Biology · Human Reproduction · NEET
Because the mother is XX, so every ovum she makes carries only an X chromosome. She has no choice to give. The father is XY, so he makes two types of sperm: half carry X and half carry Y. So the sex chromosome that comes from the sperm decides whether the baby is XX (girl) or XY (boy). The father supplies the deciding chromosome.
An X-carrying sperm makes a girl (X from mother + X from father = XX). A Y-carrying sperm makes a boy (X from mother + Y from father = XY). Simple rule: Y sperm = boy, X sperm = girl.
A girl (female) is XX. A boy (male) is XY. Out of 23 pairs of chromosomes, 22 pairs are autosomes (same in both sexes) and 1 pair is the sex chromosome pair. That last pair is XX in females and XY in males.
NCERT says the sex of the baby is decided at the moment of fertilisation itself, when the sperm nucleus fuses with the ovum nucleus to form the zygote. It is not decided later during development. Which sperm (X or Y) reaches the ovum first fixes the sex right there.
No. The human female is XX, so every ovum always carries an X chromosome and never a Y. Only sperm can carry a Y. This is exactly why the male parent, not the female, determines the sex.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. The father makes X and Y sperm in equal numbers, but he cannot choose which sperm fertilises the ovum. It is a random 50:50 chance, so about half of babies are boys and half are girls.
Because the male has two different sex chromosomes, X and Y, while the female has two same ones, XX. When the male is the one carrying two different sex chromosomes, the system is called the XY type of sex determination (also called male heterogamety).
A human body cell has 23 pairs of chromosomes: 22 pairs of autosomes plus 1 pair of sex chromosomes (XX in female, XY in male). So there are 44 autosomes and 2 sex chromosomes in total.
The idea that the father decides the sex at fertilisation is stated in Human Reproduction. The full XY mechanism, XO type, and honeybee sex determination are explained in Principles of Inheritance and Variation. NEET can ask from both chapters.
