Identify the parent genotypes.
Example: Aa × Aa
What is a monohybrid cross in AP Biology?
A monohybrid cross is a genetics cross that tracks one gene with two alleles. In a common heterozygous × heterozygous cross, Aa × Aa, the genotype ratio is 1 AA : 2 Aa : 1 aa. If A is completely dominant over a, the phenotype ratio is 3 dominant : 1 recessive.
Say it fast
Monohybrid means one gene, one trait pattern.
AP exam tip: If the prompt tracks one trait and gives parent genotypes like Aa × Aa or Aa × aa, use a 2×2 Punnett square.
Monohybrid Means One Gene
A monohybrid cross follows one gene at a time. The gene may have two allele versions, such as A and a. Each gamete receives one allele from each parent, and the offspring genotype depends on which two alleles combine during fertilization. On AP Biology exams, one-trait wording, a 2×2 square, and parent notation like Aa × Aa or Aa × aa are strong signals that you are working with a monohybrid cross rather than a two-gene dihybrid problem.
Compare: Monohybrid cross = one gene. Dihybrid cross = two genes. Do not mix them.
Before building squares, review the general method in Punnett squares and allele basics in Mendelian genetics.
How to Solve a Monohybrid Cross
List the possible gametes.
Each parent can make A or a gametes.
Build a 2×2 Punnett square.
Put one parent's gametes across the top and the other parent's gametes along the side.
Fill offspring genotypes.
Combine one allele from each parent in every box.
Count genotype ratio.
For Aa × Aa, the ratio is 1 AA : 2 Aa : 1 aa.
Convert to phenotype ratio.
Under complete dominance, AA and Aa show the dominant phenotype, while aa shows the recessive phenotype.
AP exam callout: Always separate genotype ratio from phenotype ratio. They are not the same.
Aa × Aa Monohybrid Cross
The most common AP Biology monohybrid example is Aa × Aa. Both parents are heterozygous, so each parent can pass either A or a. If you need to label genotypes first, review homozygous vs heterozygous before counting ratios.
| Parent gametes | A | a |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Results:
- 1 AA
- 2 Aa
- 1 aa
Genotype ratio: 1 AA : 2 Aa : 1 aa
Phenotype ratio under complete dominance: 3 dominant : 1 recessive
Monohybrid Cross Examples with Answers
Use this quick-reference table when an AP Biology prompt asks for monohybrid cross examples with answers, expected ratios, or what each cross means on the exam.
| Cross | Genotype Ratio | Phenotype Ratio | AP Meaning |
|---|---|---|---|
| AA × aa | 100% Aa | 100% dominant | All offspring are heterozygous. |
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 dominant : 1 recessive | Classic complete-dominance monohybrid ratio. |
| Aa × aa | 1 Aa : 1 aa | 1 dominant : 1 recessive | Common test-cross pattern. |
| aa × aa | 100% aa | 100% recessive | All offspring show recessive phenotype. |
Monohybrid Ratio Builder
Choose a parent cross to instantly see the expected genotype ratio, phenotype ratio, and AP exam clue under complete dominance.
AA × aa
- Gametes
- A and a
- Genotype ratio
- 100% Aa
- Phenotype ratio
- 100% dominant
AP clue: All offspring are heterozygous but show the dominant phenotype.
Genotype Ratio vs Phenotype Ratio
A genotype ratio counts allele combinations. A phenotype ratio counts visible or expressed traits. In complete dominance, AA and Aa usually show the same dominant phenotype, even though they are different genotypes.
Review the genotype vs phenotype guide if you need help separating allele combinations from expressed traits before counting ratios.
| Cross | Genotype Ratio | Phenotype Ratio |
|---|---|---|
| AA × aa | 100% Aa | 100% dominant |
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 dominant : 1 recessive |
| Aa × aa | 1 Aa : 1 aa | 1 dominant : 1 recessive |
| aa × aa | 100% aa | 100% recessive |
Step-by-Step Monohybrid Walkthroughs
Not every monohybrid cross is Aa × Aa. AP Biology questions may give homozygous parents, test-cross setups, or ask you to compare expected ratios across different parent pairs. Work through these three common one-gene crosses before practice MCQs.
Homozygous dominant × homozygous recessive (AA × aa)
One parent can only pass A alleles; the other can only pass a alleles. Every offspring receives one A and one a, so all offspring are heterozygous Aa.
Ratios: Genotype ratio: 100% Aa. Phenotype ratio: 100% dominant under complete dominance.
AP clue: AP clue: If a cross produces no recessive offspring and one parent is known homozygous recessive, the other parent must be homozygous dominant.
Heterozygous × homozygous recessive (Aa × aa)
The heterozygous parent makes A and a gametes. The homozygous recessive parent makes only a gametes. Half the offspring are Aa and half are aa.
Ratios: Genotype ratio: 1 Aa : 1 aa. Phenotype ratio: 1 dominant : 1 recessive under complete dominance.
AP clue: AP clue: A 1:1 phenotype ratio often signals Aa × aa, not Aa × Aa. Read the parent genotypes before choosing a ratio.
