the driver behind sex determination, such that if the male principle was insufficiently expressed during reproduction, the fetus would develop as a female. 20th century genetics nettie stevens (working with beetles) and edmund beecher wilson (working with hemiptera) are credited with independently discovering, in 1905, the chromosomal xy sex-determination system in insects: the fact that males have xy sex chromosomes and females have xx sex chromosomes. in the early 1920s, theophilus painter demonstrated that sex in humans (and other mammals) was also determined by the x and y chromosomes, and the chromosomes that make this determination are carried by the spermatozoa. the first clues to the existence of a factor that determines the development of testis in mammals came from experiments carried out by alfred jost, who castrated embryonic rabbits in utero and noticed that they all developed as female. in 1959, c. e. ford and his team, in the wake of jost's experiments, discovered that the y chromosome was needed for a fetus to develop as male when they examined patients with turner's syndrome, who grew up as phenotypic females, and found them to be x0 (hemizygous for x and no y). at the same time, jacob & strong described a case of a patient with klinefelter syndrome (xxy), which implicated the presence of a y chromosome in development of maleness. all these observations led to a consensus that a dominant gene that determines testis development (tdf) must exist on the human y chromosome. the search for this testis-determining factor (tdf) led a team of scientists in 1990 to discover a region of the y chromosome that is necessary for the male sex determination, which was named sry (sex-determining region of the y chromosome).