AQA A-Level Biology (7402)

## Learning Outcomes

- Define **genetic diversity** and explain why it matters in populations
- Describe the stages and outcome of meiosis using precise biological terminology
- Explain how **crossing over** and **independent segregation** increase genetic diversity
- Distinguish clearly between homologous chromosomes, sister chromatids and non-sister chromatids
- Apply meiosis knowledge to inheritance questions, including dihybrid crosses
- Recognise common exam errors in synoptic questions about meiosis and variation

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand how meiosis contributes to genetic diversity within a species, with a focus on the following syllabus points:

- Know that genetic diversity is the number of different alleles of genes in a population or species
- Explain that genetic diversity can arise from gene mutation, chromosome mutation, meiosis and random fertilisation
- Understand that meiosis usually produces four haploid cells from one diploid parent cell
- Identify that in meiosis I homologous chromosomes separate, while in meiosis II sister chromatids separate
- Explain how independent segregation of homologous chromosomes in meiosis I produces different allele combinations
- Explain how crossing over between non-sister chromatids in prophase I produces recombinant chromatids
- Understand that random fertilisation further increases genetic diversity
- Recognise that linked genes on the same chromosome do not always assort independently

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is meant by genetic diversity?
2. How does meiosis reduce chromosome number?
3. What is the difference between homologous chromosomes and sister chromatids?
4. How do crossing over and independent segregation create variation in gametes?
5. Why does a 9:3:3:1 ratio depend on genes being on separate chromosomes or assorting independently?

## Introduction

In AQA A-level Biology, meiosis is not just a list of stages to memorise. It is a mechanism that explains why sexually produced offspring are genetically different from one another. That idea links directly to inheritance, natural selection, evolution and population genetics, so it appears often in synoptic questions.

A strong answer must do more than say that meiosis “causes variation”. You need to explain exactly how chromosome behaviour leads to new allele combinations. That means using precise terms such as homologous chromosomes, non-sister chromatids, crossing over and independent segregation.

> **Key Term: genetic diversity**
> The number of different alleles of genes in a population or species.

### What meiosis is and why it matters

Meiosis is a type of nuclear division that reduces chromosome number. A diploid cell has two copies of each chromosome, arranged as homologous pairs. In humans, diploid cells have 46 chromosomes, or 23 homologous pairs. Meiosis produces haploid cells, each with one chromosome from each homologous pair, so human gametes contain 23 chromosomes.

This reduction is essential in sexual reproduction. If gametes were diploid, fertilisation would double the chromosome number in every generation. Instead, meiosis halves the chromosome number first, and fertilisation restores the diploid number.

> **Key Term: diploid**
> A cell containing two sets of chromosomes, one set from each parent.
>
> **Key Term: haploid**
> A cell containing one set of chromosomes.

The process involves DNA replication followed by two nuclear divisions:

- before meiosis, DNA replicates
- meiosis I separates homologous chromosomes
- meiosis II separates sister chromatids

The result is usually four haploid cells that are genetically different from each other.

> **Key Term: homologous chromosomes**
> A pair of chromosomes, one maternal and one paternal, with the same genes at the same loci, but possibly different alleles.
>
> **Key Term: sister chromatids**
> Identical copies of a chromosome produced by DNA replication and joined by a centromere.

Students often confuse homologous chromosomes with sister chromatids. They are not the same. Homologous chromosomes carry the same genes in the same places, but may carry different alleles. Sister chromatids are replicated copies of one chromosome, so they are initially genetically identical.

Why this matters in exam answers:

- if the question asks what separates in meiosis I, the answer is **homologous chromosomes**
- if it asks what separates in meiosis II, the answer is **sister chromatids**

This distinction is basic but heavily tested because it underpins every later explanation of genetic diversity.

Meiosis also matters because it creates variation. Without variation, natural selection cannot change allele frequencies. If all individuals were genetically the same, a disease or environmental change could wipe out the whole population.

Genetic diversity can increase through:

- gene mutations creating new alleles
- chromosome mutations changing chromosome number or structure
- random processes in meiosis
- random fertilisation

For this subtopic, the main focus is the last two in the middle of the life cycle: meiosis and fertilisation.

> **Test Tip:** If a question says “explain how meiosis increases genetic diversity”, do not just describe the stages. You must name **crossing over** and **independent segregation**, then explain how each changes allele combinations.

