Meiosis, fertilisation and chromosome number

Reproduction and inheritance · Cell division, variation and evolution · note 3 of 7

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Meiosis, fertilisation and chromosome numberSpec 3.30, 3.31, 3.32

In short

Meiosis is the type of cell division that makes gametes, in the ovaries and testes. One diploid cell divides to produce four haploid gametes, each with half the number of chromosomes and genetically different from each other. In humans, 46 chromosomes become 23. Random fertilisation restores 46 in the zygote and produces genetic variation.

Meiosis is the type of cell division that makes gametes (sex cells). In humans it happens in the reproductive organs: the ovaries and the testes.

One diploid cell divides to produce four cells.
Each cell has half the number of chromosomes of the parent cell.
The cells are genetically different from each other and from the parent cell.
The four cells are haploid gametes, with one set of chromosomes.

In human cells the diploid number is 46 and the haploid number is 23. Meiosis makes sperm and egg cells with 23 chromosomes each.

Body cells and gametes
	Body cell (diploid)	Gamete (haploid)
Chromosomes in a human cell	46	23
Genetically identical to the others?	Yes, if made by mitosis	No, each gamete is different
Made by	Mitosis	Meiosis

Fertilisation and variation

At fertilisation, a sperm and an egg fuse to make a zygote with 46 chromosomes again, half from each parent. Fertilisation is random: any sperm can fuse with any egg.

Each gamete is genetically different, and any gamete can fuse with any other. So random fertilisation produces genetic variation in the offspring. Brothers and sisters are not identical to each other, or to either parent.

A diploid parent cell with 4 chromosomes divides by meiosis into four genetically different haploid gametes with 2 chromosomes each; one gamete fuses with a gamete from the other parent at fertilisation to form a diploid zygote with 4 chromosomes. — Meiosis halves the chromosome number; fertilisation restores it in the zygote.

Exam tip:

The stages of meiosis are not required. Learn the outcome: four genetically different haploid gametes, each with half the number of chromosomes.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

What is the difference between mitosis and meiosis?

Mitosis produces two diploid cells that are genetically identical to the parent cell and is used for growth, repair, cloning and asexual reproduction. Meiosis produces four haploid gametes, each with half the number of chromosomes and genetically different from each other. In humans, mitosis gives cells with 46 chromosomes and meiosis gives gametes with 23.

How does meiosis create genetic variation?

Meiosis creates genetic variation because the four gametes it makes are genetically different from each other and from the parent cell. Fertilisation is also random, so any sperm can fuse with any egg. This is why brothers and sisters are not identical to each other or to either parent.

How does natural selection lead to evolution?

Natural selection leads to evolution because individuals in a population show genetic variation and compete for resources. Those with characteristics best suited to the environment are more likely to survive and reproduce, passing on their alleles. Over many generations, the proportion of the population with the advantageous alleles increases, so inherited characteristics change.

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