Human sex is determined by:

In humans, biological sex is determined by the combination of the 23rd pair of chromosomes, known as the sex chromosomes. 1. **Genetic Makeup**: Biological females have two $X$ chromosomes ($XX$), while biological males have one $X$ and one $Y$ chromosome ($XY$). 2. **Gamete Production (Meiosis)**: During meiosis, the chromosome pairs separate. - Because females are $XX$, all eggs produced by the mother carry a single $X$ chromosome. - Because males are $XY$, sperm produced by the father can carry either an $X$ chromosome (approx. 50%) or a $Y$ chromosome (approx. 50%). 3. **Fertilization**: - If a sperm carrying an $X$ chromosome fertilizes the egg ($X$), the result is an $XX$ zygote (female). - If a sperm carrying a $Y$ chromosome fertilizes the egg ($X$), the result is an $XY$ zygote (male). Since the egg always contributes an $X$, the sex of the offspring depends entirely on whether the father's sperm contributes an $X$ or a $Y$ chromosome.