AHL: Meiosis and variation

IB Biology Higher Level· Theme D: Continuity and Change, Unit 4· 25 min read

1. Crossing Over in Prophase I★★☆☆☆⏱ 6 min

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

📘 Definition

Crossing Over

n/a

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

Example:

If one homolog carries alleles $A B$ and the other carries $ab$ , crossing over can produce gametes with $A b$ and $a B$ .

📐 Worked Example

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

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

2. Independent Assortment of Homologs★★☆☆☆⏱ 7 min

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

📘 Definition

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 $2 n = 6$ ?

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

3. Random Fertilization★☆☆☆☆⏱ 5 min

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

1
1. Human haploid number $n = 23$ .
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1. Each gamete has $2^{23}$ possible combinations from independent assortment.
3
1. 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.

4. Linkage and Recombination Frequency★★★☆☆⏱ 7 min

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.

📘 Definition

Recombination Frequency

$R F$

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

📐 Worked Example

A test cross between a heterozygous plant $(B b L l)$ and a homozygous recessive plant $(bb l l)$ produces 1000 offspring: 400 parental $B b L l$ , 400 parental $bb l l$ , 100 recombinant $B b l l$ , 100 recombinant $bb L l$ . 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
1. Total offspring = 1000, number of recombinant offspring = 100 + 100 = 200.
3
1. Calculate RF:
4
$RF = \frac{200}{1000} \times 100\% = 20\%$
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This means the genes are 20 map units (centimorgans) apart on the chromosome.

5. Common Pitfalls

Wrong move:

Claiming crossing over occurs between sister chromatids

Why:

Sister chromatids are genetically identical after replication, so exchanging segments produces no new variation

Correct move:

Crossing over occurs between non-sister chromatids of homologous chromosomes, which have different alleles

Wrong move:

Confusing independent assortment with crossing over

Why:

Both occur in meiosis I, but they produce variation through different mechanisms

Correct move:

Crossing over exchanges alleles between chromosomes; independent assortment is random alignment of whole chromosome pairs

Wrong move:

Using the diploid number as $n$ in the $2^{n}$ combinations formula

Why:

The formula counts the number of homologous pairs, which equals the haploid number

Correct move:

The number of gamete combinations from independent assortment is $2^{n}$ , where $n$ = haploid number of chromosomes

Wrong move:

Stating that linked genes are always inherited together

Why:

Crossing over can separate linked genes at any point along the chromosome, producing recombinant gametes

Correct move:

Linked genes are inherited together more often than expected by chance, but recombination can produce recombinant offspring

Wrong move:

Claiming genetic variation arises in meiosis II

Why:

Meiosis II separates sister chromatids after crossing over is complete, so no new variation is generated

Correct move:

All new genetic variation from meiosis originates in meiosis I, during crossing over and independent assortment

6. Quick Reference 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%

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · Paper 1
Identify source of variation in meiosis
2024 · Paper 2
Explain three sources of variation
2023 · Paper 1
Calculate independent assortment combinations

Going deeper

official syllabusIB Biology 2025 Syllabus Theme DCheck for official AHL learning outcomes

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.