Linked genes
Genes close together on a chromosome may be inherited together.
What is a dihybrid cross in AP Biology?
A dihybrid cross is a genetics cross that tracks two genes at the same time. In the classic AaBb × AaBb cross, each parent can make four gametes: AB, Ab, aB, and ab. If both genes assort independently and show complete dominance, the expected phenotype ratio is 9:3:3:1.
Say it fast
Dihybrid means two genes, four gametes, sixteen boxes.
AP exam tip: If the prompt tracks two traits or shows a genotype like AaBb, think dihybrid cross and independent assortment.
Dihybrid Means Two Genes
A dihybrid cross follows two genes at the same time. For example, AaBb includes one gene represented by A/a and another gene represented by B/b. Each gamete must receive one allele from the A gene and one allele from the B gene.
Comparison: Monohybrid cross = one gene. Dihybrid cross = two genes.
Review Monohybrid Crosses for one-gene crosses and 3:1 ratios. For general square setup, see Punnett Squares.
How to Find Gametes with FOIL
The hardest part of a dihybrid cross is often finding the gametes. For AaBb, use FOIL to combine one allele from each gene.
- First: AB
- Outer: Ab
- Inner: aB
- Last: ab
Result: AaBb can produce AB, Ab, aB, and ab gametes.
Important: Each gamete gets one allele from each gene. A gamete should not contain both A and a from the same gene.
AaBb × AaBb 4×4 Punnett Square
Because each AaBb parent can make four gametes, the Punnett square has four gametes across the top and four along the side. That creates 16 possible offspring genotype boxes.
| Parent gametes | AB | Ab | aB | ab |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
Why AaBb × AaBb Gives a 9:3:3:1 Ratio
Under complete dominance and independent assortment, AaBb × AaBb produces four phenotype categories.
- 9 show both dominant traits: A_B_
- 3 show dominant A and recessive b: A_bb
- 3 show recessive a and dominant B: aaB_
- 1 shows both recessive traits: aabb
Notation: The underscore means the second allele can be either dominant or recessive.
AEO callout: The 9:3:3:1 ratio only applies when both genes assort independently and both traits follow complete dominance.
Dihybrid Cross Examples with Answers
Use this quick-reference table for dihybrid cross examples with answers, valid gametes, and what each cross means on the AP Biology exam.
| Cross | Valid Gametes | Expected Result | AP Meaning |
|---|---|---|---|
| AaBb × AaBb | AB, Ab, aB, ab from each parent | 9:3:3:1 phenotype ratio | Classic independent assortment example. |
| AaBb × aabb | AB, Ab, aB, ab × ab | 1:1:1:1 if genes assort independently | Common dihybrid testcross pattern. |
| AABb × AaBb | AB, Ab × AB, Ab, aB, ab | No aa offspring possible | At least one A allele appears in every offspring. |
| AaBB × aabb | AB, aB × ab | 1 AaBb : 1 aaBb | B phenotype appears in all offspring. |
Dihybrid Gamete Builder
Choose a parent genotype to see valid gamete combinations under complete dominance and independent assortment.
AaBb
- Valid gametes
- AB, Ab, aB, ab
AP clue: Four gamete types means a 4×4 square if both parents are AaBb.
Common invalid gametes: Invalid examples include Aa, Bb, AABB, and abB. A gamete must contain one allele from each gene, not both alleles from the same gene.
Why Independent Assortment Matters
Dihybrid crosses depend on independent assortment. During meiosis I, homologous chromosome pairs line up randomly, which allows alleles of different genes to enter gametes in different combinations. If the genes are linked, the classic 9:3:3:1 ratio may not appear.
Review Independent Assortment for the full explanation of how alleles of different genes enter gametes in new combinations.
Keep scope tight: Linked genes can break the 9:3:3:1 expectation, but gene mapping and recombination frequency are separate topics.
Dihybrid Probability Shortcut
You do not always need to draw a full 4×4 square. You can solve each gene separately and multiply probabilities.
Example: For AaBb × AaBb:
- Chance of dominant A phenotype = 3/4
- Chance of dominant B phenotype = 3/4
- Chance of both dominant phenotypes = 3/4 × 3/4 = 9/16
Other probabilities:
- A_bb = 3/4 × 1/4 = 3/16
- aaB_ = 1/4 × 3/4 = 3/16
- aabb = 1/4 × 1/4 = 1/16
AP exam tip: Multiplication works when the genes assort independently.
