Final answer:
The sex of a fetus is determined at fertilization when the sperm contributes either an X or a Y chromosome.
Step-by-step explanation:
The determination of the fetus's sex in humans occurs at fertilization. When the sperm fertilizes the egg, it contributes either an X or a Y chromosome. If the sperm carries an X chromosome, the resulting embryo will be female, while if the sperm carries a Y chromosome, the resulting embryo will be male. This process happens during the initial stages of pregnancy.
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