In genetics, inheritance patterns determine how traits are passed from parents to offspring. While dominant and recessive traits are commonly discussed, two other important inheritance patterns are incomplete dominance and codominance.
These two genetic concepts describe how alleles interact when neither one is completely dominant over the other. Although they may seem similar, they have distinct characteristics that influence the expression of traits in different ways.
This topic explores the differences between incomplete dominance and codominance, along with examples and real-world applications.
What Is Incomplete Dominance?
Incomplete dominance occurs when neither allele is completely dominant over the other, resulting in a blended phenotype in heterozygous individuals. Instead of one allele masking the effect of the other, the two traits mix to create an intermediate form.
Key Features of Incomplete Dominance:
Neither allele is fully dominant.
Heterozygous individuals show a blended phenotype.
The offspring’s traits are a mix of both parental traits.
Example of Incomplete Dominance
One of the most classic examples of incomplete dominance is seen in flower color in snapdragons (Antirrhinum majus).
- Red flowers (RR) × White flowers (WW) → Pink flowers (RW)
- The heterozygous RW plants do not produce red or white flowers but instead show a blended pink color.
Another example is hair texture in humans:
- Curly hair (CC) × Straight hair (SS) → Wavy hair (CS)
- The offspring inherit a wavy hair texture, which is an intermediate between curly and straight.
What Is Codominance?
Codominance occurs when both alleles in a heterozygous individual are fully expressed, meaning that both traits appear without blending. Instead of an intermediate phenotype, both parental traits are visible side by side.
Key Features of Codominance:
Both alleles are fully expressed.
No blending occurs; both traits appear distinctly.
Heterozygous individuals exhibit both parental phenotypes simultaneously.
Example of Codominance
A well-known example of codominance is seen in blood types in humans.
- The AB blood type is an example of codominance because individuals inherit one A allele and one B allele, and both are equally expressed.
- Instead of blending into a new blood type, both A and B antigens appear on the red blood cells.
Another example is seen in coat color in cattle:
- When a red-haired cow (RR) is crossed with a white-haired cow (WW), the offspring have a roan coat (RW), where red and white hairs appear together without mixing.
Key Differences Between Incomplete Dominance and Codominance
Both inheritance patterns result in unique genetic expressions, but they differ in how traits are displayed in heterozygous individuals.
| Feature | Incomplete Dominance | Codominance |
|---|---|---|
| Expression | Blended phenotype | Both traits are fully visible |
| Example | Pink snapdragons (Red × White) | AB blood type (A × B) |
| Type of Inheritance | Partial dominance | Equal dominance |
| Visual Appearance | Intermediate trait | Both traits exist side by side |
How to Identify Incomplete Dominance and Codominance in Genetics
To determine whether a trait follows incomplete dominance or codominance, scientists use Punnett squares to predict the outcomes of genetic crosses.
- If the offspring show a blend of both parental traits → Incomplete dominance.
- If the offspring express both traits distinctly → Codominance.
Why Are These Concepts Important?
Understanding incomplete dominance and codominance is essential for:
Genetic research – Helps scientists study how traits are inherited.
Medical science – Plays a role in blood transfusions and genetic disorders.
Agriculture and breeding – Used to improve crop varieties and livestock traits.
Real-World Applications
-
Medicine:
- Blood typing is critical for safe blood transfusions and organ transplants.
- Studying genetic disorders influenced by codominance (e.g., sickle cell anemia).
-
Agriculture:
- Incomplete dominance is used in breeding fruit with desirable characteristics (e.g., pink flowers in certain crops).
- Codominance is important in livestock breeding for distinct coat colors.
Incomplete dominance and codominance are two unique patterns of genetic inheritance where neither allele is completely dominant.
- Incomplete dominance results in a blended phenotype, such as pink flowers from red and white parents.
- Codominance allows both traits to be fully expressed, like AB blood type or roan cattle coats.
These concepts help scientists and researchers understand how traits are passed down, leading to advancements in medicine, agriculture, and genetics.