In humans, biological sex is determined by a pair of SEX CHROMOSOMES — one of the 23 pairs of chromosomes.
FEMALES: XX — two X chromosomes.
MALES: XY — one X chromosome and one Y chromosome.
The Y chromosome is smaller than the X chromosome and contains fewer genes. It carries the genes responsible for male development, including the SRY gene which triggers the development of testes.
All other 22 chromosome pairs are called AUTOSOMES — they are the same in males and females.
How Sex is Determined at Fertilisation
All eggs produced by a female contain ONE X chromosome (since females are XX).
Sperm produced by a male contain EITHER:
an X chromosome (approximately 50% of sperm), OR
a Y chromosome (approximately 50% of sperm).
At fertilisation:
If an X-bearing sperm fertilises the egg: XX → FEMALE.
If a Y-bearing sperm fertilises the egg: XY → MALE.
Therefore: the SPERM determines the biological sex of the offspring — not the egg.
Probability: 50% chance of a female child, 50% chance of a male child in each pregnancy.
Punnett Square for Sex Determination
We can use a Punnett square to show sex determination:
Mother (XX) × Father (XY)
Sperm: X or Y (50/50)
Eggs: X only (100%)
Offspring:
XX = female (50%)
XY = male (50%)
This shows why the sex ratio in human populations is approximately 50:50.
Key point: the sex of a child is determined RANDOMLY at fertilisation — it cannot be predicted in advance for a specific pregnancy. The 50:50 ratio is the probability, not a guarantee for any given family.
⚠️ Common Mistake
It is the SPERM that determines the sex of the child — not the egg. All eggs contain an X chromosome. Sperm can contain either X or Y. If a Y-carrying sperm fertilises the egg, the child is male (XY). Historically, women were sometimes blamed for not producing sons — this is biologically incorrect.
📌 Key Note
Female = XX. Male = XY. All eggs carry X. Sperm carry X or Y (50/50). Y-sperm → male (XY). X-sperm → female (XX). Sex determined at fertilisation by which sperm fertilises the egg. 50% probability each time.
🎯 Matching Activity — Sex Determination Match
Match each statement to the correct sex chromosome fact. — drag the symbols on the right to match the component names on the left.
Female
Drop here
Male
Drop here
All eggs
Drop here
Sperm
Drop here
50%
Drop here
Contain one X chromosome — females are XX so can only pass on X
Has sex chromosomes XX — both are X chromosomes
Probability of a male offspring — same as probability of female offspring
Contain either X or Y — determines the sex of the offspring
Has sex chromosomes XY — one X and one smaller Y chromosome
🎯 Test Yourself
Question 1 of 2
1. What determines the biological sex of a human offspring?
2. A couple have three daughters. What is the probability their next child will be a son?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
Don't get itGetting thereNailed it!
🤖 Ask Mr Badmus AI
Stuck? Just ask! 💬
I'll use FIFA for calculations and flag Higher/Triple content clearly.