Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried.

Drosophila melanogaster

$(a)$ Male $(b)$  Female

Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.

Experimental verification of the chromosomal theory of inheritance by Thomas Hunt Morgan and his colleagues, led to discovering the basis for variation that sexual reproduction produced. 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.

There was a clear differentiation of the sexes i.e. male (smaller) and female (bigger).

It has four pairs of chromosomes which differ in size.

It has many types of hereditary variations that can be seen with low power microscope.