Dihybrid Cross vs Monohybrid Cross
| Feature | Monohybrid Cross | Dihybrid Cross |
|---|---|---|
| Genes tracked | One gene | Two genes |
| Example | Aa × Aa | AaBb × AaBb |
| Gametes per heterozygous parent | A or a | AB, Ab, aB, ab |
| Punnett square | 2×2 | 4×4 |
| Classic phenotype ratio | 3:1 | 9:3:3:1 |
| AP clue | One trait | Two traits |
See Monohybrid Crosses for one-gene practice and Mendelian Genetics for allele rules behind both cross types.
When the 9:3:3:1 Ratio Changes
The classic 9:3:3:1 ratio depends on independent assortment and complete dominance. The ratio can change if genes are linked, traits are non-Mendelian, sample size is small, or observed data differ from expected data.
Non-Mendelian traits
Incomplete dominance, codominance, or multiple alleles can change phenotype categories.
Small sample size
Observed counts may not perfectly match expected ratios.
Chi-square testing
Used to test whether observed data fit expected ratios.
Links: Non-Mendelian Genetics · Chi-Square Test Genetics · Linked Genes and Recombination Frequency
AP Bio Exam Clues for Dihybrid Crosses
“Two traits”
→ dihybrid cross
“AaBb”
→ find four gametes
“AaBb × AaBb”
→ 9:3:3:1 under complete dominance
“FOIL”
→ AB, Ab, aB, ab
“4×4 Punnett square”
→ 16 boxes
“Independent assortment”
→ genes sort into gametes separately
“Linked genes”
→ expected ratio may change
“Observed vs expected 9:3:3:1”
→ chi-square may be needed
“Probability shortcut”
→ multiply independent probabilities
Common Dihybrid Cross Mistakes
Writing gametes incorrectly
Fix: Each gamete gets one allele from each gene, such as AB or Ab, not Aa or Bb.
Forgetting four gamete types
Fix: AaBb produces AB, Ab, aB, and ab.
Using a 2×2 square for AaBb × AaBb
Fix: A full AaBb × AaBb dihybrid Punnett square is 4×4.
Assuming 9:3:3:1 always applies
Fix: It only applies with independent assortment and complete dominance.
Confusing genotype and phenotype ratios
Fix: 9:3:3:1 is a phenotype ratio, not a full genotype ratio.
Ignoring linked genes
Fix: Linked genes may not assort independently, so expected ratios can change.
Forgetting to multiply independent probabilities
Fix: For independent genes, calculate each gene separately and multiply.
Identify the Dihybrid Cross Clue
Revealed: 0 of 5 scenarios
The prompt tracks seed shape and seed color.
Reveal: This is a dihybrid cross because two traits are tracked.
The genotype is AaBb.
Reveal: Possible gametes are AB, Ab, aB, and ab.
The cross is AaBb × AaBb with independent assortment and complete dominance.
Reveal: Expected phenotype ratio is 9:3:3:1.
A student writes Aa and Bb as gametes.
Reveal: Incorrect. Gametes should carry one allele from each gene, such as AB or ab.
Observed offspring do not fit 9:3:3:1.
Reveal: Possible explanations include linked genes, non-Mendelian inheritance, small sample size, or chance variation tested with chi-square.
Dihybrid Cross Practice Questions
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More drills: Unit 5 practice questions.
Dihybrid Cross FRQ Practice
Open each card, draft your response, then reveal the rubric and sample answer.
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Prompt
Two organisms with genotype AaBb are crossed. The genes assort independently and both traits show complete dominance. Identify the possible gametes from each parent and predict the expected phenotype ratio.
Scoring rubric
- Each AaBb parent can produce AB, Ab, aB, and ab gametes.
- Independent assortment allows different allele combinations.
- The cross AaBb × AaBb produces 16 possible offspring combinations.
- Under complete dominance, the expected phenotype ratio is 9:3:3:1.
- The four phenotype categories are both dominant, A dominant only, B dominant only, and both recessive.
