Meiosis produces haploid daughter cells from a diploid parent cell, halving the number of chromosomes so that fertilisation restores the diploid number. Genetic variation arises from independent segregation of homologous chromosomes and from crossing over between homologous chromosomes during meiosis, and the number of possible combinations can be calculated.

What this dot point is asking

AQA wants you to explain how meiosis reduces a diploid cell to haploid gametes through two divisions, and to identify the two sources of genetic variation that arise during meiosis (independent segregation and crossing over). You must be able to calculate the number of possible chromosome combinations.

Why meiosis halves the chromosome number

Body cells are diploid ($2n$): they carry two copies of each chromosome, one from each parent, forming homologous pairs. Gametes must be haploid ($n$), carrying one of each chromosome, so that when two gametes fuse at fertilisation the diploid number is restored. If gametes were diploid, the chromosome number would double every generation.

Meiosis achieves this with two divisions:

• Meiosis I (reduction division). Homologous chromosomes are separated, halving the chromosome number. The daughter cells are haploid.
• Meiosis II. Sister chromatids are separated (like mitosis), giving four haploid cells in total.

One diploid parent cell therefore produces four haploid daughter cells.

Source 1: independent segregation

During metaphase I, the homologous pairs line up along the equator of the cell. Crucially, the orientation of each pair is random: whether the maternal or the paternal chromosome of a pair faces a given pole is independent of every other pair.

When the pairs separate in anaphase I, each daughter cell receives a random mix of maternal and paternal chromosomes.

Source 2: crossing over

During prophase I, homologous chromosomes pair up closely (forming a bivalent). Where the chromatids of the homologous chromosomes touch, at points called chiasmata, they may break and rejoin, exchanging sections.

This crossing over swaps alleles between the maternal and paternal chromatids, producing new combinations of alleles on the same chromosome that did not exist in either parent.

Random fertilisation

Variation from meiosis is multiplied at fertilisation, because any one of the many genetically distinct sperm can fuse with any one of the many genetically distinct eggs. The total number of chromosome combinations in offspring from two parents is therefore $(2^n)^2 = 2^{2n}$ from independent segregation alone, before crossing over is even counted.

Meiosis versus mitosis

Common mistakes

Try this

Q1. Explain why the daughter cells produced by meiosis are described as haploid. [2 marks]

• Cue. They contain one chromosome from each homologous pair, half the chromosome number of the diploid parent cell.

Q2. A plant has a diploid number of 14. Calculate the number of different chromosome combinations possible in its gametes from independent segregation. [2 marks]

• Cue. Haploid number $n = 7$, so $2^7 = 128$ combinations.

Q3. Crossing over increases genetic variation. Explain how. [2 marks]

• Cue. Homologous chromosomes exchange sections of chromatid at chiasmata, producing new combinations of alleles on a chromosome.