Difference Between Incomplete Dominance and Codominance

What is Incomplete dominance?

Incomplete dominance is a genetic phenomenon where neither allele in a heterozygote is completely dominant over the other. Instead, the phenotype of the heterozygote is an intermediate blend of the two alleles. This condition is also known as partial dominance or semi-dominance. Below is a detailed explanation of incomplete dominance:

• Definition:
  • Incomplete dominance occurs when the dominant allele does not fully mask the effect of the recessive allele, resulting in an intermediate phenotype in the heterozygote.
  • This intermediate phenotype represents a blend of the traits associated with each allele.
• Phenotypic Expression:
  • The heterozygous phenotype is a mix of both the dominant and recessive traits, rather than the complete expression of just one.
  • For example, if red is the dominant allele and white is the recessive allele, a heterozygous plant may produce pink flowers due to incomplete dominance.
• Genetic Mechanism:
  • Incomplete dominance is distinguished from codominance, where both alleles are fully expressed simultaneously.
  • The resulting phenotype in incomplete dominance appears as a combination of the two parent traits, rather than a distinct expression of either allele.
• Historical Context:
  • Gregor Mendel’s experiments with pea plants did not include examples of incomplete dominance, as the traits he studied either exhibited complete dominance or were not intermediate.
  • Although Mendel’s model explains the inheritance patterns for dominant and recessive traits, it does not account for incomplete dominance. Nevertheless, the principles of inheritance still apply.
• Genotypic and Phenotypic Ratios:
  • When incomplete dominance is present, the genotypic ratio in the F1 generation typically follows a 1:2:1 ratio, with the phenotypic ratio reflecting the mix of both alleles.
  • For instance, crossing a red-flowered plant (C1C1) with a white-flowered plant (C2C2) results in pink-flowered offspring (C1C2), and further crosses will show a phenotypic ratio of 1:2:1 (red:pink) in the F2 generation.
• Examples:
  • Flower Color in Mirabilis jalapa: Crossing a red-flowered plant with a white-flowered plant yields pink flowers in the F1 generation. The F2 generation displays a 1:2:1 ratio of red, pink, and white flowers.
  • Hair Texture in Humans: Wavy hair represents an intermediate phenotype resulting from the incomplete dominance between curly (dominant) and straight (recessive) hair traits.

What is Co-dominance?

Co-dominance is a genetic phenomenon where both alleles of a gene in a heterozygote are expressed simultaneously and independently. Unlike traditional dominance where one allele masks the effect of another, co-dominance allows both alleles to be fully visible in the phenotype. Below is a detailed explanation of co-dominance:

• Definition:
  • Co-dominance occurs when two different alleles contribute equally and distinctly to the phenotype of the heterozygote.
  • In this case, neither allele is dominant or recessive. Instead, both alleles exhibit their effects simultaneously.
• Phenotypic Expression:
  • The heterozygote displays distinct features corresponding to both alleles. This is different from incomplete dominance, where the phenotype is an intermediate blend of the two alleles.
  • For instance, if one allele codes for red flowers and the other for white flowers, a co-dominant phenotype would show both red and white regions in the flower, rather than a pink blend.
• Genetic Symbols:
  • Co-dominant alleles are often represented by uppercase letters with different superscripts (e.g., A1A^1A1 and A2A^2A2).
  • Each allele is expressed to some degree even in the presence of its alternative allele, which visually distinguishes co-dominance from other inheritance patterns.
• Historical Context:
  • Gregor Mendel’s original experiments did not include examples of co-dominance, as his model organisms exhibited either complete dominance or did not express co-dominance traits.
  • Despite this, Mendel’s principles can still apply to the analysis of co-dominant traits. For instance, the genotypic ratio for co-dominant traits often follows Mendelian ratios, such as 1:2:1, while the phenotypic expression is distinctly different from either homozygote.
• Examples:
  • Human Blood Types: The ABO blood group system in humans is a classic example of co-dominance. The A and B alleles are co-dominant, so an individual with both A and B alleles (genotype AB) will have blood type AB, expressing both A and B antigens on red blood cells.
  • Livestock: In animals, co-dominance can result in a mixed coat color. For example, breeding a white-feathered chicken with a black-feathered chicken produces offspring with feathers displaying both black and white colors. Similarly, crossing black and white cows can result in a mixed coat pattern known as roan.

