Testis Determining Factor (TDF) is a protein encoded by the SRY gene located on the Y chromosome. It plays a crucial role in male sex determination by initiating the development of the testes during embryonic development. TDF triggers a cascade of genetic events that lead to the formation of male reproductive structures. In the absence of TDF, the default pathway leads to the development of female characteristics. This means that if the SRY gene is not present or functional, the embryo will typically develop ovaries and female reproductive anatomy. TDF is essential for the differentiation of male gonads and the subsequent production of male hormones.