What is the difference between incomplete dominance and codominance?

In incomplete dominance the heterozygote shows an intermediate, blended phenotype, whereas in codominance both alleles are fully and separately expressed, as with the A and B antigens in AB blood type.

What does it mean for a trait to be incompletely penetrant?

Incomplete penetrance means that not everyone carrying the relevant genotype displays the associated phenotype, so a trait can appear to skip a generation even though the allele is present.

Extensions to Mendelian Inheritance

Many traits depart from the simple dominant-recessive pattern Mendel described, because alleles can blend, both be expressed, exist in many forms, interact across genes, or affect several traits at once.

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Definition

Extensions to Mendelian inheritance are the phenomena that modify or obscure the simple dominant-recessive ratios, including complex dominance relationships, interactions among alleles and genes, and variable expression of genotypes.

Scope

This topic covers incomplete dominance and codominance, multiple alleles and allelic series, lethal alleles and their effect on ratios, epistasis and other gene interactions that modify the 9:3:3:1 dihybrid ratio, pleiotropy, and the concepts of penetrance and expressivity. It treats these as modifications of single-gene transmission; the inheritance of continuously varying quantitative traits is covered under quantitative genetics.

Core questions

How do incomplete dominance and codominance differ in the heterozygous phenotype they produce?
How do epistatic interactions reshape the classical 9:3:3:1 dihybrid ratio?
Why can a single gene affect many seemingly unrelated traits?
What do penetrance and expressivity describe, and why do they complicate predictions?

Key concepts

Incomplete dominance and codominance
Multiple alleles and allelic series
Lethal alleles and modified ratios
Epistasis and gene interaction
Pleiotropy, penetrance, and expressivity

Mechanisms

These departures arise from molecular features of gene products: incomplete dominance from dosage effects of partially functional proteins, codominance from independent expression of two allele products, epistasis from genes acting in a shared pathway, and pleiotropy from a single product functioning in multiple tissues or processes.

Clinical relevance

Recognizing these patterns explains why some inherited conditions vary in severity (expressivity) or skip apparently at-risk individuals (incomplete penetrance), and why ABO blood typing follows codominance and multiple alleles, all of which inform genetic counseling and transfusion practice.

History

Soon after Mendel's rediscovery, breeders found traits that did not fit clean dominant-recessive ratios: Cuénot's work on coat-colour lethals in mice and Bateson and Punnett's studies of comb shape in fowl revealed allelic series and epistasis, broadening Mendelism without overturning it.

Key figures

William Bateson
Lucien Cuénot
Reginald Punnett

Seminal works

griffiths2020

Frequently asked questions

What is the difference between incomplete dominance and codominance?: In incomplete dominance the heterozygote shows an intermediate, blended phenotype, whereas in codominance both alleles are fully and separately expressed, as with the A and B antigens in AB blood type.
What does it mean for a trait to be incompletely penetrant?: Incomplete penetrance means that not everyone carrying the relevant genotype displays the associated phenotype, so a trait can appear to skip a generation even though the allele is present.