How is the sex of the child determined in human beings?
In human beings, the sex of the child is genetically determined. Humans have 22 pairs of chromosomes that have maternal and paternal copies. Additionally, there is one pair called the sex chromosomes which is not always a perfect pair.
Women have a perfect pair of sex chromosomes, both called X. Thus, women are XX. Men have a mismatched pair where one is a normal-sized X and the other is a short one called Y. Thus, men are XY.
All children inherit an X chromosome from their mother regardless of whether they are boys or girls. The sex of the child is determined by what they inherit from their father. A child who inherits an X chromosome from the father will be a girl, and one who inherits a Y chromosome from the father will be a boy.
Explanation
The textbook explains that sex determination in humans is genetic. The key concept is that females are XX (homogametic) and males are XY (heterogametic). Since all mothers can only pass on an X chromosome, the father's contribution determines the sex - X for female offspring and Y for male offspring. This results in approximately 50% probability for each sex.