Linkage and Crossing Over

CIE A-Level Biology· Unit 17: Inheritance, 17.5 Linkage and crossing over· 25 min read

1. What is Gene Linkage?★★☆☆☆⏱ 8 min

📘 Definition

Autosomal Linkage

The tendency of two or more genes located on the same autosome (non-sex chromosome) to be inherited together, as they cannot assort independently during meiosis

Example:

Body colour and wing shape genes in Drosophila are commonly used as examples of linked autosomal genes

Unlinked genes are located on different chromosomes, so they assort independently during meiosis, producing the expected 9:3:3:1 phenotypic ratio in a dihybrid cross between two heterozygotes. Linked genes do not follow this ratio because they are physically connected on the same chromosome.

✓ Quick check

Check your understanding of the core definition:

Which statement correctly describes linked genes?
- They are always located on sex chromosomes
- They are located on the same chromosome
- They always assort independently
- They never cross over during meiosis
Reveal answer
1 —
Correct. Linked genes are on the same chromosome so tend to be inherited together.

📐 Worked Example

Distinguish between linked and unlinked genes in terms of independent assortment.

1
Unlinked genes are found on different homologous chromosome pairs.
2
During metaphase I of meiosis, they align randomly at the cell equator, so they assort independently into gametes.
3
Linked genes are located on the same physical chromosome, so they are inherited together and do not assort independently.

2. Crossing Over and Recombinant Gametes★★★☆☆⏱ 10 min

📘 Definition

Crossing Over

The exchange of homologous chromosomal segments between non-sister chromatids during prophase I of meiosis, which breaks the linkage between linked genes to produce new allele combinations

Example:

A single cross-over between two linked genes produces two new recombinant allele combinations

When crossing over occurs between two linked genes, it breaks the original physical linkage. The result is two types of gamete: parental gametes that retain the original allele combination from the parent, and recombinant gametes that have a new combination of alleles.

📐 Worked Example

A Drosophila heterozygous for two linked genes has genotype $r a c GN g n$ (G = grey body, g = black body, N = normal wings, n = vestigial wings). What gametes are produced after crossing over between these two genes?

1
Homologous chromosomes line up in prophase I, with alleles arranged as shown:
2
$\text{Chromosome 1: } G---N \\ \text{Chromosome 2: } g---n$
3
A cross-over occurs between the two gene loci, swapping the ends of the non-sister chromatids:
4
$\text{Recombinant chromatids after crossing over:} \\ G---n \quad \text{and} \quad g---N$
5
After meiosis is complete, four gametes are produced:
6
1. Parental gamete: $GN$ (original combination)
2. Parental gamete: $g n$ (original combination)
3. Recombinant gamete: $G n$ (new combination)
4. Recombinant gamete: $g N$ (new combination)

3. Analysing Test Crosses for Linked Genes★★★★☆⏱ 7 min

✓ Calculator OK

A test cross (crossing a heterozygous parent with a homozygous recessive tester) lets you count parental and recombinant offspring directly. From this count you can calculate recombination frequency, which measures the distance between two genes on a chromosome.

📐 Worked Example

A test cross between $r a c A B ab$ and $r a c ab ab$ produces the following offspring: 420 $A B / ab$ , 40 $ab / ab$ , 38 $A b / ab$ , 402 $a B / ab$ . Calculate the recombination frequency between the two genes.

1
First, identify recombinant offspring: they have non-parental phenotype combinations. The parental combinations are $A B$ and $ab$ , so recombinants are $A b$ and $a B$ .
2
Calculate the total number of offspring: $420 + 40 + 38 + 402 = 900$ .
3
Sum the total number of recombinant offspring: $38 + 40 = 78$ .
4
Calculate recombination frequency as the percentage of recombinant offspring:
5
$\text{Recombination frequency} = \frac{\text{Total recombinants}}{\text{Total offspring}} \times 100 = \frac{78}{900} \times 100 = 8.67\%$
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This means the two genes are 8.67 map units apart on the chromosome.

4. Common Pitfalls

Wrong move:

Writing linked genotypes as AaBb instead of showing linkage arrangement

Why:

Examiners require you to show which alleles are on each chromosome, this standard format loses marks

Correct move:

Write linked genotypes as $\frac{A B}{ab}$ to show A is linked to B and a to b

Wrong move:

Counting only one recombinant class when calculating recombination frequency

Why:

Crossing over produces two recombinant gamete classes, both must be counted

Correct move:

Add the number of offspring from both recombinant classes before dividing by total offspring

Wrong move:

Claiming linked genes can never be separated

Why:

Crossing over regularly separates linked genes, just at a lower frequency

Correct move:

State that linked genes tend to be inherited together, but can be separated by crossing over during prophase I

Wrong move:

Expecting a 9:3:3:1 ratio for linked gene dihybrid crosses

Why:

Linkage prevents independent assortment, so parental phenotypes are far more common

Correct move:

Predict that most offspring will have parental phenotypes, with a small number of recombinant offspring

5. Quick Reference Cheatsheet

Concept	Key Fact	Formula/Note
Linked genes	Located on same chromosome, do not assort independently	N/A
Crossing over	Occurs in prophase I, breaks linkage to make recombinants	N/A
Parental offspring	Original allele combination, more numerous	N/A
Recombinant offspring	New allele combination, less numerous	N/A
Recombination frequency	Equals distance between genes in map units	$\frac{Total recombinants}{Total offspring} \times 100$

6. Frequently Asked

What is the difference between autosomal linkage and sex linkage?

Autosomal linkage (covered here) refers to linkage of genes on non-sex chromosomes, while sex linkage refers to linkage of genes on X or Y sex chromosomes, which is covered in a separate sub-topic.

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.

2022 · 22
Calculate recombination frequency between genes
2021 · 12
Explain why ratio is not 9:3:3:1
2023 · 23
Identify linked genes from cross data

Going deeper

syllabusCIE 9700 A-Level Biology Official SyllabusCheck section 17.5 for full learning outcomes

What's Next

Understanding linkage and crossing over is the foundation for genetic mapping and explains how genetic variation is generated during meiosis. It connects to all core inheritance concepts, is frequently tested alongside dihybrid cross problems, and recombination frequency calculations are used to map disease-causing genes in human genetics. This topic also leads directly into the study of sex linkage, a specific form of linkage that is very commonly tested in CIE exams. Mastering the rules for linked gene crosses will help you gain full marks on the 10-15% of genetic cross marks typically found on paper 2.