Sample response
Each AaBb parent can produce four gamete types: AB, Ab, aB, and ab. These gametes form because each gamete receives one allele from the A gene and one allele from the B gene. Since the genes assort independently and both traits show complete dominance, a 4×4 Punnett square produces a 9:3:3:1 phenotype ratio. The largest category shows both dominant phenotypes, while the smallest category shows both recessive phenotypes.
Status: Draft your answer first—then open the rubric or sample.
Prompt
A student expects a 9:3:3:1 ratio from a dihybrid cross, but the observed offspring counts do not fit the expected ratio well. Give two biological or statistical explanations and describe how the student could test whether the difference is significant.
Scoring rubric
- Linked genes could cause offspring ratios to differ from independent assortment expectations.
- Non-Mendelian inheritance could change phenotype categories.
- Small sample size or chance variation could cause observed data to differ from expected data.
- A chi-square test can compare observed counts with expected counts.
- The student should compare the calculated chi-square value with a critical value.
Sample response
One explanation is that the genes may be linked, so they do not assort independently and the 9:3:3:1 ratio would not be expected. Another explanation is that the traits may not follow complete dominance, which could change the phenotype categories. Chance variation or small sample size could also affect observed counts. The student could use a chi-square test to compare observed counts with expected 9:3:3:1 counts and determine whether the difference is statistically significant.
Status: Draft your answer first—then open the rubric or sample.
Continue the Unit 5 Heredity Path
Dihybrid Cross FAQs
What is a dihybrid cross in AP Biology?
A dihybrid cross is a genetics cross that tracks two genes at the same time. In the classic AaBb × AaBb cross, each parent can make four gametes: AB, Ab, aB, and ab. If both genes assort independently and show complete dominance, the expected phenotype ratio is 9:3:3:1.
What does dihybrid mean?
Dihybrid means two genes. In AP Biology, a dihybrid cross follows two allele pairs at once, such as A/a and B/b in genotype AaBb.
How do you solve a dihybrid cross?
Identify parent genotypes, list gametes from each parent using FOIL for heterozygotes, fill a 4×4 Punnett square, count genotype combinations, and convert to phenotype ratios under complete dominance.
What gametes does AaBb produce?
AaBb produces four gamete types: AB, Ab, aB, and ab. Each gamete carries one allele from the A gene and one allele from the B gene.
What is the phenotype ratio for AaBb × AaBb?
Under complete dominance and independent assortment, AaBb × AaBb produces a 9:3:3:1 phenotype ratio.
Why is the dihybrid ratio 9:3:3:1?
Independent assortment creates four gamete combinations from each parent. When combined in a 4×4 square, complete dominance groups offspring into four phenotype categories with probabilities 9/16, 3/16, 3/16, and 1/16.
What is the FOIL method in genetics?
FOIL helps find all gamete combinations from AaBb: First (AB), Outer (Ab), Inner (aB), and Last (ab). It ensures you list every possible one-allele-from-each-gene combination.
What is the biggest mistake in dihybrid crosses?
The biggest mistake is writing gametes with both alleles from the same gene, such as Aa or Bb. A valid dihybrid gamete gets one allele from each gene, such as AB, Ab, aB, or ab.
What is the difference between a monohybrid and dihybrid cross?
A monohybrid cross tracks one gene and often uses a 2×2 square with a 3:1 phenotype ratio. A dihybrid cross tracks two genes and often uses a 4×4 square with a 9:3:3:1 phenotype ratio.
When does the 9:3:3:1 ratio apply?
The 9:3:3:1 ratio applies when both genes assort independently and both traits follow complete dominance.
Why might a dihybrid cross not fit 9:3:3:1?
Linked genes, non-Mendelian inheritance, small sample size, or chance variation can cause observed ratios to differ from 9:3:3:1.
How is independent assortment related to dihybrid crosses?
Independent assortment allows alleles of different genes to enter gametes in different combinations during meiosis I. That is why AaBb can produce AB, Ab, aB, and ab gametes.
How should I answer dihybrid cross FRQs?
Identify parent genotypes, list gametes, explain independent assortment if relevant, predict the phenotype ratio, and connect gamete logic to the ratio. If observed data differ, mention linked genes, non-Mendelian patterns, or chi-square testing.