Non-Mendelian Genetics

AP Biology· AP Bio CED Unit 5, Topic 5.4· 25 min read

1. Incomplete Dominance vs Codominance★★☆☆☆⏱ 5 min

📘 Definition

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.

2. Multiple Alleles and Sex-Linked Inheritance★★★☆☆⏱ 6 min

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.

📘 Definition

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.

3. Polygenic and Multifactorial Inheritance★★★☆☆⏱ 5 min

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.

📘 Definition

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.

4. Pleiotropy and Epistasis★★★★☆⏱ 6 min

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.

📘 Definition

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.

5. Common Pitfalls

Wrong move:

Confusing incomplete dominance with codominance

Why:

Assuming any non-Mendelian heterozygote phenotype is the same for both patterns

Correct move:

Incomplete dominance = blended intermediate phenotype; codominance = both parental traits fully and separately expressed

Wrong move:

Confusing multiple alleles with polygenic inheritance

Why:

Mixing up the number of genes vs number of alleles

Correct move:

Multiple alleles = one gene, more than two alleles in a population; polygenic = multiple genes, one trait

Wrong move:

Stating males can be carriers of X-linked recessive traits

Why:

Forgetting males only have one X chromosome

Correct move:

Only females can be carriers; any male with the recessive allele will express the trait

Wrong move:

Calling epistasis a form of gene linkage

Why:

Confusing gene interaction with physical arrangement on chromosomes

Correct move:

Epistasis is a phenotypic interaction between genes, not physical linkage on the same chromosome

Wrong move:

Applying Mendelian 9:3:3:1 or 3:1 ratios to all crosses

Why:

Assuming all genetic crosses follow Mendelian rules

Correct move:

Always work through the cross step-by-step, accounting for the specific non-Mendelian mechanism to get the correct ratio

6. Quick Reference 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

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · FRQ
Non-Mendelian pedigree analysis
2022 · MCQ
Multiple alleles cross prediction
2021 · FRQ
Epistasis in flower color

Going deeper

official syllabusAP Biology CED Unit 5Topic 5.4 learning objectives

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.