The sex (boy or girl) of a baby is decided at the moment of fertilisation.
The decision is made by chromosomes present in the fertilised egg (zygote).
What are Chromosomes?
- Thread-like structures inside the nucleus of cells
- Carry instructions (genes)
- Humans have 23 pairs of chromosomes (total 46)
Special Pair: Sex Chromosomes
Out of 23 pairs:
- 22 pairs → body chromosomes
- 1 pair → sex chromosomes (X and Y)
Sex Chromosomes in Parents
| Person | Sex Chromosomes |
| Female | XX |
| Male | XY |
This is the key to understanding sex determination.
Chromosomes in Gametes
Gametes have only one chromosome from each pair.
Egg (ovum)
- Always carries X chromosome
- Only one type of egg
Sperm
- Two types of sperms:
- One carries X
- One carries Y
This difference is very important.
Sex Determination Flow Chart
Mother’s egg → always X
+
Father’s sperm → X or Y
↓
Case 1: X (egg) + X (sperm) → XX → Girl baby 👧
Case 2: X (egg) + Y (sperm) → XY → Boy baby 👦
Simple Diagram
Father (XY)
/ \
Sperm X Sperm Y
| |
Mother X + X → XX (Girl) |
Mother X + Y → XY (Boy)
Who Determines the Sex of the Baby?
Father determines the sex of the baby
Because:
- Mother always gives X
- Father gives X or Y
Common Myth (Very Important)
Wrong belief: Mother is responsible for the baby’s sex
Scientific truth: Father’s sperm decides the sex
Therefore, blaming the mother is completely unjustified and incorrect.
Key Points to Remember
Humans have 23 pairs of chromosomes
One pair is sex chromosomes (X and Y)
Females are XX
Males are XY
Egg always carries X
Sperm carries X or Y
Father determines the sex of the baby
Mother is NOT responsible
In Simple Words
Every egg from the mother has an X chromosome. The father’s sperm may carry either X or Y. If an X-sperm fertilises the egg, the baby is a girl (XX). If a Y-sperm fertilises the egg, the baby is a boy (XY). So, the father’s chromosome decides the baby’s sex.
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