# AHL: Meiosis and variation

> IB Biology Higher Level · IB Biology HL 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-meiosis-and-variation/

This sub-topic explores how meiosis and sexual reproduction generate genetic variation, the raw material for evolution. We cover three core mechanisms of variation and how linkage affects recombination in gametes.

**Prerequisites:** [Basic meiosis structure and stages](https://www.owlsprep.com/study/ib-biology-hl-u4-meiosis-stages/); [Chromosome structure and homologous chromosomes](https://www.owlsprep.com/study/ib-biology-hl-u3-chromosome-structure/)

## Learning objectives

- Explain how meiosis and sexual reproduction generate genetic variation
- Identify key meiotic events that produce genetic variation
- Distinguish between different sources of genetic variation
- Calculate recombination frequencies for linked genes

## Crossing Over in Prophase I

The first major source of genetic variation in meiosis is crossing over, which occurs during the extended prophase I of meiosis.

**Crossing Over** — The process by which non-sister chromatids of homologous chromosomes break and exchange corresponding segments of DNA, producing new combinations of alleles.

*Notation:* n/a

*Example:* If one homolog carries alleles $AB$ and the other carries $ab$, crossing over can produce gametes with $Ab$ and $aB$.

**Worked example:** A homologous chromosome pair carries alleles for seed shape $(R/r)$ and seed color $(Y/y)$: one homolog has $RY$, the other has $ry$. Show how crossing over produces recombinant gametes.

1. 1. During prophase I, the homologous chromosomes pair up tightly, forming a bivalent.
2. 2. A chiasma forms between the loci for seed shape and seed color on non-sister chromatids.
3. 3. Segments of DNA are exchanged between the two non-sister chromatids.
4. 4. After meiosis is complete, four distinct gametes are formed: two parental, two recombinant.
5. Parental gametes:
6. $$RY \text{ and } ry$$
7. Recombinant gametes:
8. $$Ry \text{ and } rY$$

> **tip**
>
> Always specify that crossing over occurs between non-sister chromatids, not sister chromatids, in exam answers.

## Independent Assortment of Homologs

The second source of genetic variation arises from the random alignment of homologous chromosome pairs during metaphase I of meiosis.

**Independent Assortment** — The random orientation of each homologous chromosome pair along the metaphase plate, meaning the segregation of alleles for one gene does not affect the segregation of alleles for another, unlinked gene.

**Worked example:** How many distinct gamete genotypes can be produced by independent assortment alone in an organism with a diploid number of $2n = 6$?

1. 1. Recall that the number of possible combinations from independent assortment is $2^n$, where $n$ is the haploid number of chromosomes.
2. 2. For $2n = 6$, the haploid number $n = 3$.
3. 3. Calculate the number of combinations:
4. $$2^n = 2^3 = 8$$
5. 4. So 8 distinct gamete genotypes can be produced from independent assortment alone.

> **tip**
>
> Remember: $n$ in the formula $2^n$ is always the haploid number, not the diploid number. This is a common exam mistake.

## Random Fertilization

Sexual reproduction adds a third layer of genetic variation after meiosis, through the random fusion of gametes.

Any male gamete can fertilize any female gamete, regardless of their genotype, multiplying the number of possible genetic combinations in the zygote.

**Worked example:** In humans, $2n = 46$. How many possible diploid combinations can be produced by random fertilization of two human gametes, ignoring crossing over?

1. 1. Human haploid number $n = 23$.
2. 2. Each gamete has $2^{23}$ possible combinations from independent assortment.
3. 3. Random fertilization multiplies the combinations from the two gametes:
4. $$(2^{23}) \times (2^{23}) = 2^{46} \approx 7 \times 10^{13}$$
5. That is over 7 trillion possible distinct zygotes, just from independent assortment and random fertilization.

## Linkage and Recombination Frequency

Genes located on the same chromosome are called linked genes, and are typically inherited together. Crossing over can separate linked genes, producing recombinant gametes at a frequency proportional to the distance between the genes.

**Recombination Frequency** — The proportion of recombinant offspring produced in a cross, used to estimate the distance between two linked genes on a chromosome.

*Notation:* $RF$

**Worked example:** A test cross between a heterozygous plant $(BbLl)$ and a homozygous recessive plant $(bbll)$ produces 1000 offspring: 400 parental $BbLl$, 400 parental $bbll$, 100 recombinant $Bbll$, 100 recombinant $bbLl$. Calculate the recombination frequency between the $B/b$ and $L/l$ genes.

1. 1. Recombination frequency is calculated as (number of recombinant offspring / total number of offspring) × 100%
2. 2. Total offspring = 1000, number of recombinant offspring = 100 + 100 = 200.
3. 3. Calculate RF:
4. $$RF = \frac{200}{1000} \times 100\% = 20\%$$
5. This means the genes are 20 map units (centimorgans) apart on the chromosome.

> **exam_tip**
>
> 1% recombination frequency equals 1 map unit (centimorgan) of distance between two genes, a common conversion question in exams.

## Common pitfalls

- **Wrong:** Claiming crossing over occurs between sister chromatids
  - Why it fails: Sister chromatids are genetically identical after replication, so exchanging segments produces no new variation
  - Correct: Crossing over occurs between non-sister chromatids of homologous chromosomes, which have different alleles
- **Wrong:** Confusing independent assortment with crossing over
  - Why it fails: Both occur in meiosis I, but they produce variation through different mechanisms
  - Correct: Crossing over exchanges alleles between chromosomes; independent assortment is random alignment of whole chromosome pairs
- **Wrong:** Using the diploid number as $n$ in the $2^n$ combinations formula
  - Why it fails: The formula counts the number of homologous pairs, which equals the haploid number
  - Correct: The number of gamete combinations from independent assortment is $2^n$, where $n$ = haploid number of chromosomes
- **Wrong:** Stating that linked genes are always inherited together
  - Why it fails: Crossing over can separate linked genes at any point along the chromosome, producing recombinant gametes
  - Correct: Linked genes are inherited together more often than expected by chance, but recombination can produce recombinant offspring
- **Wrong:** Claiming genetic variation arises in meiosis II
  - Why it fails: Meiosis II separates sister chromatids after crossing over is complete, so no new variation is generated
  - Correct: All new genetic variation from meiosis originates in meiosis I, during crossing over and independent assortment

## Cheatsheet

| Source of Variation | Stage Occurs | Core Mechanism | Key Formula/Note |
| --- | --- | --- | --- |
| Crossing over | Prophase I | Exchange of segments between non-sister homolog chromatids | Generates new allele combinations |
| Independent Assortment | Metaphase I | Random alignment of homologous pairs | Gamete combinations = $2^n$ ($n$ = haploid) |
| Random Fertilization | Post-meiosis | Random fusion of male and female gametes | Zygote combinations = $4^n$ ($n$ = haploid) |
| Linked gene recombination | Prophase I | Crossing over separates linked alleles | RF = (recombinants / total) × 100% |

## What's next

Meiosis and genetic variation are the foundation of all genetics and evolution topics for IB Biology HL. Understanding how new allele combinations arise connects chromosome behavior during meiosis to observable inheritance patterns, and explains how populations have the genetic variation needed for natural selection to act. This topic is frequently tested alongside inheritance questions, and its principles are required for understanding speciation and biodiversity. Mastering the mechanisms of variation will help you answer both short answer and extended response questions across multiple syllabus themes.

- [AHL: Inheritance extensions](https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-inheritance-extensions/)
- [AHL: Mutation and gene editing](https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-mutation-and-gene-editing/)
- [AHL: Speciation](https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-speciation/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-biology-hl-u4-ahl-meiosis-and-variation/