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D3.2.17 (HL)—Dihybrid crosses

Dihybrid crosses track inheritance of two genes at once, using gamete combinations to predict offspring ratios in inheritance problems in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.17
Level
HL

Exam analysis

Chance of appearing12%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1–2

Common command terms

  • Determine
  • Identify
  • State
  • Explain
  • Predict

Recent exam appearances

November 2025Paper2 ["HL"] · TZ36(a)[ 3 ]D3.2.17 (HL)—Dihybrid crosses
November 2024Paper2 ["HL"] · TZ03(b)(i)[ 2 ]D3.2.17 (HL)—Dihybrid crosses
November 2023Paper2 ["HL"] · TZ24(c)[ 1 ]D3.2.17 (HL)—Dihybrid crosses
November 2023Paper2 ["HL"] · TZ24(b)[ 3 ]D3.2.17 (HL)—Dihybrid crosses
November 2023Paper2 ["HL"] · TZ24(a)[ 1 ]D3.2.17 (HL)—Dihybrid crosses

Coverage 2012–2025 · Updated 16 Jul 2026

Derive Dihybrid Gametes One Allele from Each Gene at a Time

HL only

Every gamete from AaBb must contain one allele from the A/a locus and one from the B/b locus. For unlinked loci, multiply the independent probabilities.

A-locus choice B-locus choice Gamete Probability
A, 1/2 B, 1/2 AB 1/4
A, 1/2 b, 1/2 Ab 1/4
a, 1/2 B, 1/2 aB 1/4
a, 1/2 b, 1/2 ab 1/4

A valid gamete list has one allele per locus, no duplicate types and probabilities summing to 1. Writing A, a, B and b as four separate gametes loses one gene from each gamete.

The same multiplication rule scales: an individual heterozygous at n independently assorting loci can make 2ⁿ gamete types, before considering linkage or unequal viability.

Two Unlinked Heterozygotes Produce a 9:3:3:1 F2 Expectation

HL only

Assume both genes are autosomal and unlinked, complete dominance acts at each locus, both parents are AaBb, the four gamete types are equally frequent, fertilization is random and offspring classes survive equally.

Phenotype class Probability calculation Expected fraction
A_B_ 3/4 × 3/4 9/16
A_bb 3/4 × 1/4 3/16
aaB_ 1/4 × 3/4 3/16
aabb 1/4 × 1/4 1/16

The ratio is two independent 3:1 monohybrid probabilities multiplied together. A 4 × 4 grid lists all 16 genotype combinations; the product rule reaches the same phenotype expectation more efficiently.

An AaBb by AaBb dihybrid Punnett grid groups offspring into four phenotype classes in a 9:3:3:1 ratio and contrasts an AaBb by aabb test cross with a 1:1:1:1 ratio.

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.

The Unlinked Model Runs from Meiosis to Dihybrid Ratios

HL only

homologs separate alleles → independently oriented bivalents combine alleles from unlinked genes → AaBb makes AB, Ab, aB and ab gametes equally → random fertilization produces 9:3:3:1 in an F2 or 1:1:1:1 in a test cross

Prediction Required conditions
1:1 allele segregation normal meiosis and equal gamete contribution
four equal AaBb gametes loci unlinked or recombination effectively 50%
9:3:3:1 F2 complete dominance, random fertilization and equal survival
1:1:1:1 test cross double heterozygote × double recessive tester

A database locates the genes; the cross tests their behaviour. If observed offspring depart systematically from the unlinked expectation, inspect chromosome location, parental phase, crossing over and viability before blaming random sampling alone.

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.

Dihybrid crosses

HL only

[ 3 ]

The expected ratio of phenotypes in the offspring of a cross between a plant with narrow, yellow leaves and a plant heterozygous for the genes for leaf width and colour is 1: 1: 1: 1.

Justify this expected ratio using a Punnett grid or other diagram.

Concept essentials

  • A dihybrid cross follows two genes simultaneously.
  • Independent assortment creates different gamete combinations for unlinked genes.
  • The 9:3:3:1 ratio applies only to particular heterozygous unlinked crosses.