The egg of every species of animal or plant carries a definite number of bodies called chromosomes. The sperm carries the same number. Consequently, when the sperm unites with the egg, the fertilized egg will contain the double number of chromosomes.

In 1915 Morgan Sturtevant Calvin Bridges and H. George Beadle returned to Caltech to replace Morgan as chairman of the department in 1946. E.

These discoveries formed the basis of the modern science of genetics. In his famous Fly Room at Columbia University Morgan demonstrated that genes are carried on chromosomes and are the mechanical bases of heredity. Following the rediscovery of Mendelian inheritance in 1900 Morgan's research moved to the study of mutation in the fruit fly Drosophila melanogaster.