Thomas Hunt Morgan (Lexington, Kentucky, 1866 – Pasadena, 1945), a zoologist at Columbia University, demonstrates through experiments with Drosophila melanogaster (fruit fly) that genes are physically located on chromosomes and that gene linkage — the tendency of certain genes to be inherited together — is explained by their proximity on the same chromosome. He publishes the results in 1910 in Science ("Sex Limited Inheritance in Drosophila"). His laboratory — Columbia's "Fly Room" — builds, with his students Alfred Sturtevant, Calvin Bridges, and Hermann Muller, the first genetic maps in history, establishing the distance between genes as a function of recombination frequency. Morgan receives the Nobel Prize in Physiology or Medicine in 1933. Chromosomal theory articulates the physical substrate of Mendel's laws: chromosomes are the material support of hereditary factors.