Heterozygote is intermediate
Incomplete dominance is likely.
Where Non-Mendelian Genetics Fits in Unit 5
The previous guide, Punnett Squares, showed how to model inheritance probabilities. This page explains why not every trait fits a simple dominant-recessive Punnett square. After this page, study Chi-Square Test for Genetics to compare expected inheritance ratios with observed data.
- 1 Unit 5 Hub
- 2 Meiosis
- 3 Mitosis vs Meiosis
- 4 Crossing Over
- 5 Independent Assortment
- 6 Mendelian Genetics
- 7 Punnett Squares
- 8 Non-Mendelian Genetics You are here
- 9 Chi-Square Test for Genetics
- 10 Unit 5 Practice Questions
What is non-Mendelian genetics in AP Biology?
Non-Mendelian genetics includes inheritance patterns that do not follow simple complete dominance. In these patterns, heterozygotes may show blended traits, both alleles may be expressed, more than two alleles may exist in a population, or multiple genes may influence one trait. AP Biology uses non-Mendelian genetics to test whether students can identify the inheritance pattern before predicting outcomes.
Say it fast
Non-Mendelian genetics changes how traits are predicted.
Non-Mendelian Pattern Explorer
Non-Mendelian pattern explorer — tap each pattern
Complete dominance is the simple Mendelian pattern where a dominant allele masks a recessive allele in a heterozygote. This is the baseline pattern students compare against.
Incomplete dominance occurs when the heterozygote has an intermediate phenotype. The heterozygote does not look exactly like either homozygote.
Codominance occurs when both alleles are fully expressed in the heterozygote. The traits are shown together rather than blended.
Multiple alleles means more than two allele versions exist in the population. ABO blood type is the classic AP Biology example.
Polygenic traits are influenced by multiple genes. These traits often show continuous variation rather than simple categories.
Sex-linked traits are controlled by genes on sex chromosomes. X-linked recessive traits often appear more often in males.
Mendelian vs Non-Mendelian Genetics
Mendelian genetics often assumes complete dominance, two alleles, and independent assortment. Non-Mendelian genetics modifies one or more of those assumptions. The Punnett square may still be useful, but the way you interpret heterozygotes and phenotype ratios changes. For the classic 3:1 monohybrid pattern under complete dominance, see the monohybrid crosses guide.
| Pattern | Heterozygote phenotype | AP clue |
|---|---|---|
| Complete dominance | Dominant phenotype | Dominant masks recessive |
| Incomplete dominance | Intermediate phenotype | Blended or in-between |
| Codominance | Both traits expressed | Both alleles show |
| Multiple alleles | Depends on allele pair | More than two alleles in population |
| Polygenic inheritance | Range of phenotypes | Many genes affect one trait |
| Sex-linked inheritance | Differs by sex | X-linked or sex chromosome clue |
Incomplete Dominance
Incomplete dominance occurs when the heterozygote phenotype is intermediate between the two homozygous phenotypes. A common example is red and white flowers producing pink heterozygotes. The important AP Biology idea is that the heterozygote has its own phenotype instead of simply showing the dominant trait.
Direct answer: Incomplete dominance means the heterozygote shows an intermediate phenotype.
Codominance
Codominance occurs when both alleles are expressed in the heterozygote. Unlike incomplete dominance, the traits are not blended into an intermediate phenotype. Instead, both versions are visible or detectable.
Direct answer: Codominance means both alleles show at the same time.
Multiple Alleles and ABO Blood Type
Multiple alleles means a gene has more than two allele versions in the population. ABO blood type is the classic example because the population has IA, IB, and i alleles. An individual still inherits only two alleles, but more than two versions exist in the population.
| Genotype | Blood type phenotype | Pattern |
|---|---|---|
| IAIA or IAi | Type A | A allele expressed |
| IBIB or IBi | Type B | B allele expressed |
| IAIB | Type AB | Codominance |
| ii | Type O | Recessive phenotype |
Polygenic Traits
Polygenic traits are controlled by multiple genes. Instead of producing simple categories, these traits often show a range of phenotypes. Human height and skin pigmentation are common examples used to show continuous variation.
