Morgan's work is based on fruit flies (Drosophila melanogaster). He formulated the chromosomal theory of linkage. He defined linkage as the co-existence of two or more genes in the same chromosome and performed dihybrid crosses in Drosophila to show that linked genes are inherited together and are located on $X-$ chromosome. His experiments have also proved that tightly linked genes show very low recombination while loosely linked genes show higher recombination.