American zoologist whose study of the mechanisms of heredity led to his 1901 hypothesis that an extra, or accessory, chromosome was the determiner of sex (sex chromosome). The discovery of the sex-determining chromosome provided some of the earliest evidence that a given chromosome carries a definable set of hereditary traits. He also studied how the behaviour of chromosomes in the sex cells of different organisms affects their heredity.

McClung was educated at the University of Kansas (Ph.D., 1902), where he became a professor and later dean of the medical school. In 1912 he went to the University of Pennsylvania as head of the zoological laboratories, a position he held until his retirement in 1940.