Dominant phenotype
At least one dominant allele may be present.
Where Mendelian Genetics Fits in Unit 5
The previous guide, Independent Assortment, explained how chromosome pairs sort into gametes. Mendelian genetics turns that chromosome movement into inheritance predictions by tracking alleles from parents to offspring. After this page, study Punnett Squares to practice calculating genotype and phenotype probabilities.
What is Mendelian genetics in AP Biology?
Mendelian genetics is the study of how alleles are inherited from parents and how those alleles predict traits in offspring. It focuses on patterns such as dominant and recessive inheritance, segregation of alleles into gametes, and independent assortment of unlinked genes. AP Biology uses Mendelian genetics to connect meiosis, Punnett squares, probability, and trait prediction.
Say it fast
Mendelian genetics tracks alleles to predict traits.
Mendel's Experiment Pattern: P, F1, and F2
Mendel often started with true-breeding P generation parents. The F1 generation received one allele from each parent and often showed the dominant phenotype. When F1 offspring self-crossed, the F2 generation could reveal the recessive phenotype again. In a simple complete-dominance monohybrid cross, the F2 phenotype ratio is often 3 dominant : 1 recessive.
| Generation | Meaning | AP Biology Clue |
|---|---|---|
| P generation | Original true-breeding parents | Often homozygous |
| F1 generation | First offspring generation | Often heterozygous |
| F2 generation | Offspring from F1 cross | Recessive phenotype can reappear |
| 3:1 ratio | Common F2 phenotype ratio | Suggests simple dominant-recessive inheritance |
For step-by-step ratio work on one gene, see monohybrid crosses.
Mendelian Genetics Explorer
Interactive Mendelian genetics explorer — tap each concept
A gene is a DNA sequence that helps determine a trait. Different versions of a gene are called alleles.
An allele is a version of a gene. Offspring inherit alleles from parents through gametes.
A genotype is the allele combination an organism has, such as AA, Aa, or aa.
A phenotype is the expressed trait. In simple dominant-recessive inheritance, AA and Aa may show the same dominant phenotype.
Mendelian genetics uses allele combinations to predict likely offspring outcomes. Punnett squares are one tool for modeling those probabilities.
Alleles: Versions of a Gene
An allele is a version of a gene. For many Mendelian problems, each organism has two alleles for a gene, one inherited from each parent. During meiosis, alleles separate into gametes, which is why offspring receive one allele from each parent.
Direct answer: A gene is the trait-related DNA region; an allele is a version of that gene.
Dominant vs Recessive Alleles
In simple Mendelian inheritance, a dominant allele can be expressed when only one copy is present. A recessive allele is expressed only when two recessive copies are present. This is why a heterozygous genotype can show the dominant phenotype.
| Genotype | Allele combination | Common phenotype result |
|---|---|---|
| AA | Two dominant alleles | Dominant phenotype |
| Aa | One dominant, one recessive | Dominant phenotype |
| aa | Two recessive alleles | Recessive phenotype |
AP callout: Dominant does not mean more common, stronger, or better. It only describes expression in a heterozygote.
Law of Dominance
The law of dominance states that, in a simple complete-dominance pattern, one dominant allele can mask a recessive allele in a heterozygote. For example, Aa shows the dominant phenotype even though the recessive allele is still present.
Warning: Do not assume every trait follows complete dominance. Incomplete dominance and codominance are Non-Mendelian patterns.
Genotype vs Phenotype
Genotype refers to the allele combination an organism has. Phenotype refers to the observable or measurable trait that results from genotype and sometimes environment. AP Biology often asks students to distinguish genotype ratios from phenotype ratios.
For a dedicated deep dive on definitions, AA/Aa/aa examples, and the Genotype-to-Phenotype Decoder, see the genotype vs phenotype guide.
| Term | Meaning | Example |
|---|---|---|
| Genotype | Allele combination | AA, Aa, aa |
| Phenotype | Expressed trait | purple flower, white flower |
| Homozygous | Two matching alleles | AA or aa |
| Heterozygous | Two different alleles | Aa |
AP Biology often asks you to report both genotype ratio vs phenotype ratio. Genotype ratio tracks allele combinations; phenotype ratio tracks visible traits after dominance is applied.
Homozygous vs Heterozygous
Homozygous means two matching alleles, such as AA or aa. Heterozygous means two different alleles, such as Aa. In simple Mendelian inheritance, a heterozygous organism shows the dominant phenotype. For AA, Aa, and aa naming with practice MCQs, see the homozygous vs heterozygous guide.
| Term | Allele pattern | Typical Mendelian result |
|---|---|---|
| Homozygous dominant | AA | Dominant phenotype |
| Heterozygous | Aa | Dominant phenotype |
| Homozygous recessive | aa | Recessive phenotype |
Monohybrid Cross Ratios
A monohybrid cross tracks one gene. In a typical heterozygous × heterozygous cross, the genotype ratio is 1:2:1 and the phenotype ratio is 3:1 when one allele is completely dominant.
