red-eyed male, however, the results were not what he expected. In this group of offspring the females all had red eyes and the males all had white. This lead him to the logical conclusion that the Boveri-Sutton theory about genes being passed through chromosomes was correct. The white-eyed males of the second pairing showed that the white gene was only found on the X chromosome, meaning that chromosome was where the genetic information was stored.