16.1.5Inheritance

Meiosis — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 7

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

0%read

Part 1 of 7What meiosis is and where it happens

Meiosis is a type of cell division that produces gametes.

Ovaries

In humans it takes place only in the reproductive organs, the ovaries and testes.

A body cell with two sets of chromosomes (diploid, 46 in humans) divides to make four cells, each with a single set (haploid, 23 chromosomes).

The cells made are genetically different from each other and from the parent cell.

This is different from mitosis, which makes two identical diploid cells for growth and repair.

Meiosis is sometimes called a reduction division because it reduces the chromosome number by half. Without it the number of chromosomes would double with every new generation, which would be impossible for the cell to manage. The process begins with a diploid cell that has already copied its DNA, in the same way as before mitosis.

Part 2 of 7The stages in outline

Before the division begins, the cell copies its genetic material (DNA), so each chromosome is doubled.

2First division

In the first division, the chromosomes line up in pairs and the pairs are pulled apart.

Genetic material may be exchanged between chromosomes.
The cell divides in two, and each new cell has a mixture of chromosomes, with one from each pair.

3Second division

In the second division, the copies of each chromosome are pulled apart.

4Four haploid cells

The result is four haploid cells, each genetically different.

The gametes then develop

in males, the four cells become sperm;
- in females, usually one of them becomes an egg.

Part 3 of 7Why meiosis causes variation

Meiosis leads to variation in two ways.

First, the chromosomes in each pair are shared out at random,

so each gamete receives a different combination of the maternal and paternal chromosomes.

Second, the exchange of genetic material between chromosomes gives new combinations of alleles.

Humans can make about 8 million different chromosome combinations from independent assortment alone, and fertilisation joins two such random gametes,

so the number of different genetic combinations in offspring is enormous.

This explains why siblings, apart from identical twins, differ from each other.

Part 4 of 7Meiosis compared with mitosis

Feature	Mitosis	Meiosis
Number of divisions	One	Two
Cells produced	Two	Four
Chromosome number	Diploid (same as parent)	Haploid (half)
Genetically	Identical	Different
Where	Most body cells	Ovaries, testes
Purpose	Growth, repair, asexual reproduction	Making gametes

Part 5 of 7Fertilisation and links

When a sperm and egg fuse at fertilisation, the full chromosome number is restored (23 + 23 = 46), and the zygote then divides by mitosis to form an embryo, so each cell of the embryo has the same genes.

Fertilisation and links

If meiosis did not halve the number, the chromosome number would double every generation.

Errors in meiosis can cause the wrong number of chromosomes, as in Down's syndrome, where there are three copies of chromosome 21.

This topic links to sexual reproduction (6.1.1), DNA and genes (6.1.4), and genetic inheritance (6.1.6), since the gametes produced determine the possible offspring in a genetic cross.

Part 6 of 7Drawing and interpreting the stages

Exam questions often give a diagram of a cell with a few chromosomes, for example a parent cell with four chromosomes (two pairs).

Using this simplified model, the four cells formed at the end of meiosis each contain two chromosomes, one from each pair,

but they are not all the same: depending on which chromosome of each pair goes where, there are four possible combinations for the gametes.

Mitosis on the same cell gives two cells each with four chromosomes identical to those of the parent.

When labelling a diagram, state the number of chromosomes in the parent cell, the number in each new cell, and whether the cells are the same or different. In a calculation, use the rule that the number of possible combinations is 2 raised to the power of the number of chromosome pairs, which for humans is 2²³, about 8.4 million, before the exchange of genetic material is even considered.

It is also worth remembering that meiosis in females is unusual: only one of the four cells becomes a functional egg, and the others, called polar bodies, are discarded, which allows the egg to hold a large food store for the early embryo.

Part 7 of 7Exam-style questions

Describe what happens to the chromosome number in meiosis and in fertilisation. [3]3 marks
State two differences between mitosis and meiosis. [2]2 marks
Explain how meiosis leads to genetic variation. [3]3 marks

Key terms

Term	Meaning
Meiosis	Cell division that makes four genetically different haploid gametes.
Haploid	Having a single set of chromosomes.
Diploid	Having two sets of chromosomes.
Gamete	A sex cell.
Chromosome	A structure of coiled DNA carrying many genes.
Allele	A different version of the same gene.
Zygote	The cell formed by fertilisation.

Avoid these mistakes

Saying meiosis happens in all body cells; it is only in the reproductive organs.

Saying meiosis produces two cells.

Saying the cells produced are identical.

Writing that meiosis doubles the chromosome number.

Confusing the two divisions of meiosis with two rounds of mitosis.

Last updated: 20 October 2026