Biology 1 · Genetics & Inheritance
Non-Mendelian Genetics
On this page 5 sections
The college version
Core Explanation
Mendel's laws describe the fundamental behavior of chromosomes and alleles, but they do not represent the full complexity of inheritance. Many traits exhibit patterns that extend beyond simple dominant/recessive relationships. These patterns do NOT "break" Mendel's laws — they reflect additional layers of biology such as protein structure, biochemical pathways, gene interactions, and environmental effects.
Incomplete Dominance
In Incomplete dominance Heterozygote has intermediate phenotype; alleles remain discrete, the heterozygote has an intermediate phenotype — neither allele is fully dominant. There is no "blending" of the alleles themselves (they remain discrete units), but the phenotype appears blended.
Example: Snapdragon flower color
- CRCR = red flowers
- CWCW = white flowers
- CRCW = pink flowers (intermediate)
Crossing two pink snapdragons (CRCW × CRCW) produces a 1:2:1 ratio: ¼ red, ½ pink, ¼ white. This is the same genotype ratio as a monohybrid cross — the phenotype ratio simply reflects the intermediate heterozygote rather than dominance.
Critical distinction: Incomplete dominance is NOT "blending inheritance." The alleles remain intact and segregate normally in subsequent generations. If blending occurred, the red and white phenotypes would be permanently lost after the first cross — but they reappear in the F2 generation.
Codominance
In Codominance Both alleles fully expressed in heterozygote (e.g., AB blood type), both alleles are fully expressed in the heterozygote — neither is masked or intermediate. The phenotype shows both traits simultaneously.
Example: ABO blood groups
- IA allele → A antigen on red blood cells
- IB allele → B antigen on red blood cells
- i allele → no antigen (O)
- IAIB genotype → both A and B antigens expressed (type AB blood)
This is distinct from incomplete dominance: AB blood is not "intermediate" between A and B — both antigens are present at full levels.
Multiple Alleles
While each individual carries only two alleles for a given gene (one on each homologous chromosome), a gene may have more than two alleles segregating in the population.
Example: ABO blood groups involve three alleles: IA, IB, and i.
| Genotype | Blood type (phenotype) |
|---|---|
| IAIA or IAi | A |
| IBIB or IBi | B |
| IAIB | AB (codominant) |
| ii | O |
Pleiotropy
Pleiotropy Single gene affects multiple traits occurs when a single gene affects multiple seemingly unrelated phenotypic traits. This is common because a single protein may function in different tissues or participate in multiple pathways.
Example: The CFTR gene — mutations cause cystic fibrosis, which affects the lungs (thick mucus, infections), pancreas (digestive enzyme blockage), sweat glands (salty sweat), and male reproductive tract (vas deferens obstruction). One gene → multiple organ system effects.
Epistasis
Epistasis Expression of one gene masks/modifies expression of another occurs when the expression of one gene affects or masks the expression of another gene. It is gene-gene interaction, not dominance between alleles of the same gene.
Example: Coat color in Labrador retrievers involves two genes:
- B gene: B (black pigment) is dominant to b (brown pigment)
- E gene: E (pigment deposition enabled) is dominant to e (pigment deposition blocked)
If a dog is ee (homozygous recessive at the E locus), pigment cannot be deposited in the fur — the dog is yellow, regardless of its B alleles. The ee genotype is epistatic to the B locus.
This modifies the expected dihybrid ratio from 9:3:3:1 to 9 black : 3 brown : 4 yellow — a hallmark of recessive epistasis.
Polygenic Inheritance
Many traits are influenced by multiple genes (polygenic), each contributing a small, additive effect. The result is continuous variation rather than discrete categories.
Examples: Human height, skin pigmentation, body mass. These traits typically show a bell-shaped (normal) distribution in the population because many genes + environmental factors produce a smooth range of phenotypes.
Environmental Effects on Phenotype
The phenotype is not solely determined by genotype — the environment can profoundly influence gene expression:
- Temperature: Siamese cats and Himalayan rabbits have temperature-sensitive pigment production — cooler extremities (ears, nose, paws) are darker because the tyrosinase enzyme functions at lower temperatures.
- pH: Hydrangea flower color depends on soil pH (affects aluminum availability, which influences pigment).
- Nutrition: Human height is influenced by both genetics and childhood nutrition.
- Light: Plant chlorophyll production requires light — seedlings grown in darkness are etiolated (pale, elongated).
Sex-Related Inheritance Patterns
| Pattern | Description | Example |
|---|---|---|
| Sex-linked | Gene is located on a sex chromosome (usually X) | Red-green color blindness, hemophilia A (X-linked recessive) |
| Sex-limited | Trait is expressed in only one sex, though both sexes carry the gene | Milk production in mammals, beard growth in humans |
| Sex-influenced | Dominance relationship differs between sexes | Pattern baldness: dominant in males, recessive in females (influenced by testosterone) |
Common Misconceptions and Exam Traps
- Exam trap: Calling incomplete dominance "blending inheritance." Alleles are NOT blended — they remain discrete and segregate in meiosis.
