born April 6, 1870, Clayton, Calif., U.S.
died Jan. 17, 1946, Swarthmore, Pa.
U.S. zoologist.
He received his Ph.D. from the University of Kansas. His study of the mechanisms of heredity led to his hypothesis (1901) that an extra, or accessory, chromosome determined sex. The discovery of the sex-determining chromosome provided some of the earliest evidence that a given chromosome carries a definable set of hereditary traits. McClung also studied how the behaviour of chromosomes in the sex cells of different organisms affects their heredity.