Incomplete dominance and codominance are two types of genetic inheritance patterns that describe how different alleles interact to affect the phenotype of an organism. Both are variations from the classic Mendelian inheritance patterns, where one allele is dominant over the other. Understanding the difference between incomplete dominance and codominance is essential for grasping how traits are inherited and expressed in genetics.
Definition of Incomplete Dominance
Incomplete dominance is a type of genetic inheritance where the phenotype of a heterozygous individual is a blend or intermediate of the phenotypes of the two homozygous parents. In this type of inheritance, neither allele is completely dominant over the other, resulting in a third, distinct phenotype.
- Key Characteristics:
- Intermediate Phenotype: In incomplete dominance, the offspring’s phenotype is a mixture of the parental phenotypes. This results in a new, intermediate phenotype that is distinct from either parent.
- Allele Interaction: Both alleles contribute to the phenotype, but neither allele is fully expressed or completely masks the other, leading to partial expression of each.
- Genetic Representation: In genetic notation, incomplete dominance is represented by using different letters for each allele (e.g., R for red and W for white) to indicate that neither allele is dominant.
- Examples:
- Flower Color in Snapdragons: When a red-flowered snapdragon (RR) is crossed with a white-flowered snapdragon (WW), the offspring (RW) have pink flowers, which is an intermediate color between red and white.
- Hair Texture in Humans: When one parent has curly hair (CC) and the other has straight hair (SS), their offspring (CS) may have wavy hair, a blend of both curly and straight textures.
Definition of Codominance
Codominance is a type of genetic inheritance where both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that clearly shows both parental traits simultaneously. In codominance, neither allele is recessive, and both traits appear together in the phenotype without blending.
- Key Characteristics:
- Simultaneous Expression: In codominance, both alleles contribute equally and visibly to the phenotype. The resulting phenotype shows both traits distinctly and simultaneously.
- Allele Interaction: Both alleles are equally strong and fully expressed, meaning that neither allele can mask the effect of the other.
- Genetic Representation: In genetic notation, codominance is often represented using different capital letters (e.g., B for black and W for white) to signify that both alleles are equally dominant.
- Examples:
- Blood Type in Humans: The ABO blood group system exhibits codominance. For example, individuals with genotype IAIB have blood type AB, where both A and B antigens are equally and fully expressed on the surface of red blood cells.
- Coat Color in Cattle: In certain breeds of cattle, a cross between a white-coated individual (WW) and a red-coated individual (RR) produces offspring (WR) with a roan coat, where both red and white hairs are present and fully expressed.
Core Differences
Phenotypic Expression
- Incomplete Dominance: The phenotype of a heterozygous individual is an intermediate blend of the two homozygous parents’ phenotypes. The traits mix to create a new, unique phenotype.
- Codominance: The phenotype of a heterozygous individual displays both parental traits simultaneously without blending. Each trait is fully and distinctly expressed.
Genetic Interaction
- Incomplete Dominance: Both alleles partially contribute to the phenotype, resulting in a mixed expression where neither allele is completely dominant.
- Codominance: Both alleles fully contribute to the phenotype, and both traits are equally and visibly expressed without one being dominant over the other.
Visual Representation
- Incomplete Dominance: Results in a phenotype that is a mixture, such as pink flowers from red and white parents.
- Codominance: Results in a phenotype where both traits are distinctly visible, such as a roan coat in cattle with both red and white hairs.
Core Similarities
Non-Mendelian Inheritance
Both incomplete dominance and codominance are types of non-Mendelian inheritance, meaning they do not follow the classic Mendelian patterns of dominant and recessive alleles.
Expression of Both Alleles
In both incomplete dominance and codominance, both alleles in a heterozygous individual are expressed, though in different ways (blended versus distinct).