- Exam trap: Confusing codominance with incomplete dominance. Codominance = both alleles visible (AB blood). Incomplete dominance = intermediate (pink flowers).
- Misconception: "Epistasis is the same as dominance." Dominance is interaction between alleles of the SAME gene. Epistasis is interaction between DIFFERENT genes.
- Exam trap: Forgetting that X-linked recessive traits CAN appear in females if the female is homozygous recessive or if X-inactivation patterns produce expression.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Mendel's rules are like the basic grammar of inheritance, but real life has a lot more nuance. Sometimes two different instructions blend their effects (like mixing red and white to make pink). Sometimes both instructions shout at once (like Type AB blood showing both A and B markers). Sometimes one instruction controls several different things — one gene affects your lungs AND your pancreas. And sometimes multiple genes team up, each adding a tiny effect — which is why height isn't just "tall" or "short" but a smooth range from one person to the next.
Key takeaways
- Incomplete dominance: intermediate phenotype, NOT blending — alleles segregate normally
- Codominance: both alleles expressed (AB blood); distinct from incomplete dominance
- Epistasis: gene-gene interaction; modifies dihybrid ratios (e.g., 9:3:4)
- Polygenic traits: continuous variation, bell-shaped distribution
- Environment can alter phenotype expression (norm of reaction)
- X-linked recessive: more common in males (only one X); females are carriers
- Incomplete dominance: intermediate phenotype (e.g., pink snapdragons); NOT blending
- Codominance: both alleles expressed (e.g., ABO blood type AB)
- Pleiotropy: one gene → multiple traits (e.g., cystic fibrosis)
- Epistasis: one gene masks another → modified dihybrid ratios (e.g., 9:3:4 in Labradors)
- Polygenic traits: multiple genes → continuous variation (height, skin color)
- Environment influences phenotype expression
- How would you distinguish incomplete dominance from codominance in an experimental cross?
- In Labrador retrievers, the B gene controls pigment (B=black, b=brown) and the E gene controls deposition (E=deposition, e=blocked). What color is a dog with genotype Bb ee?
- Why are X-linked recessive disorders more common in males than females?
- Cross two F1 heterozygotes. If incomplete dominance, the F2 will show a 1:2:1 phenotype ratio (the heterozygote is intermediate). If codominance, the F2 will also show 1:2:1, but the heterozygote will express both parental phenotypes simultaneously (not an intermediate). The distinction is in the HETEROZYGOTE phenotype: intermediate = incomplete dominance; both traits visible = codominance. At the molecular level, codominance means both gene products are produced and detectable.
- Yellow. The dog has at least one B allele (so it CAN make black pigment), but it is ee — homozygous recessive at the E locus, which blocks pigment deposition entirely. Without the E gene product, pigment cannot be deposited in the fur regardless of what the B gene encodes. The dog will have a yellow coat.
- Males have only one X chromosome (XY). If a male inherits an X chromosome carrying a recessive disease allele, he HAS no second X chromosome with a potentially normal allele to mask it — the trait is expressed. Females (XX) must inherit TWO copies of the recessive allele (one from each parent) to express the trait; with one copy, they are typically unaffected carriers.
Study tools & related lessonsYou’ll learn to · Key vocabulary · Related
You’ll learn to
- Compare incomplete dominance, codominance, and multiple alleles with classic Mendelian dominance
- Explain pleiotropy and epistasis and how they modify phenotypic ratios
- Describe polygenic inheritance and how it produces continuous variation
- Explain why environmental factors can influence phenotype
- Distinguish between sex-linked, sex-limited, and sex-influenced traits
Key vocabulary
- Incomplete dominance
- Heterozygote has intermediate phenotype; alleles remain discrete
- Codominance
- Both alleles fully expressed in heterozygote (e.g., AB blood type)
- Multiple alleles
- More than two allelic forms of a gene exist in the population
- Pleiotropy
- Single gene affects multiple traits
- Epistasis
- Expression of one gene masks/modifies expression of another
- Polygenic inheritance
- Multiple genes contribute additively to a trait → continuous variation
- Sex-linked
- Gene located on a sex chromosome (usually X)
- Pedigree
- Family tree showing inheritance patterns across generations
Sources & references
- OpenStax. (2018). *Biology 2e*. Chapter 12: Section 12.3: Laws of Inheritance — Extensions.
- Online Mendelian Inheritance in Man (OMIM). Johns Hopkins University.
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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