Biology 1 - Lesson 14: Extensions of Mendelian Inheritance

While Mendel’s laws of segregation and independent assortment provide a foundational understanding of heredity, many traits do not follow simple dominant–recessive patterns. Complex genetic phenomena—such as incomplete dominance, codominance, epistasis, polygenic inheritance, and sex-linked traits—demonstrate that real-world genetics often involves multiple alleles, gene interactions, and environmental influences. These “extensions” to Mendelian inheritance deepen our appreciation for genetic diversity and trait complexity.

Incomplete Dominance

In incomplete dominance, neither allele is fully dominant. Instead, the heterozygous phenotype is an intermediate blend of the two homozygous forms. For example, crossing red (RR) and white (rr) snapdragon flowers yields pink (Rr) offspring. When two pink flowers are crossed, the F₂ generation shows a 1:2:1 ratio (red:pink:white).

Codominance

In codominance, both alleles in a heterozygote are fully expressed. A classic example is the human ABO blood group system, where the I^A and I^B alleles are codominant. Individuals with genotype I^AI^B express both A and B antigens on red blood cells, resulting in the AB blood type. This differs from incomplete dominance, as both alleles are simultaneously observable rather than blended.

Multiple Alleles

Although an individual can carry only two alleles of a gene, multiple alleles can exist at the population level. The ABO blood group again serves as an example: three main alleles (I^A, I^B, i) govern four blood types (A, B, AB, and O). Multiple-allele systems greatly expand genetic diversity within populations.

flowchart TB A((Blood Type Genotypes)) --> B{"I^A I^A or I^A i = Type A"} A --> C{"I^B I^B or I^B i = Type B"} A --> D{"I^A I^B = Type AB (codominant)"} A --> E{"i i = Type O"}

A simplified Mermaid flowchart summarizing how different ABO genotypes lead to distinct blood types.

Epistasis

Epistasis occurs when one gene affects the phenotypic expression of another. In Labrador retrievers, for instance, the B gene determines coat color (black vs. brown), but a separate E gene determines whether pigment is deposited at all. A recessive genotype (ee) at the E locus can mask the B locus, resulting in a yellow lab regardless of the B gene’s alleles. Such gene interactions can modify Mendelian ratios (e.g., 9:3:4 or 12:3:1) in dihybrid crosses.

Polygenic (Quantitative) Traits

Some characteristics, like human height, skin color, or grain yield in crops, are controlled by multiple genes contributing to a continuous distribution of phenotypes. Each gene typically has a small additive effect, and environmental factors often further influence the trait. This type of inheritance leads to bell-shaped distributions in populations, illustrating how polygenic traits generate continuous variation.

Example Polygenic Trait Effects

Trait	Number of Genes	Phenotypic Distribution	Influence of Environment
Human Height	Multiple loci (perhaps hundreds)	Continuous range	Significant (nutrition, health care)
Skin Pigmentation	Several major genes + minor modifiers	Continuous or near-continuous range	Moderate (sun exposure)
Grain Yield (Crops)	Many interacting QTLs (quantitative trait loci)	Continuous yield variations	High (soil, climate, farming practices)

Sex-Linked Inheritance

Certain genes reside on sex chromosomes (X or Y in many animals). Males (XY) only have one X chromosome, so recessive alleles on the X chromosome can be expressed more frequently in males. Examples include:

• Red–Green Color Blindness: An X-linked recessive trait often seen more in males than females.
• Hemophilia: Another X-linked recessive condition affecting blood clotting.

In humans, females (XX) can be heterozygous carriers for such recessive conditions without expressing them, whereas a single copy in males results in the phenotype.

Linkage and Recombination

Mendel’s Law of Independent Assortment applies to genes on different chromosomes or those sufficiently far apart on the same chromosome. Genes located close together on the same chromosome (linked genes) tend to be inherited together. Crossing over in meiosis can reshuffle these linked genes if they are not too tightly linked. Linkage analysis and recombination frequency remain vital tools for genetic mapping and understanding chromosome organization.

Pedigree Analysis

In humans and other species where controlled crosses are impractical or unethical, pedigree analysis helps track inheritance patterns across generations. By analyzing family trees—where squares represent males, circles represent females, filled symbols represent affected individuals—geneticists can infer:

• Whether a trait is dominant, recessive, or sex-linked
• Likely genotypes of individuals
• Risk probabilities for offspring inheriting a condition

Conclusion

Genetics extends far beyond the simple monogenic, dominant–recessive scenarios Mendel described. Real inheritance patterns often involve multiple alleles, incomplete dominance, codominance, gene interactions (epistasis), polygenic control, and environmental factors. Recognizing these complexities allows us to more accurately predict phenotypes, diagnose genetic conditions, enhance breeding programs, and explore the vast diversity observed in biological systems.