In the fetus there are two primitive gonads and whether they are activated as male or as female gonads depends on the chromosomes. By inference from the fruit fly (Drosophila), it was thought that the X chromosomes were the active sex-determining chromosomes. In Drosophila, the autosomes are "male," and two X chromosomes counter them to result in a female, one being insufficient to do so. The Y chromosome is inactive. In man, however, the Y is the sex-determining chromosome. XX is thus female, and XXY or XXXY (types of Klinefelter's syndrome) is male. In the seventh week of fetal life, there is gonad differentiation, and, at that time, sex can also be determined by the presence of chromatin bodies in the cells.
Once having activated the gonads, the chromosomes apparently cease to influence normal sex development, and subsequent sexual differentiation in the fetus seems to be under the control of hormones from the fetal gonads.
In a series of classical experiments, Jost (1953) demonstrated