Mendel began with true-breeding parents in the P generation, crossed them to produce the F1 generation, then self-crossed F1 plants to observe the F2 generation. That F2 pattern is where the classic 3:1 phenotype ratio and 1:2:1 genotype ratio appear in a monohybrid cross AP Biology problem.
AP exam clue: A 3:1 phenotype ratio often suggests simple dominant-recessive Mendelian inheritance. A 1:2:1 genotype ratio shows the allele combinations behind that phenotype pattern.
Test cross: A test cross helps determine an unknown genotype by crossing the organism with a homozygous recessive individual.
For step-by-step probability work, use the Punnett squares guide—this page focuses on Mendelian rules, not full square construction.
Mendel's Law of Segregation
Mendel's law of segregation states that the two alleles for a gene separate during gamete formation. This happens because homologous chromosomes separate during meiosis I. As a result, each gamete receives only one allele for a gene.
Direct answer: Segregation explains why each gamete carries one allele from each allele pair.
See meiosis for the full cell-division context and independent assortment for how different genes can sort into gametes.
Mendel's Law of Independent Assortment
Mendel's law of independent assortment states that alleles for different genes can be passed into gametes independently. This works best when genes are unlinked, meaning they are on different chromosomes or far apart on the same chromosome. AP Biology often connects this idea to dihybrid crosses.
| Mendel's law | What it means | Meiosis connection |
|---|---|---|
| Segregation | Allele pairs separate | Homologous chromosomes separate |
| Independent assortment | Different genes can sort independently | Chromosome pairs orient randomly |
| Dominance | One allele can mask another | Heterozygote phenotype |
Continue with independent assortment, dihybrid crosses, and Punnett squares for two-gene prediction practice.
Mendelian Pattern Sorter
Select a clue to see the Mendelian concept it points to.
Concept:
How Mendelian Genetics Connects to Punnett Squares
Punnett squares are diagrams that use Mendelian allele logic to predict possible offspring outcomes. They do not guarantee exact offspring numbers; they show probability. To use a Punnett square well, students must know the parent genotypes, possible gametes, and how genotypes translate into phenotypes.
Callout: Mendelian genetics gives the rules. Punnett squares model the probabilities.
Next guide: Punnett Squares
Mendelian vs Non-Mendelian Inheritance
Mendelian inheritance uses predictable dominant-recessive allele patterns. Non-Mendelian inheritance includes incomplete dominance, codominance, multiple alleles, polygenic traits, and sex-linked traits.
When offspring ratios do not match a 3:1 phenotype pattern or a 1:2:1 genotype pattern, check whether non-Mendelian inheritance fits the data better than simple dominance.
How AP Biology Tests Mendelian Genetics
Recessive phenotype
Usually two recessive alleles are required.
Heterozygous
The genotype has two different alleles.
Alleles separate into gametes
Law of segregation is involved.
Genes on different chromosomes
Independent assortment may apply.
Genotype ratio
Track allele combinations, not just visible traits.
How to Answer Mendelian Genetics FRQs
Identify the inheritance pattern
Decide whether simple dominance, recessive expression, or non-Mendelian patterns fit the data.
Assign allele symbols
Use consistent letters and define which allele is dominant or recessive.
Predict genotypes and phenotypes
List gametes or build a Punnett square, then report ratios.
Justify with evidence
Connect predictions to segregation, dominance, or data from the prompt.
AP FRQ writing frame
Because ___ is dominant/recessive, the genotype ___ produces ___. The expected offspring outcome is ___ because ___.
Common AP Bio Mendelian Genetics Mistakes
Saying dominant means common
Fix: Dominant means expressed in a heterozygote, not more frequent.
Confusing genotype and phenotype
Fix: Genotype is allele combination; phenotype is expressed trait.
Forgetting gametes carry one allele
Fix: Segregation puts one allele into each gamete.
Treating all traits as Mendelian
Fix: Some traits follow non-Mendelian inheritance patterns.
Ignoring parent genotypes
Fix: You need parent genotypes before predicting offspring.
Reporting only phenotype ratios
Fix: AP questions may ask for genotype ratios too.
Mendelian Genetics Clue Lab
Revealed: 0 of 4 scenarios
An organism has genotype Aa.
Answer: The organism is heterozygous and may show the dominant phenotype in simple Mendelian inheritance.
A trait appears only when genotype is aa.
Answer: The trait is likely recessive in this simple inheritance model.
Alleles A and a separate into different gametes.
