T.H. Morgan worked with the tiny fruit flies Drosophila melanogaster for experimental verification of chromosomal theory of inheritance, as they are very suitable for such studies because

Morgan worked with the tiny fruit flies, Drosophila melanogaster, which were found very suitable for such studies. They could be grown on simple synthetic medium in the laboratory. They complete their life cycle in about two weeks, and a single mating could produce a large number of progeny flies. Also, there was a clear differentiation of the sexes – the male and female flies are easily distinguishable. Also, it has many types of hereditary variations that can be seen with low power microscopes.