Homozygous recessive × homozygous recessive (aa × aa)
Both parents pass only a alleles. Every offspring is aa regardless of sample size.
Ratios: Genotype ratio: 100% aa. Phenotype ratio: 100% recessive.
AP clue: AP clue: This cross confirms recessive inheritance but does not reveal dominance by itself—you need a cross that includes at least one dominant allele.
Alleles segregate during meiosis so each gamete carries one allele per gene. That segregation is why parent genotypes determine which gametes appear on the Punnett square edges.
Complete Dominance in Monohybrid Crosses
Complete dominance means one allele masks the other allele in a heterozygote. If A is completely dominant over a, both AA and Aa show the dominant phenotype. Only aa shows the recessive phenotype.
Key ratio: The 3:1 phenotype ratio appears in Aa × Aa only when the trait follows complete dominance.
Review Mendelian Genetics for Mendel's laws and dominant-recessive allele logic.
Test Cross: Finding an Unknown Genotype
A test cross helps determine whether an organism with a dominant phenotype is homozygous dominant or heterozygous. The unknown organism is crossed with a homozygous recessive organism.
For the full unknown-genotype evidence workflow, see the dedicated test cross AP Biology guide.
Example: Unknown dominant phenotype × aa
- If all offspring show dominant phenotype: unknown parent may be AA.
- If some recessive offspring appear: unknown parent must be Aa.
AP exam clue: A test cross uses a homozygous recessive individual because recessive offspring reveal hidden recessive alleles.
Punnett Squares Show Probability, Not Certainty
A Punnett square predicts possible outcomes and expected ratios. It does not guarantee that a small family will exactly match the ratio. For example, Aa × Aa predicts a 3:1 phenotype ratio, but four actual offspring may not perfectly show three dominant and one recessive.
AP exam wording: Use "expected," "probability," or "predicted," not "guaranteed." Small samples—especially families of three or four offspring—often deviate from textbook ratios even when the genetic model is correct.
When observed counts differ from expected ratios, use a chi-square test to test whether the difference is likely due to chance.
When Monohybrid Ratios Change
The classic 3:1 phenotype ratio assumes complete dominance. If the trait shows incomplete dominance, codominance, multiple alleles, or sex-linked inheritance, the phenotype pattern may change. When a prompt describes blended heterozygotes, both alleles visible, or sex-specific inheritance, switch to non-Mendelian reasoning instead of forcing a 3:1 answer.
- Incomplete dominance can make heterozygotes intermediate.
- Codominance can show both alleles.
- Sex-linked traits require X and Y notation.
Review Non-Mendelian Genetics when ratios do not match classic Mendelian patterns. For two-gene prediction practice, see dihybrid crosses.
AP Bio Exam Clues for Monohybrid Crosses
“One trait”
→ monohybrid cross
“Aa × Aa”
→ 1:2:1 genotype ratio
“Complete dominance”
→ 3:1 phenotype ratio
“Aa × aa”
→ 1:1 phenotype ratio
“Unknown dominant genotype”
→ test cross
“Homozygous recessive tester”
→ test cross
“Probability”
→ expected, not guaranteed
“Genotype ratio”
→ allele combinations
“Phenotype ratio”
→ expressed traits
Read whether the question asks for genotype or phenotype
Underline the exact wording. Genotype ratio counts AA, Aa, and aa. Phenotype ratio groups visible traits. Many AP Biology errors come from reporting 3:1 when the rubric asks for 1:2:1.
Write gametes before you draw the square
List each parent's possible gametes in the margin. For monohybrid crosses, each gamete carries one allele. If gametes are wrong, every box in the Punnett square will be wrong.
Use Mendelian vocabulary precisely
Homozygous means two matching alleles (AA or aa). Heterozygous means two different alleles (Aa). Dominant describes which allele is expressed in a heterozygote, not how common a trait is in a population.
Connect crosses to meiosis when asked
Gamete diversity comes from segregation during meiosis. Each parent contributes one allele per gene at fertilization. Linking meiosis to Punnett square logic earns full reasoning credit on many FRQs.
Common Monohybrid Cross Mistakes
Confusing genotype ratio and phenotype ratio
Fix: Genotype counts allele combinations. Phenotype counts expressed traits.
Saying 3:1 is always the genotype ratio
Fix: For Aa × Aa, 1:2:1 is genotype ratio and 3:1 is phenotype ratio under complete dominance.
Forgetting heterozygotes show dominant phenotype
Fix: Under complete dominance, Aa shows the dominant phenotype.
Treating Punnett square results as guaranteed
Fix: Punnett squares show probability, not exact offspring counts.
Using a 4×4 square for one gene
Fix: A monohybrid cross usually uses a 2×2 square.
Forgetting the test cross uses homozygous recessive
Fix: Use aa to reveal whether the unknown parent carries a recessive allele.
Identify the Monohybrid Cross Clue
Revealed: 0 of 5 scenarios
The prompt tracks one trait controlled by A and a.
Reveal: Use a monohybrid cross.
The cross is Aa × Aa.
Reveal: Genotype ratio is 1 AA : 2 Aa : 1 aa.
The cross is Aa × aa.
Reveal: Expected genotype ratio is 1 Aa : 1 aa.
