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D3.2.20 (HL)—Recombinants

Recombinants have allele combinations different from parental combinations because crossing over reshuffles linked genes during meiosis in inheritance problems in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.20
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2022
Most common paperPaper1
Typical marks1

Common command terms

  • Deduce
  • Identify

Recent exam appearances

May 2022Paper1 ["HL"] · TZ136[ 1 ]D3.2.20 (HL)—Recombinants
May 2021Paper1 ["HL"] · TZ136[ 1 ]D3.2.20 (HL)—Recombinants
November 2019Paper2 ["HL"] · TZ02(b)[ 1 ]D3.2.20 (HL)—Recombinants
November 2015Paper2 ["HL"] · TZ04(c)[ 1 ]D3.2.20 (HL)—Recombinants
November 2014Paper1 ["HL"] · TZ034[ 1 ]D3.2.20 (HL)—Recombinants

Coverage 2012–2022 · Updated 16 Jul 2026

A Dihybrid Test Cross Reads Gamete Frequencies Directly

HL only

Cross the double heterozygote AaBb with aabb. The tester can contribute only ab, so each offspring genotype reveals which gamete came from the heterozygote.

AaBb gamete Tester gamete Offspring Phenotype class
AB ab AaBb both dominant
Ab ab Aabb A dominant only
aB ab aaBb B dominant only
ab ab aabb both recessive

If A/a and B/b assort independently, AB, Ab, aB and ab gametes each occur with probability 1/4, so the test-cross phenotype expectation is 1:1:1:1.

A strong excess of two reciprocal classes and a shortage of the other two suggests linkage: parental chromosome combinations are being transmitted more often than recombinant combinations.

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.

Recombinants exam focus

HL only

[ 1 ]

An individual is heterozygous for two linked genes ABab‾\frac{\mathrm{AB}}{\overline{\mathrm{ab}}}.

To investigate the frequency of crossing over, a test cross is carried out between the individual and another that is homozygous recessive for both genes. What are the possible recombinants in the offspring of this cross?

Concept essentials

  • Recombinants differ from parental allele combinations.
  • Crossing over produces recombinant chromosomes during meiosis.
  • Recombination frequency can indicate distance between linked loci.