Question

in humans, clear sex differentiation occurs not at fertilization but after the second month of gestation. what is the first event of this differentiation?
formation of testosterone in male embryos
formation of estrogens in female embryos
anatomical differentiation of a penis in male embryos
activation of the sry gene in male embryos and development of testes

Explanation:

In human development, the SRY (Sex - determining Region Y) gene on the Y chromosome is crucial for male sex differentiation. It is activated in male embryos (around 6 - 7 weeks of gestation, which is within the second month). The activation of the SRY gene leads to the development of testes. Testosterone formation (from the testes) and anatomical differentiation of male structures (like the penis) occur after the development of testes. Estrogen formation in female embryos is not the first event in sex differentiation as the default pathway in the absence of SRY - induced testes development would lead to female - like development, but the first key event in clear sex differentiation (towards male) is SRY activation and testes development.

Answer:

activation of the SRY gene in male embryos and development of testes