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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

Coverage 2013–2025 · Updated 16 Jul 2026

Linked Genes Must Be Written as Allele Packages on Homologous Chromosomes

HL only
Same AaBb genotype Alleles on one homolog Alleles on the other Name
AB/ab A with B a with b coupling phase
Ab/aB A with b a with B repulsion phase

Writing only AaBb records which alleles are present but loses which combinations share a chromosome. For linked genes, parental phase determines which gametes are common and which require crossing over.

Autosomal linkage means two loci share a non-sex chromosome. Sex linkage is different: it refers to a locus on a sex chromosome, usually X.

A diagram contrasts linked A and B loci on the same chromosome, unlinked loci on different chromosomes and crossing over that creates Ab and aB recombinant chromatids from AB and ab parental chromatids.

A Crossover between Linked Loci Creates Reciprocal Recombinants

HL only
1

Start with a heterozygote in coupling phase: parental homologs carry AB and ab. After DNA replication, each homolog has two sister chromatids.

2

A chiasma between the A and B loci exchanges matching segments between non-sister chromatids. Two chromatids remain parental, while two become Ab and aB.

3
Chromatid or gamete class Allele combination Origin
parental AB and ab no exchange between the loci
recombinant Ab and aB crossover between the loci

A crossover elsewhere on the chromosome does not recombine these two loci. The exchange must occur between their positions to separate their parental allele combination.

A Linked Test Cross Exposes Parental and Recombinant Gamete Classes

HL only

Cross AB/ab with ab/ab. Because the tester contributes only ab, each offspring's phenotype or genotype identifies the gamete produced by the double heterozygote.

Heterozygote gamete Offspring Classification for AB/ab parent
AB AaBb parental
ab aabb parental
Ab Aabb recombinant
aB aaBb recombinant
  • two largest reciprocal classes identify the parental haplotypes
  • two smaller reciprocal classes identify recombinant gametes
  • an exact 1:1:1:1 pattern supports independent assortment, not strong linkage

Classify by comparison with the heterozygous parent's chromosome combinations, not by whether an offspring looks common or unusual in the population.

Recombinant Frequency Measures How Often Crossing Over Separates Two Loci

HL only

r=RN×100%r=\frac{R}{N}\times 100\%

Here, R is the number of recombinant offspring and N is the total. If a test cross gives 420 AB, 400 ab, 90 Ab and 90 aB offspring, r = (90 + 90) / 1000 × 100% = 18%.

Observation Inference
recombinants much below 50% loci are linked
lower recombinant frequency loci are usually closer, so fewer chiasmata fall between them
about 50% recombinants loci behave as unlinked; they may be on different chromosomes or far apart

Recombinant frequency does not exceed 50% in an ordinary two-locus test. Multiple crossovers can restore parental combinations, so frequency underestimates large physical distances.

Inheritance Models Are Built from Chromosomes and Tested with Offspring Counts

HL only

meiosis predicts gamete combinations → chromosome loci determine whether genes assort independently or travel together → crossing over creates reciprocal recombinants → test-cross counts reveal parental phase and recombinant frequency → chi-squared tests whether departures from a chosen ratio exceed chance expectation

Offspring pattern First model to examine
9:3:3:1 F2 two unlinked genes with complete dominance
1:1:1:1 test cross four equally frequent gametes from an unlinked double heterozygote
two large and two small reciprocal test-cross classes linked loci with crossing over
significant χ² departure assumptions, linkage, viability, fertilization or scoring require review

Keep prediction and evidence separate: define the genetic model first, calculate expected counts from it, compare observations with the expectation, and change the model only when chromosome location or statistical evidence requires it.

Autosomal gene linkage

HL only

[ 3 ]

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

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.