Final answer:
A mother's estrogen does not masculinize a fetus; it is the androgens, such as testosterone, secreted by the testes in male fetuses that lead to masculinization. The presence of the SRY gene on the Y chromosome triggers this process.
Step-by-step explanation:
During fetal development, it is not a mother's estrogen that masculinizes a fetus, but rather the presence of androgens such as testosterone. In the case of genetically male (XY) embryos, around the seventh week after conception, the SRY gene on the Y chromosome initiates the production of proteins that cause gonadal tissue to develop into testes. These testes then secrete hormones, including testosterone, which lead to the development of male reproductive systems.
Hence, a mother's estrogen does not masculinize a fetus. Instead, if androgens are present in excessive amounts, they can lead to masculinization in females. Estrogen typically leads to the formation of female reproductive systems in the absence of the SRY gene.