# Non-Mendelian Genetics

> AP Biology · Unit 5: Heredity
> Source: https://www.owlsprep.com/study/ap-biology-u5-non-mendelian-genetics/

This module covers inheritance patterns that deviate from Mendel’s laws of segregation and independent assortment. You will learn to recognize, explain, and predict outcomes for all common non-Mendelian patterns tested on the AP Biology exam.

**Prerequisites:** [Mendelian genetics and Punnett square analysis](https://www.owlsprep.com/study/ap-biology-u5-mendelian-genetics/)

## Learning objectives

- Distinguish between Mendelian and non-Mendelian inheritance patterns
- Explain mechanisms of common non-Mendelian inheritance patterns
- Predict offspring genotypic and phenotypic ratios for non-Mendelian crosses
- Identify non-Mendelian patterns from experimental or pedigree data

## Incomplete Dominance vs Codominance

**Non-Mendelian Inheritance** — Inheritance patterns that do not follow Mendel’s laws, typically due to allele interactions, multiple genes, or non-nuclear DNA.

Incomplete dominance produces a blended intermediate phenotype in heterozygotes, while codominance results in both parental traits being fully expressed simultaneously in heterozygotes. Both produce a 1:2:1 phenotypic ratio (same as genotypic ratio) in crosses of two heterozygotes.

**Worked example:** In snapdragons, flower color follows incomplete dominance. Homozygous $C^R C^R$ = red, $C^W C^W$ = white, heterozygotes = pink. What is the phenotypic ratio of offspring from a cross between two pink snapdragons?

1. Identify parental genotypes: both pink parents are heterozygous:
2. $$C^R C^W \times C^R C^W$$
3. Each parent produces two types of gametes, $C^R$ and $C^W$, in equal proportion. Complete the Punnett square:
4. Offspring genotypes and phenotypes: 1 $C^R C^R$ (red) : 2 $C^R C^W$ (pink) : 1 $C^W C^W$ (white)
5. Final phenotypic ratio is 1 red : 2 pink : 1 white

**Worked example:** In cattle, coat color is codominant. Homozygous $C^R C^R$ = red, $C^W C^W$ = white, heterozygotes = roan (both red and white hairs). What offspring phenotypes result from crossing a roan bull and a red cow?

1. Parental genotypes: roan bull = $C^R C^W$, red cow = $C^R C^R$
2. Gametes: bull produces $C^R$ and $C^W$; cow produces only $C^R$
3. Offspring: 50% $C^R C^R$ (red), 50% $C^R C^W$ (roan)

> **Exam tip:** AP multiple choice questions almost always test the difference between these two patterns: remember blended = incomplete, both traits expressed = codominance.

## Multiple Alleles and Sex-Linked Inheritance

Multiple alleles means there are more than two alleles for a gene in a population (though each individual only inherits two). Sex-linked traits are carried on the X chromosome, so inheritance patterns differ between XY males and XX females.

**Sex-Linked Inheritance** — Inheritance of genes located on the X chromosome (most common in humans) leads to different phenotypic rates in males vs females. Males only have one X chromosome, so any recessive allele is always expressed.

**Worked example:** Red-green color blindness is an X-linked recessive trait. A carrier woman ($X^N X^n$) has a child with a man with normal vision ($X^N Y$). What is the probability their son is colorblind?

1. Sons inherit their X chromosome from their mother and Y from their father.
2. The mother has one $X^N$ (normal) and one $X^n$ (colorblind) allele. The probability of passing $X^n$ to any child is 1/2.
3. The son will express the trait if he inherits $X^n$, so the final probability is 50% (1/2).

> **Exam tip:** X-linked recessive traits are much more common in males because they cannot be carriers. Always note this in FRQ explanations.

## Polygenic and Multifactorial Inheritance

Polygenic traits are controlled by multiple independent genes, leading to a continuous range of phenotypes rather than discrete categories. Multifactorial traits are polygenic traits also influenced by environmental factors. Common examples include human height, skin color, and risk for many diseases.

**Polygenic Inheritance** — Inheritance of a trait controlled by the additive effect of multiple genes, resulting in a continuous, bell-shaped (normal) distribution of phenotypes in a population.

**Worked example:** Human skin color is controlled by three unlinked additive genes. Each dominant allele adds one unit of darkness. How many distinct phenotype classes are possible from a cross of two heterozygotes (AaBbCc × AaBbCc)?

