The sex of the fetus will be decided at

Options

A. fertilisation by male gamete
B. implantation
C. fertilisation by female gamete
D. the start of cleavage

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks when the sex of the fetus is determined during human reproduction.

2. **Role of Chromosomes**: The sex of the fetus is determined by the combination of sex chromosomes contributed by the male and female gametes. The male gamete (sperm) can carry either an X or a Y chromosome, while the female gamete (egg) always carries an X chromosome.

3. **Fertilization Process**: During fertilization, if the sperm carrying the Y chromosome fertilizes the egg, the resulting zygote will have XY chromosomes, leading to the development of a male fetus (boy). Conversely, if the sperm carrying the X chromosome fertilizes the egg, the zygote will have XX chromosomes, resulting in a female fetus (girl).

4. **Conclusion**: Therefore, the sex of the fetus is determined at the moment of fertilization, based on which type of male gamete (X or Y) successfully fertilizes the female gamete (X).

5. **Final Answer**: The sex of the fetus will be decided at fertilization.

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