Although gene maps are a relatively recent (past thirty years) means of locating genes, geneticists of the early twentieth century had prototype gene maps. American zoologist and geneticist Edmund Beecher Wilson (1856–1939) and his colleagues were the first to demonstrate that the genetic differences between males and females were due to a special pair of chromosomes in the cell. American evolutionary biologist, geneticist, and embryologist Thomas Hunt Morgan (1866–1945), American geneticist Calvin Bridges (1889–1938), and their colleagues were able to place a gene known to be inherited differently by males and females onto one of the sex chromosomes. This was the beginning of the first gene map.