Curt Stern provided the first cytological evidence of genetic crossing over in 1931 through experiments with Drosophila. He used a female fly with two distinct X chromosomes - one normal and one broken into segments. These chromosomes contained different alleles for eye color and shape. His analysis of offspring from crosses showed recombinant chromosome types could only arise through exchange of segments between homologous non-sister chromatids, proving that genetic crossing over involves an actual exchange of chromosome segments. Similar results were later found in maize by Creighton and McClintock, firmly establishing the cytological basis of genetic recombination.