Answer: This is Mendel's law of segregation.
Two genes are on different chromosomes.
Answer: They may assort independently during meiosis.
Mendelian Genetics MCQ Practice
Answer all eight questions. Choices shuffle on reload—focus on mechanism, not letter memorization.
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More drills: Unit 5 practice questions, practice by topic, or daily AP Biology practice.
Mendelian Genetics FRQ Practice
Open each card, draft your response, then reveal the rubric and sample. For more free-response practice, open the Unit 5 practice questions.
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Prompt
A plant trait follows simple dominant-recessive inheritance. A heterozygous plant is crossed with a homozygous recessive plant.
- A. Identify the possible gametes from each parent.
- B. Predict the expected genotype ratio.
- C. Explain how the phenotype ratio depends on dominance.
Scoring rubric
- 1 pt — Lists gametes from heterozygous parent (two allele types).
- 1 pt — Lists gametes from homozygous recessive parent (one allele type).
- 1 pt — Predicts 1:1 genotype ratio (e.g., Aa : aa).
- 1 pt — Connects dominance to phenotype expression in heterozygotes.
- 1 pt — Explains expected phenotype ratio with complete dominance.
- 1 pt — Uses Mendelian segregation logic (one allele per gamete).
Sample response
The heterozygous parent produces gametes with A or a; the homozygous recessive parent produces only a gametes. Offspring genotypes are expected in a 1 Aa : 1 aa ratio. With complete dominance, Aa shows the dominant phenotype and aa shows the recessive phenotype, so the phenotype ratio also depends on how dominance is defined in the prompt.
Self-check
Status: Draft your answer first—then open the rubric or sample.
Prompt
A student claims that a dominant allele must be more common in a population.
- A. Correct the student's claim.
- B. Explain what dominance actually means.
- C. Describe why allele frequency and dominance are different concepts.
Scoring rubric
- 1 pt — States dominance does not mean more common in the population.
- 1 pt — Defines dominance as expression in a heterozygote for that pattern.
- 1 pt — Contrasts phenotype expression with allele frequency.
- 1 pt — Gives that recessive alleles can be common or rare.
- 1 pt — Explains allele frequency is a population genetics measure.
- 1 pt — Uses clear Mendelian vocabulary (allele, genotype, phenotype).
Sample response
The student is incorrect: a dominant allele is not necessarily more common. Dominance describes how an allele affects phenotype when paired with a recessive allele in a heterozygote. Allele frequency measures how often an allele appears in a population, which is separate from whether that allele is dominant or recessive in a given inheritance pattern.
Self-check
Status: Draft your answer first—then open the rubric or sample.
Mendelian Genetics FAQs
What is the difference between homozygous and heterozygous?
Homozygous means two matching alleles, such as AA or aa. Heterozygous means two different alleles, such as Aa. In simple Mendelian inheritance, a heterozygous organism shows the dominant phenotype.
What is a monohybrid cross?
A monohybrid cross tracks inheritance for one gene. AP Biology uses it to predict offspring genotypes and phenotypes when parents differ for a single trait.
Why does a monohybrid cross produce a 3:1 phenotype ratio?
In a typical heterozygous × heterozygous cross with complete dominance, three-fourths of offspring show the dominant phenotype and one-fourth show the recessive phenotype, giving a 3:1 phenotype ratio.
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 expressed traits, which may collapse genotypes when dominance masks recessive alleles.
What is a test cross?
A test cross helps determine an unknown genotype by crossing the organism with a homozygous recessive individual.
What does true-breeding mean in Mendelian genetics?
True-breeding organisms consistently produce the same trait in offspring when self-crossed. Mendel's P generation parents were true-breeding lines with homozygous genotypes.
What is the difference between Mendelian and non-Mendelian inheritance?
Mendelian inheritance uses predictable dominant-recessive allele patterns. Non-Mendelian inheritance includes incomplete dominance, codominance, multiple alleles, polygenic traits, and sex-linked traits.
What is the law of dominance?
The law of dominance says that, in simple complete-dominance inheritance, one dominant allele can mask a recessive allele in a heterozygote. For example, Aa shows the dominant phenotype even though the recessive allele is still present.
What are P, F1, and F2 generations in Mendelian genetics?
The P generation is the original parent generation, often true-breeding. The F1 generation is the first offspring generation. The F2 generation comes from crossing F1 individuals and can reveal recessive traits again, often producing a 3:1 phenotype ratio in simple monohybrid crosses.
Are all traits Mendelian?
No. Many traits are non-Mendelian, including incomplete dominance, codominance, multiple alleles, polygenic traits, sex-linked traits, and linked genes. Mendelian genetics is a foundation, but AP Biology also tests exceptions.