The probability of a girl or a boy being born in a family is 50% chances for each. This is because the chances of inheriting an X or Y chromosome from the father are equally likely, making both outcomes equally possible.
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There are 50% chances that a girl may be born and 50% chances that a boy may be born. It can be explained as follows:
Most human chromosomes have a maternal copy and a paternal copy. We have 22 such chromosomes. One pair of chromosomes called sex chromosomes is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. (XX)
But men have a mismatched pair of sex chromosomes in which one is normal sized – X chromosome while the other is a short one called Y chromosome. (XY)
A child receives one chromosome from mother which is essentially X chromosome. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.