(a) Mendel crossed two pea plants with visible contrasting characteristics and found that there were no half-way characteristics in the plants of $F_1$ progeny. Explain the reason for this observation of Mendel.

In human beings, the sex of a child is genetically determined by the sex chromosomes inherited from the parents:

1. Maternal Contribution: Females have a perfect pair of sex chromosomes, both of which are $X$ ($XX$). Therefore, all eggs (female gametes) carry only an $X$ chromosome.
2. Paternal Contribution: Males have a mismatched pair of sex chromosomes, one normal-sized $X$ and one shorter $Y$ ($XY$). Consequently, half of the sperms (male gametes) carry an $X$ chromosome, and the other half carry a $Y$ chromosome.

Determination of Sex:

• If a sperm carrying an $X$ chromosome fertilizes the egg, the zygote will be $XX$, developing into a girl.
• If a sperm carrying a $Y$ chromosome fertilizes the egg, the zygote will be $XY$, developing into a boy.

Since the mother always contributes an $X$ chromosome, the sex of the child depends entirely on whether the father's sperm contributes an $X$ or a $Y$ chromosome.