Sex determination

Species decide sex in different ways. In a few reptiles, the temperature at which the eggs are kept decides it. In human beings, sex is largely decided by genes.

Human body cells have 23 pairs of chromosomes. The first 22 pairs match in males and females, and are called autosomes. The last pair are the sex chromosomes, also called allosomes. A woman is XX. A man is XY, with one X and one short Y.

A small example

• Egg (X) + sperm with X = XX, a girl.
• Egg (X) + sperm with Y = XY, a boy.

Every egg carries an X. Half the sperm carry an X, and half carry a Y. So each birth has an equal chance of either, and the father's sperm decides which. The mother cannot give a Y, because she has none.

Where marks go

CBSE's 2026-27 sample paper has no question on sex determination. In a 1-mark question, the marking scheme wants the exact fact: which parent, which gamete, and whether it carries an X or a Y. The facts are few, so each word counts.

In ICSE, the 2024 Biology paper asked what a zygote grows into if it got a Y chromosome from the father. The answer was a boy. CISCE's report on the 2024 ICSE Biology paper names "either boy or a girl" as the wrong option chosen. Once the Y is present, the sex is already fixed.

The 2027 specimen paper asks why haemophilia is uncommon in females. The same X and Y chromosomes explain that, through sex-linked inheritance.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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