A dominant phenotype organism is crossed with aa to find its genotype.
Reveal: This is a test cross.
A student says 3:1 is the genotype ratio for Aa × Aa.
Reveal: Incorrect. 3:1 is the phenotype ratio under complete dominance. The genotype ratio is 1:2:1.
Monohybrid Cross Practice Questions
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Monohybrid Cross FRQ Practice
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Prompt
Two heterozygous pea plants are crossed for a trait showing complete dominance. Predict the genotype and phenotype ratios and explain why the two ratios are different.
Scoring rubric
- Parent genotypes are Aa × Aa.
- Genotype ratio is 1 AA : 2 Aa : 1 aa.
- Phenotype ratio is 3 dominant : 1 recessive.
- AA and Aa have the same phenotype under complete dominance.
- Genotype counts allele combinations; phenotype counts expressed traits.
Sample response
The cross is Aa × Aa. Each parent can produce A or a gametes. The offspring genotypes are AA, Aa, Aa, and aa, so the genotype ratio is 1 AA : 2 Aa : 1 aa. Under complete dominance, AA and Aa show the dominant phenotype, while aa shows the recessive phenotype. Therefore, the phenotype ratio is 3 dominant : 1 recessive. The ratios differ because genotype counts allele combinations, while phenotype counts visible traits.
Status: Draft your answer first—then open the rubric or sample.
Prompt
A plant with a dominant phenotype is crossed with a homozygous recessive plant. Some offspring show the recessive phenotype. Explain what this reveals about the dominant-phenotype parent.
Scoring rubric
- This is a test cross.
- Homozygous recessive parent contributes only recessive alleles.
- Recessive offspring must receive a recessive allele from both parents.
- Therefore, the dominant-phenotype parent must be heterozygous.
- Use genotype notation such as Aa × aa.
Sample response
This is a test cross because the dominant-phenotype plant is crossed with a homozygous recessive plant. The recessive parent can only contribute a recessive allele. If some offspring show the recessive phenotype, those offspring must have received a recessive allele from both parents. Therefore, the dominant-phenotype parent must carry a recessive allele and is heterozygous, Aa.
Status: Draft your answer first—then open the rubric or sample.
Continue the Unit 5 Heredity Path
Monohybrid Cross FAQs
What is a monohybrid cross in AP Biology?
A monohybrid cross is a genetics cross that tracks one gene with two alleles. In a common heterozygous × heterozygous cross, Aa × Aa, the genotype ratio is 1 AA : 2 Aa : 1 aa. If A is completely dominant over a, the phenotype ratio is 3 dominant : 1 recessive.
What does monohybrid mean?
Monohybrid means one gene, one trait pattern. A monohybrid cross follows inheritance for a single gene rather than two genes at once.
How do you solve a monohybrid cross?
Identify parent genotypes, list possible gametes, build a 2×2 Punnett square, fill offspring genotypes, count the genotype ratio, and convert to phenotype ratio under complete dominance.
What is the genotype ratio for Aa × Aa?
The expected genotype ratio for Aa × Aa is 1 AA : 2 Aa : 1 aa, also written as 1:2:1.
What is the phenotype ratio for Aa × Aa?
Under complete dominance, Aa × Aa produces a 3 dominant : 1 recessive phenotype ratio because AA and Aa show the dominant phenotype while aa shows the recessive phenotype.
What is the difference between Aa × Aa and Aa × aa?
Aa × Aa is a heterozygous × heterozygous cross that produces a 1:2:1 genotype ratio and a 3:1 phenotype ratio under complete dominance. Aa × aa is a heterozygous × homozygous recessive cross that produces a 1:1 genotype ratio and a 1:1 phenotype ratio.
Why is the genotype ratio 1:2:1 but the phenotype ratio 3:1?
Genotype ratio counts allele combinations (AA, Aa, aa). Phenotype ratio counts expressed traits. Under complete dominance, AA and Aa both show the dominant phenotype, so three of four genotypes look dominant.
What is a test cross?
A test cross mates an individual with unknown genotype to a homozygous recessive individual (aa). Recessive offspring reveal that the unknown parent carries a recessive allele.
What is the difference between genotype ratio and phenotype ratio?
Genotype ratio counts allele combinations such as 1 AA : 2 Aa : 1 aa. Phenotype ratio counts visible or expressed traits such as 3 dominant : 1 recessive.
Are monohybrid cross ratios guaranteed?
No. Punnett squares show expected probability, not guaranteed offspring counts. Small samples may not match the predicted ratio exactly.
When does the 3:1 phenotype ratio apply?
The 3:1 phenotype ratio applies to a heterozygous × heterozygous monohybrid cross (Aa × Aa) when the trait follows complete dominance.
What is the difference between monohybrid and dihybrid crosses?
A monohybrid cross tracks one gene with two alleles. A dihybrid cross tracks two genes at once and uses a larger Punnett square. Monohybrid crosses focus on one-trait inheritance patterns.
How should I answer monohybrid cross FRQs?
State parent genotypes, list gametes, describe offspring genotypes, report genotype and phenotype ratios separately, and explain why the ratios differ under complete dominance.