16.1.2Inheritance

The Inheritance of Sex — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 7

📚 Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Hub / Topical Revision

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Part 1 of 7Sex chromosomes

Human cells have 23 pairs of chromosomes.

Autosomes

Twenty-two pairs are autosomes, which have the same appearance in males and females.

Sex chromosomes

The 23rd pair is the pair of sex chromosomes.

Females have two X chromosomes (XX), and males have one X and one much smaller Y chromosome (XY).

SRY

The Y chromosome carries a gene called SRY which triggers the development of testes in the embryo; without it the embryo develops ovaries.

The testes then produce testosterone that drives further male development.

Part 2 of 7Gametes and the 50:50 ratio

Every egg has one X chromosome, because the mother is XX and meiosis gives each egg a single X.

Gametes and the 50:50 ratio

A sperm cell can carry either an X or a Y chromosome, in equal numbers, because the father is XY.

Which sperm fertilises the egg

The sex of a baby is therefore determined by which sperm fertilises the egg: an X sperm gives a girl (XX) and a Y sperm gives a boy (XY).

The father's sperm determines the sex of the child.

	X (sperm)	Y (sperm)
X (egg)	XX female	XY male
X (egg)	XX female	XY male

The probability of any child being male is 1 in 2 (50%), and female is 50%. The ratio of male to female is 1 : 1.

Part 3 of 7Understanding probability correctly

Each fertilisation is independent, so the sex of previous children has no effect on the next.

Understanding probability correctly

A family with three girls still has a 50% chance of a fourth being a girl.

Across large populations, births are close to half and half (slightly more boys are born, but boys have a higher mortality rate in infancy).

A common mistake is to say that a family of four children will have exactly two boys and two girls; this is only the expected value,

and many families will not match it.

If you toss a coin to represent a fertilisation, results from 10 tosses vary, but results from 1000 are very close to half heads and half tails. In the same way, family-sized samples show random variation.

Part 4 of 7Other sex-determination systems

Not all species use the human system.

In birds, females are the ones with two different sex chromosomes (ZW) and males have ZZ, so the egg determines the sex of the offspring.

Many reptiles, such as crocodiles and some turtles, have sex determined by the temperature at which eggs are incubated.

In some fish individuals can change sex during life.

Bees are determined by whether eggs are fertilised: unfertilised eggs develop into males.

These examples show that the human XY mechanism is one of several possibilities.

Part 5 of 7Sex chromosomes and genes

The X chromosome is large and carries many genes unrelated to sex, such as those for blood clotting and colour vision, whereas the Y carries very few genes.

Sex-linked

This difference leads to sex-linked inheritance, covered in a later sub-topic: because males have only one X, a recessive allele on the X chromosome will show in a male even when he has just one copy.

Chromosome errors can also affect sex chromosomes; for example, females with only one X chromosome (Turner syndrome, XO) and males with an extra X chromosome (Klinefelter syndrome, XXY) show differences in development.

Part 6 of 7Chromosome counts and sex in practice

Cells in the human body that are not gametes contain 46 chromosomes, so a sperm cell has 22 autosomes plus either an X or a Y, giving 23, and an egg has 22 autosomes plus an X.

Chromosome counts and sex in practice

A karyotype is a picture of all the chromosomes of a cell arranged in pairs by size,

and it allows doctors to check the sex chromosomes and to detect extra or missing chromosomes.

In a karyotype of a female the 23rd pair has two similar large X chromosomes, whereas in a male the 23rd pair has a large X and a tiny Y.

The 1:1 ratio is also important in populations. Because equal numbers of male and female babies are born, in a species with two sexes most individuals can find a mate. Natural selection favours parents who produce equal numbers of sons and daughters, which helps explain why the ratio stays close to 1:1 in many species.

Part 7 of 7Exam-style questions

Draw a genetic diagram to show the inheritance of sex in humans and give the probability of a baby being a girl. [4]4 marks
A couple have had four daughters. Explain why the chance that their next child is a boy is still 50%. [2]2 marks
Explain why the sex of a baby is determined by the father and not the mother. [2]2 marks

Key terms

Term	Meaning
Autosome	A chromosome that is not a sex chromosome.
Sex chromosomes	The pair of chromosomes that determine sex: XX in females, XY in males.
SRY gene	A gene on the Y chromosome that triggers testis development.
Gamete	A haploid sex cell.
Fertilisation	The fusion of sperm and egg.
Probability	The chance of an event occurring.
Sex-linked	Controlled by a gene on a sex chromosome.

Avoid these mistakes

Saying that the mother determines sex; her eggs always carry X.

Thinking that a family will have equal numbers of boys and girls exactly.

Writing that females have XY. Females are XX.

Forgetting that gametes carry only one sex chromosome.

Believing the Y chromosome carries as many genes as X.

Last updated: 20 October 2026