### How meiosis creates genetic diversity

There are two main meiotic sources of variation: crossing over and independent segregation.

First, in prophase I, homologous chromosomes pair up. This pairing is called synapsis. The homologous pair forms a structure sometimes called a bivalent. At this point, non-sister chromatids can come into contact and exchange sections of DNA.

> **Key Term: synapsis**
> The pairing of homologous chromosomes during prophase I of meiosis.
>
> **Key Term: non-sister chromatids**
> Chromatids from different homologous chromosomes.
>
> **Key Term: chiasmata**
> The points where non-sister chromatids of homologous chromosomes are in contact and crossing over occurs.

Crossing over happens at chiasmata. A section of one non-sister chromatid breaks off and swaps with the equivalent section of the other. This does not usually create new genes. Instead, it creates new combinations of existing alleles on the chromatids.

For example, imagine one homologous chromosome carries alleles A and B, while the other carries a and b. After crossing over, chromatids may carry combinations such as A b or a B. That means the gametes produced later can contain allele combinations not present on the original parental chromosomes.

> **Key Term: crossing over**
> Exchange of DNA between non-sister chromatids of homologous chromosomes during prophase I, producing new allele combinations.

Why is “non-sister chromatids” essential? Because sister chromatids are copies of the same chromosome. Exchanging DNA between them would not usually create new allele combinations. The variation comes from exchange between chromatids from different homologous chromosomes, which may carry different alleles.

Second, independent segregation occurs in meiosis I. Homologous pairs line up randomly at the equator in metaphase I. The orientation of each pair is independent of the others. When the homologous chromosomes separate in anaphase I, either the maternal or paternal chromosome from each pair can move into a given daughter cell.

> **Key Term: independent segregation**
> The random alignment and separation of homologous chromosome pairs in meiosis I, producing different chromosome combinations in gametes.

This matters because each daughter cell receives a random mixture of maternal and paternal chromosomes. Even if no crossing over happened, the chromosome combinations in gametes would still vary.

For a species with haploid number n, the number of possible chromosome combinations in gametes from independent segregation alone is:

2^n

Worked example for humans:

- human haploid number, n = 23
- possible gamete combinations = 2^23
- 2^23 = 8,388,608

So one person can produce over 8 million genetically different gametes from independent segregation alone. This does not even include crossing over, so the real number is much greater.

If considering possible zygote combinations from two parents through independent segregation alone:

- possible zygotes = 2^46
- 2^46 = 70,368,744,177,664

This shows why siblings from the same parents can be genetically different.

A common weak answer says “chromatids line up randomly”. That is imprecise for meiosis I. The correct A-level wording is that **homologous chromosome pairs** line up randomly at the equator in metaphase I.

There is one important limit to remember. Genes on the same chromosome are **linked genes**. They do not assort independently in the same way as genes on different chromosomes. Crossing over can separate linked alleles, but linked genes tend to be inherited together more often than genes on separate chromosomes.

> **Key Term: linked genes**
> Genes located on the same chromosome, so they do not always assort independently.
>
> **Test Tip:** When explaining crossing over, include the missing mark many students lose: homologous chromosomes must first pair by synapsis in prophase I. Do not jump straight to “chromatids break and rejoin”.

### Random fertilisation and synoptic links to inheritance

Meiosis produces genetically different gametes, but variation increases further because fertilisation is random. Any sperm can fertilise any egg, so the combination of gametes that form a zygote is a matter of chance.

> **Key Term: random fertilisation**
> The chance fusion of any male gamete with any female gamete, producing different allele combinations in offspring.

This means variation comes from two levels:

- meiosis makes the gametes different
- fertilisation combines those different gametes at random

That is why exam answers often phrase the full explanation as “genetic diversity arises from crossing over, independent segregation and random fertilisation”.

This also explains dihybrid inheritance patterns. In Mendel’s classic pea plant cross, one gene controlled seed shape and another controlled seed colour. Because the genes assorted independently, the F₁ plants could produce four gamete types: RG, Rg, rG and rg. Random fertilisation between these gametes produced the familiar 9:3:3:1 F₂ phenotypic ratio.

