Whose research first identified the X and Y chromosomes as sex chromosomes in humans?

Thomas Hunt Morgan’s fruit-fly research in the early 1900s helped establish that chromosomes carry heredity and that one pair can determine sex. In humans, the X and Y chromosomes are the sex chromosomes, with typical female karyotypes being XX and typical male karyotypes being XY. The chromosome basis of sex determination was clarified in the decades after the rediscovery of Mendel’s work and became a central part of modern genetics. The human Y chromosome is much smaller than the X chromosome and carries key genes such as SRY, which triggers male developmental pathways.