Comparison Table
| Feature | Incomplete Dominance | Codominance |
|---|---|---|
| Phenotypic Expression | Intermediate blend of parental traits | Both parental traits fully and distinctly expressed |
| Genetic Interaction | Neither allele is completely dominant; both partially expressed | Both alleles are equally dominant; both fully expressed |
| Visual Representation | Mixed or blended phenotype (e.g., pink flowers) | Distinct phenotype showing both traits (e.g., roan coat) |
| Examples | Pink snapdragon flowers from red and white parents | AB blood type showing both A and B antigens |
| Allele Notation | Different letters (e.g., R and W) | Different capital letters (e.g., B and W) |
Pros and Cons
Incomplete Dominance
- Pros:
- Provides a clear example of how genetic traits can blend and result in new, intermediate phenotypes.
- Helps illustrate genetic diversity and variation within a population.
- Cons:
- Can be confusing when predicting outcomes using Mendelian principles, as the resulting phenotype is not like either parent.
- Intermediate traits may not be as advantageous as dominant traits in certain environments.
Codominance
- Pros:
- Demonstrates how multiple alleles can contribute to the phenotype, providing a straightforward example of genetic variation.
- Useful in understanding complex traits and genetic conditions where multiple alleles are expressed simultaneously.
- Cons:
- Can complicate genetic predictions, as both traits are fully expressed, requiring more complex calculations.
- May lead to conflicting traits in the phenotype, potentially affecting the organism’s fitness depending on the environment.
Use Cases and Scenarios
When to Focus on Incomplete Dominance
- Genetic Crosses in Plants: Use incomplete dominance to study plant breeding and the inheritance of flower colors, where intermediate colors provide insights into genetic variation.
- Animal Breeding: Apply incomplete dominance principles in breeding programs to predict and achieve specific traits, such as hair texture in domestic animals.
When to Focus on Codominance
- Human Blood Types: Focus on codominance when studying human genetics, particularly the ABO blood group system, to understand blood type inheritance and compatibility.
- Coat Color in Animals: Use codominance to study patterns of coat color in animals, such as cattle, where distinct color patterns result from both alleles being fully expressed.
Summary
In summary, the main difference between incomplete dominance and codominance lies in how the alleles are expressed in a heterozygous individual. Incomplete dominance results in an intermediate phenotype that is a blend of the two parental traits, while codominance results in both parental traits being fully and distinctly expressed without blending. Both inheritance patterns are important for understanding genetic diversity and the mechanisms of heredity in various organisms.
FAQs
Q: Can a single gene exhibit both incomplete dominance and codominance?
A: No, a single gene typically exhibits either incomplete dominance or codominance, depending on how the alleles interact with each other. However, different genes within an organism can show different patterns of inheritance.
Q: Are there other types of non-Mendelian inheritance besides incomplete dominance and codominance?
A: Yes, other types include multiple alleles, polygenic inheritance, and pleiotropy, which all contribute to the diversity of genetic expression beyond simple dominant-recessive patterns.
Q: How can you tell if a trait is due to incomplete dominance or codominance?
A: You can determine the type of inheritance by examining the phenotype of heterozygous individuals. If the phenotype is intermediate between the two parents, it’s incomplete dominance. If both parental traits are fully visible, it’s codominance.
Q: Are there any human traits that show incomplete dominance?
A: Yes, examples of human traits that show incomplete dominance include hair texture (wavy hair as a blend of curly and straight hair) and certain skin pigmentation patterns.
Q: Is codominance more common than incomplete dominance?
A: Neither is more common universally; the prevalence depends on the species and the specific traits being considered. Both incomplete dominance and codominance are equally important in genetic diversity and inheritance.
References
- https://bio.libretexts.org/Bookshelves/Genetics/Classical_Genetics_(Khan_Academy)/02%3A_Non-Mendelian_inheritance/2.01%3A_Co-dominance_and_Incomplete_Dominance#:~:text=In%20complete%20dominance%2C%20only%20one,is%20seen%20in%20the%20phenotype.
- https://blog.prepscholar.com/difference-between-incomplete-dominance-and-codominance
- https://www.britannica.com/science/incomplete-dominance