Similarities Between Incomplete Dominance and Codominance

Incomplete dominance and codominance are two forms of non-Mendelian inheritance that deviate from the classical Mendelian principles of genetics. Here is a detailed comparison of their similarities:

• Non-Mendelian Inheritance:
  • Both incomplete dominance and codominance do not adhere to the classic Mendelian 3:1 phenotypic ratio. Incomplete dominance results in a blending of traits, while codominance displays both traits simultaneously. Consequently, these inheritance patterns reveal a more complex interaction between alleles than the straightforward dominant-recessive model proposed by Gregor Mendel.
• Allelic Interaction:
  • Both mechanisms involve interactions between alleles of the same gene. In incomplete dominance, the heterozygous phenotype is a mix of the two parental phenotypes, reflecting a blending of the alleles. Conversely, in codominance, both alleles are fully expressed, leading to a phenotype where both traits are visible. This interaction between alleles leads to phenotypic expressions that deviate from the typical dominant-recessive relationships.
• Dominant Traits:
  • In both incomplete dominance and codominance, there is an element of dominance among the alleles. However, the nature of this dominance varies. In incomplete dominance, neither allele is completely dominant over the other, resulting in an intermediate phenotype. In codominance, both alleles are equally dominant and expressed simultaneously, so neither allele completely overshadows the other in the phenotype.

Differences Between Incomplete dominance vs Co-dominance

Below is a comprehensive comparison of their features:

• Definition:
  • Incomplete Dominance: Incomplete dominance occurs when neither allele is fully dominant. This results in a heterozygote exhibiting an intermediate phenotype that blends the traits of both parental alleles.
  • Codominance: Codominance involves both alleles being fully expressed in the heterozygote, where each allele contributes distinctly to the phenotype without blending.
• Phenotypic Expression:
  • Incomplete Dominance: The phenotype of heterozygotes is a blend of the traits from the two alleles. There is no clear dominance of one allele over the other.
  • Codominance: Both alleles are expressed simultaneously and independently in the phenotype. There is no blending of traits; instead, both traits are visibly distinct.
• Example in Humans:
  • Incomplete Dominance: The wavy hair observed in the offspring of a straight-haired parent and a curly-haired parent is an example of incomplete dominance.
  • Codominance: The AB blood type in humans illustrates codominance, where both A and B alleles are equally expressed.
• Genetic Inheritance Pattern:
  • Incomplete Dominance: There is no clear dominant or recessive allele. The phenotypic outcome is a unique intermediate form.
  • Codominance: Both alleles exhibit equal dominance, and neither allele masks the effect of the other.
• Allele Interaction:
  • Incomplete Dominance: The interaction of alleles results in an intermediate phenotype, which is a mix of both parental traits.
  • Codominance: Both alleles independently contribute to the phenotype, showing their effects distinctly and without blending.
• Genetic Symbolism:
  • Incomplete Dominance: Alleles are often represented by superscript letters to indicate their interaction (e.g., RW for wavy hair).
  • Codominance: Alleles are denoted with separate symbols to represent their independent expression (e.g., IA and IB for the AB blood type).
• Phenotypic Ratio in Offspring:
  • Incomplete Dominance: The phenotypic ratio in offspring typically follows a 1:2:1 pattern, representing homozygous dominant, heterozygous, and homozygous recessive phenotypes.
  • Codominance: The phenotypic ratio is also 1:2:1, but it reflects the presence of both alleles without blending (e.g., the blood types A, AB, and B).
• Overall Outcome:
  • Incomplete Dominance: The resulting phenotypes show a range or spectrum of traits that are intermediate between the parental types.
  • Codominance: The phenotypes display both parental traits simultaneously, maintaining their distinctiveness without creating a new intermediate form.
• Genetic Diversity:
  • Incomplete Dominance: Contributes to genetic diversity by producing intermediate phenotypes that enhance variability within a population.
  • Codominance: Enhances genetic diversity by preserving and displaying distinct phenotypic traits from both alleles.
• Common Examples:
  • Incomplete Dominance: Observed in flower color of snapdragons and hair structure in humans.
  • Codominance: Seen in the ABO blood group system and in certain animals with spotted coats.
• Organisms Affected:
  • Incomplete Dominance: Found in various organisms, including plants, animals, and humans.
  • Codominance: Present across a range of organisms, including humans, livestock, and other animals.