Science
Answer
In humans, the males have 'XY' chromosomes and females have 'XX' chromosome as sex chromosomes. Therefore, 50% male gametes have X chromosome and other 50% have Y chromosome. The females have all gametes with X chromosome. The sex of the child is determined at the time of fertilisation. If male gamete with X chromosome fertilizes with female gamete, it leads to a female child whereas if a gamete with Y chromosome fertilizes with female gamete, it leads to a male child.

Related Questions
How do Mendel's experiments show that traits are inherited independently?
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