Callout: Polygenic does not mean multiple alleles. Polygenic means multiple genes influence one trait.
Sex-Linked Traits
Sex-linked traits are controlled by genes located on sex chromosomes. X-linked recessive traits often appear more frequently in males because males typically have only one X chromosome. AP Biology may test this using pedigrees or Punnett squares that track X and Y chromosomes.
AP callout: If males are affected more often, check for X-linked inheritance.
For a focused deep dive, see sex-linked traits.
Linked Genes
Linked genes are located close together on the same chromosome and tend to be inherited together. This can make offspring ratios differ from what independent assortment predicts. Crossing over can sometimes separate linked genes, and recombination frequency can be used to estimate how far apart genes are.
See linked genes and recombination frequency for map-distance reasoning.
How AP Biology Tests Non-Mendelian Genetics
Both traits appear
Codominance is likely.
AB blood type
Codominance and multiple alleles are involved.
More than two alleles in the population
Multiple alleles are being tested.
Continuous range of phenotypes
Polygenic inheritance is likely.
More males affected
Consider X-linked inheritance.
Genes inherited together
Linked genes may be involved.
Expected ratio does not appear
Check whether Mendelian assumptions are violated.
How to Answer Non-Mendelian Genetics FRQs
Identify the inheritance pattern
Read heterozygote and offspring clues before assigning alleles.
Define heterozygote behavior
State whether the heterozygote blends, shows both alleles, or follows sex linkage.
Predict genotypes and phenotypes
Use the correct pattern with a Punnett square or pedigree logic.
Justify with evidence
Cite prompt data that supports your pattern choice and ratios.
AP FRQ writing frame
This pattern is ___ because the heterozygote ___. The expected genotypes are ___, and the phenotypes are ___ because ___.
Common AP Bio Non-Mendelian Genetics Mistakes
Treating every trait as complete dominance
Fix: Read the prompt for heterozygote clues.
Confusing incomplete dominance with codominance
Fix: Incomplete dominance blends; codominance shows both.
Thinking multiple alleles means an individual has many alleles
Fix: A population can have many allele versions, but an individual usually inherits two.
Confusing polygenic traits with multiple alleles
Fix: Polygenic means many genes; multiple alleles means many versions of one gene.
Ignoring sex-linked inheritance
Fix: Check whether the trait pattern differs between males and females.
Applying 3:1 ratio automatically
Fix: A 3:1 ratio usually assumes complete dominance and simple Mendelian inheritance.
Non-Mendelian Genetics Clue Lab
Revealed: 0 of 5 scenarios
Red flowers crossed with white flowers produce pink flowers.
Answer: This is incomplete dominance because the heterozygote has an intermediate phenotype.
A heterozygote shows both black and white patches.
Answer: This is codominance because both alleles are expressed.
A gene has IA, IB, and i alleles in the population.
Answer: This is multiple alleles, and ABO blood type also includes codominance.
A trait appears across a continuous range.
Answer: This suggests polygenic inheritance because multiple genes affect the phenotype.
Males are affected more often than females in a pedigree.
Answer: This can suggest X-linked inheritance.
Non-Mendelian Genetics MCQ Practice
Answer all eight questions. Choices shuffle on reload—focus on mechanism, not letter memorization.
Question 1 of 8
Start
Correct: 0
Answered: 0
Accuracy: 0%
More drills: Unit 5 practice questions, practice by topic, or daily AP Biology practice.
Non-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 FRQ guide.
0 of 2 FRQs opened
Prompt
In a flower species, red flowers crossed with white flowers produce pink offspring.
- A. Identify the inheritance pattern.
- B. Explain how the heterozygote phenotype supports your answer.
- C. Predict the phenotype ratio from a cross between two pink flowers.
Scoring rubric
- 1 pt — Identifies incomplete dominance.
- 1 pt — Explains heterozygote is intermediate (pink), not dominant masking.
