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Testis-determining factor (TDF) is a general term for the gene (or product thereof) that results in maleness in humans and some other species. TDF can also apply to several genes, depending on the specific sex determination
Current thinking is that certain genes will cause the chemical reactions that result in the development of testes. Embryos are gonadally identical, regardless of genetic sex, until a certain point in development; then the testis-determining factor causes male sex organs to develop, while lack of this factor will cause the embryo to develop as physically female.
The TDF factor is encoded by the SRY gene located in the Y chromosome. It is a DNA binding protein, that enhances other transmission factors, or is a transcription factor itself. Its expression directly or indirectly causes the development of primary sex chords, which will later develop to seminiferous tubules. These chords form in the central part of the undifferentiated-yet gonad, turning it into a testis. The testis then starts secreting testosterone and the Anti-müllerian hormone.
Older texts were discussing the role of the HY antigen in the control of testicular development, but this was later disproven.
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