Why exactly four gamete types? The F₁ genotype was RrGg. During meiosis:

- the R or r allele can enter a gamete
- independently, the G or g allele can enter a gamete

So the possible combinations are:

- RG
- Rg
- rG
- rg

If those gametes combine randomly at fertilisation, 16 genotype combinations are possible in the Punnett square. Grouping the resulting phenotypes gives:

- 9 round yellow
- 3 round green
- 3 wrinkled yellow
- 1 wrinkled green

This ratio supports the law of independent assortment, but only when the genes are not strongly linked.

Synoptic questions may use this idea in disguised forms. For example, you might be given unfamiliar organisms, chromosome diagrams, or data from breeding experiments. The task is still the same: connect chromosome behaviour in meiosis to allele combinations in gametes and then to offspring phenotypes.

You may also need to link meiosis to mutation. Gene mutation changes the DNA base sequence and can create a new allele. Chromosome mutation can involve non-disjunction, where chromosomes fail to separate properly.

Non-disjunction can occur:

- in anaphase I if homologous chromosomes fail to separate
- in anaphase II if sister chromatids fail to separate

This produces gametes with abnormal chromosome numbers. That is a chromosome mutation, not a normal source of variation like independent segregation.

A final common synoptic comparison is meiosis versus mitosis:

- meiosis: diploid to haploid, two divisions, four cells, genetically different
- mitosis: chromosome number maintained, one division, two cells, genetically identical

This contrast helps explain why meiosis is linked to sexual reproduction and variation, while mitosis is linked to growth, repair and asexual reproduction.

> **Test Tip:** In Paper 3, read the command word closely. “Describe meiosis” needs stage-based detail. “Explain how meiosis increases genetic diversity” needs cause-and-effect language about allele combinations. Those are not the same answer.

### Writing high-scoring exam answers on genetic diversity and meiosis

High-level answers are precise, selective and applied to the question stem. Examiners often report GCSE-style responses such as “meiosis makes cells different” without enough mechanism. AQA rewards accurate biological language and clear causal chains.

A strong 3-mark answer to “Explain how crossing over increases genetic diversity” would include:

- homologous chromosomes pair up in prophase I
- non-sister chromatids exchange sections of DNA at chiasmata
- this produces new combinations of alleles on chromatids, so resulting gametes are genetically different

A strong answer on independent segregation would include:

- homologous chromosome pairs align randomly at the equator in meiosis I
- maternal and paternal homologues separate independently
- different combinations of chromosomes, and therefore alleles, enter gametes

Common student errors include:

- saying crossing over happens between sister chromatids
- saying chromatids rather than homologous pairs separate in meiosis I
- saying crossing over creates new alleles rather than new combinations of alleles
- forgetting to link chromosome movement to the genetic outcome
- assuming all genes assort independently even when linkage may matter

A useful way to structure an explanation is:

1. state the meiotic event
2. state what physically happens to chromosomes or chromatids
3. state the genetic consequence in the gamete
4. state the final consequence for offspring diversity

For example:

- In metaphase I, homologous pairs line up randomly.
- In anaphase I, one chromosome from each pair enters each daughter cell.
- This gives different combinations of maternal and paternal chromosomes in gametes.
- Therefore offspring may inherit different combinations of alleles.

That chain is much better than simply writing “independent segregation causes variation”.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Genetic diversity is the number of different alleles in a population or species
- Genetic diversity can arise from gene mutation, chromosome mutation, meiosis and random fertilisation
- Meiosis usually produces four haploid daughter cells from one diploid parent cell
- Homologous chromosomes have the same genes at the same loci but may carry different alleles
- Sister chromatids are identical copies of a chromosome joined by a centromere
- In meiosis I, homologous chromosomes separate; in meiosis II, sister chromatids separate
- Crossing over occurs between non-sister chromatids in prophase I at chiasmata
- Crossing over produces new combinations of alleles, not usually new genes
- Independent segregation is the random alignment and separation of homologous chromosome pairs in meiosis I
- In humans, independent segregation alone can produce 2^23 different gamete chromosome combinations
- Random fertilisation increases variation because any sperm can fuse with any egg
- Linked genes are on the same chromosome and do not always assort independently

## Key Terms and Concepts

- genetic diversity
- diploid
- haploid
- homologous chromosomes
- sister chromatids
- synapsis
- non-sister chromatids
- chiasmata
- crossing over
- independent segregation
- linked genes
- random fertilisation


Genetic information, variation and relationships - Genetic diversity and meiosis