- 1 pt — Uses correct allele symbols (e.g., CR, CW or R, W).
- 1 pt — Predicts 1 red : 2 pink : 1 white phenotype ratio from pink × pink.
- 1 pt — Connects ratio to gamete combinations in a Punnett square.
- 1 pt — Does not treat pink as a third dominant allele without explanation.
Sample response
The pattern is incomplete dominance because red × white produces pink heterozygotes with an intermediate phenotype. A pink × pink cross gives 1 red : 2 pink : 1 white offspring if allele symbols are used consistently and phenotypes are assigned to genotypes correctly.
Self-check
Status: Draft your answer first—then open the rubric or sample.
Prompt
In a blood type problem, one parent has type AB blood and the other has type O blood.
- A. Identify the alleles each parent can pass to offspring.
- B. Predict possible offspring blood types.
- C. Explain why ABO blood type is an example of non-Mendelian inheritance.
Scoring rubric
- 1 pt — AB parent can pass IA or IB; O parent passes i only.
- 1 pt — Predicts type A (IAi) and type B (IBi) offspring; no AB or O from this cross.
- 1 pt — Notes multiple alleles (IA, IB, i) in the population.
- 1 pt — Notes codominance in type AB (IA and IB both expressed).
- 1 pt — Explains phenotype depends on allele pair, not simple complete dominance only.
- 1 pt — Uses ABO terminology correctly.
Sample response
The AB parent can contribute IA or IB; the O parent contributes i. Offspring can be type A (IAi) or type B (IBi). ABO blood type is non-Mendelian because the population has multiple alleles and IA and IB are codominant in type AB individuals.
Self-check
Status: Draft your answer first—then open the rubric or sample.
Non-Mendelian Genetics FAQs
What is non-Mendelian genetics in AP Biology?
Non-Mendelian genetics includes inheritance patterns that do not follow simple dominant-recessive rules. These patterns can involve incomplete dominance, codominance, multiple alleles, polygenic traits, sex-linked traits, or linked genes. AP Biology tests whether you can identify the pattern before predicting outcomes.
How is non-Mendelian genetics different from Mendelian genetics?
Mendelian genetics often assumes complete dominance and predictable dominant-recessive ratios. Non-Mendelian genetics changes those assumptions, so heterozygotes or offspring ratios may look different. The Punnett square may still work, but the interpretation changes.
What is incomplete dominance?
Incomplete dominance occurs when the heterozygote has an intermediate phenotype. For example, red and white flowers may produce pink heterozygotes. The key clue is that one allele does not fully mask the other.
What is codominance?
Codominance occurs when both alleles are expressed in the heterozygote. The traits appear together rather than blending. ABO blood type includes codominance because IA and IB are both expressed in type AB blood.
What are multiple alleles?
Multiple alleles means more than two allele versions exist for a gene in the population. An individual usually still inherits only two alleles. ABO blood type is the classic example because IA, IB, and i exist in the population.
What is polygenic inheritance?
Polygenic inheritance occurs when multiple genes influence one trait. These traits often show continuous variation instead of simple categories. Height and skin pigmentation are common examples.
What are sex-linked traits?
Sex-linked traits are controlled by genes on sex chromosomes. X-linked recessive traits often appear more often in males because males typically have only one X chromosome. AP Biology may test this pattern using pedigrees.
What are linked genes?
Linked genes are genes located close together on the same chromosome. They tend to be inherited together more often than genes that assort independently. Crossing over can sometimes separate linked alleles.
Can Punnett squares be used for non-Mendelian traits?
Yes, Punnett squares can still show possible genotypes. The difference is that phenotype interpretation depends on the inheritance pattern. Incomplete dominance, codominance, and sex-linked traits require different reasoning than complete dominance.
How should I answer non-Mendelian genetics FRQs?
Start by identifying the inheritance pattern from the prompt. Then explain how the heterozygote or offspring data support that pattern. Finish by predicting genotypes and phenotypes using the correct pattern.