that the X chromosome determines sex, was wrong and that sex determination is, in fact, due to the presence or absence of the Y chromosome. In the early 1920s, Theophilus Painter determined that X and Y chromosomes determined sex in humans (and other mammals).

The chromosome was given the name "Y" simply to follow on from Henking's "X" alphabetically. The idea that the Y chromosome was named after its similarity in appearance to the letter "Y"