Final answer:
The Testis-determining SRY protein on the Y chromosome instigates the development of male sexual characteristics by initiating testes formation and testosterone production in human XY embryos.
Step-by-step explanation:
In human XY embryos, the Testis-determining SRY protein activates the development of embryonic testes which then secrete hormones that trigger the development of male sex organs. The SRY gene on the Y chromosome begins the complex process of male sex differentiation during the second month after fertilization. This gene is essential for initiating a cascade of genetic activities that lead to the formation of testes from the bipotential gonadal tissue. These testes then produce testosterone, which is vital for the development of male sexual organs. Without the presence of the SRY gene and subsequent testosterone production, the default pathway leads to the development of female reproductive structures.