Q1.
What determines a baby's biological sex? So far, you have learnt that every person has two sex chromosomes. Females have XX and males have XY chromosomes. The mother always contributes an X chromosome to a baby and the father contributes either an X (Female: XX) or a Y (Male: XY) chromosome. Now, from the above information, can you predict who determines the sex of a baby?
Answer
The father — more precisely, which of his two kinds of sperm happens to fertilise the egg. The mother cannot influence it at all.
mother is XX → meiosis gives eggs that all carry X
father is XY → meiosis gives two kinds of sperm: half carry X, half carry Y
father is XY → meiosis gives two kinds of sperm: half carry X, half carry Y
| Sperm carries X | Sperm carries Y | |
|---|---|---|
| Egg carries X | XX → girl | XY → boy |
Since the two kinds of sperm are made in equal numbers, the expected chance is 50% girl and 50% boy at every conception — and each conception is independent of the last, exactly like a fresh toss of a coin.
Why the mother cannot decide: she has no Y chromosome to give. Every egg she makes carries an X, because meiosis in an XX cell can only put an X into each gamete. The sex of the baby is therefore settled entirely by which sperm arrives first — and that is a matter of chance, not of anyone's wish, diet or effort.
Why this matters: in many families a woman is still blamed for the sex of her child. That belief is simply biologically wrong. The chapter's later section on the same theme is worth reading beside this box: self-selective abortion driven by a preference for one sex distorts the sex ratio of society, which is why prenatal sex determination is strictly prohibited by law in India.