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D3.2.19 (HL)—Autosomal gene linkage

Autosomal gene linkage occurs when genes on the same non-sex chromosome tend to be inherited together unless crossing over separates them.

Syllabus
First assessment 2025
Objective
D3.2.19
Level
HL

Exam analysis

Chance of appearing14%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–2

Common command terms

  • Identify
  • Define
  • Compare
  • Explain
  • Distinguish
  • Outline

Recent exam appearances

November 2025Paper2 ["HL"] · TZ36(b)[ 2 ]D3.2.19 (HL)—Autosomal gene linkage
November 2025Paper1A ["HL"] · TZ131[ 1 ]D3.2.19 (HL)—Autosomal gene linkage
May 2023Paper1 ["HL"] · TZ232[ 1 ]D3.2.19 (HL)—Autosomal gene linkage
November 2022Paper2 ["HL"] · TZ02(b)[ 2 ]D3.2.19 (HL)—Autosomal gene linkage
November 2021Paper2 ["HL"] · TZ02(c)[ 2 ]D3.2.19 (HL)—Autosomal gene linkage
Practice this objective

Coverage 2013–2025 · Updated 16 Jul 2026

Linked Autosomal Genes Do Not Assort Independently

HL only

Autosomal linkage occurs when two genes lie on the same autosome and are therefore inherited together more often than expected by independent assortment.

The chromosome, not each allele, moves as a unit during meiosis unless crossing over occurs between the loci. Closer loci have a lower chance of being separated by a chiasma.

Show linked alleles beside two vertical lines representing homologous chromosomes—for example AB/ab for coupling or Ab/aB for repulsion—rather than writing only AaBb.

A test cross producing many AB and ab offspring but few Ab and aB offspring supports linkage in the AB/ab heterozygote.

Gene linkage means loci share a chromosome; sex linkage specifically means a locus lies on a sex chromosome. They are not synonyms.

Autosomal gene linkage

HL only

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Define / Compare.

Command terms

Identify / Define / Compare / Explain / Distinguish / Outline

What earns marks

Build the answer around this relationship: Linked autosomal genes are on the same non-sex chromosome.

Representative question

Question 1

[Maximum number: 3]

Outline how it can be shown that the genes for shell base colour (Cc) and presence or absence of bands (Bb) are linked.

Retrieve the HL Inheritance Route

HL only

HL D3.2 is secure when chromosome behaviour explains the ratios: segregation and independent assortment produce unlinked dihybrid expectations, gene loci explain linkage, recombinants reveal crossing over, and chi-squared decides whether observed counts fit the expected model.

  • homologous chromosomes separate and random bivalent orientation assort unlinked genes
  • unlinked autosomal genes can produce 9:3:3:1 or 1:1:1:1 ratios
  • linked genes give more parental types and fewer recombinants after crossing over
  • observed counts are compared with expected ratios using df and p = 0.05

Solve HL Linkage and Chi-Squared Questions

HL only

HL inheritance transfer is about deciding whether the expected ratio should be Mendelian or linked, then testing the evidence. Start from meiosis and gene location, predict gametes or ratios, identify parental and recombinant classes, and use chi-squared when observed counts need a statistical conclusion.

  • Explain segregation and independent assortment from meiosis before using dihybrid ratios.
  • Use gene loci, linkage, crossing over, and recombinant frequency to interpret offspring classes.
  • Apply chi-squared with observed/expected values, degrees of freedom, p = 0.05, and a null-hypothesis conclusion.

Concept essentials

  • Linked autosomal genes are on the same non-sex chromosome.
  • Linked genes tend to be inherited together more often than expected by independent assortment.
  • Crossing over can produce recombinant offspring from linked genes.