1. The minimum number of dominant alleles an offspring can inherit is 0 (all recessive aabbcc), and the maximum is 6 (all dominant AABBCC).
2. Every whole number of dominant alleles between 0 and 6 produces a distinct phenotype. The number of distinct classes is 6 - 0 + 1 = 7.
3. Most offspring fall in the middle of the range, producing the characteristic bell curve distribution of polygenic traits.

> **Exam tip:** On FRQs, always mention continuous variation as the key indicator of polygenic inheritance, versus discrete traits from single-gene Mendelian inheritance.

## Pleiotropy and Epistasis

Pleiotropy occurs when one gene affects multiple unrelated traits (e.g., the sickle cell anemia gene affects red blood cell shape, resistance to malaria, and organ function). Epistasis is an interaction where one gene masks or modifies the expression of another gene at a separate locus, modifying the standard 9:3:3:1 dihybrid ratio.

**Epistasis** — A genetic interaction where one gene alters the phenotypic expression of a second independent gene.

**Worked example:** In mice, coat color is controlled by two genes: gene B (B = black, b = brown) determines pigment color, and gene E (E = deposit pigment, e = no pigment) controls pigment deposition. Any mouse with ee is albino regardless of B/b genotype. What is the phenotypic ratio from a cross of two BbEe heterozygous mice?

1. Start with the standard 16-square dihybrid Punnett square for BbEe × BbEe. The genotypic ratio is 9 B_E_ : 3 bbE_ : 3 B_ee : 1 bbee.
2. All mice with ee genotype are albino, so combine B_ee and bbee: 3 + 1 = 4 albino.
3. Final phenotypic ratio: 9 black (B_E_) : 3 brown (bbE_) : 4 albino (any ee)

> **Exam tip:** If an exam question gives you a dihybrid cross ratio that is not 9:3:3:1, it is almost certainly epistasis.

## Common pitfalls

- **Wrong:** Confusing incomplete dominance with codominance
  - Why it fails: Assuming any non-Mendelian heterozygote phenotype is the same for both patterns
  - Correct: Incomplete dominance = blended intermediate phenotype; codominance = both parental traits fully and separately expressed
- **Wrong:** Confusing multiple alleles with polygenic inheritance
  - Why it fails: Mixing up the number of genes vs number of alleles
  - Correct: Multiple alleles = one gene, more than two alleles in a population; polygenic = multiple genes, one trait
- **Wrong:** Stating males can be carriers of X-linked recessive traits
  - Why it fails: Forgetting males only have one X chromosome
  - Correct: Only females can be carriers; any male with the recessive allele will express the trait
- **Wrong:** Calling epistasis a form of gene linkage
  - Why it fails: Confusing gene interaction with physical arrangement on chromosomes
  - Correct: Epistasis is a phenotypic interaction between genes, not physical linkage on the same chromosome
- **Wrong:** Applying Mendelian 9:3:3:1 or 3:1 ratios to all crosses
  - Why it fails: Assuming all genetic crosses follow Mendelian rules
  - Correct: Always work through the cross step-by-step, accounting for the specific non-Mendelian mechanism to get the correct ratio

## Cheatsheet

| Pattern | Key Feature | Common Ratio/Distribution |
| --- | --- | --- |
| Incomplete Dominance | Heterozygote has blended phenotype | 1:2:1 |
| Codominance | Both alleles fully expressed | 1:2:1 |
| Multiple Alleles | >2 alleles per gene in population | Variable |
| X-Linked Recessive | More common in males | 50% of sons of carriers affected |
| Polygenic Inheritance | Continuous phenotype range | Bell curve |
| Pleiotropy | One gene affects multiple traits | N/A |
| Recessive Epistasis | One gene masks another | 9:3:4 |

## What's next

Non-Mendelian genetics is a high-frequency topic on the AP Biology exam, appearing in both multiple choice and free response questions. Mastery of these patterns is essential for interpreting pedigree data, explaining genetic variation, and answering questions about complex human genetic traits. These concepts build directly on Mendelian genetics and prepare you for topics like gene linkage, chromosomal inheritance, and population genetics, which are also core to Unit 5 and beyond.

- [Environmental Effects on Phenotype](https://www.owlsprep.com/study/ap-biology-u5-environmental-effects-on-phenotype/)
- [Gene Expression and Regulation Overview](https://www.owlsprep.com/study/ap-biology-u6-overview/)

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