Codominance vs. Incomplete Dominance

Attribute	Codominance	Incomplete Dominance
Definition	Both alleles are fully expressed in the phenotype	Neither allele is fully dominant, resulting in a blended phenotype
Allele Interaction	Both alleles contribute equally to the phenotype	One allele is not completely dominant over the other
Phenotypic Ratio	1:2:1	1:2:1
Example	ABO blood groups (A and B alleles)	Pink flowers (red and white alleles)
Genotype	Both homozygous and heterozygous genotypes exhibit the same phenotype	Homozygous genotypes exhibit a different phenotype than heterozygous genotype
Expression	Both alleles are expressed independently and simultaneously	Blended phenotype due to incomplete dominance of one allele over the other

Introduction

When studying genetics, it is essential to understand the various patterns of inheritance that exist. Two such patterns are codominance and incomplete dominance. While both involve the expression of multiple alleles, they differ in how these alleles interact and manifest in the phenotype. In this article, we will explore the attributes of codominance and incomplete dominance, highlighting their similarities and differences.

Codominance

Codominance occurs when both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that shows both traits simultaneously. This means that neither allele is dominant or recessive over the other. A classic example of codominance is the ABO blood group system in humans. In this system, individuals can have type A, type B, or type AB blood. Type A individuals have the A antigen on their red blood cells, type B individuals have the B antigen, and type AB individuals have both A and B antigens. This demonstrates how both the A and B alleles are equally expressed, leading to the codominant phenotype.

Another example of codominance can be observed in certain flower colors. For instance, in snapdragons, the alleles for red and white flowers are codominant. When a plant with red flowers (RR) is crossed with a plant with white flowers (WW), the resulting offspring (RW) will have flowers that are neither red nor white but rather a blend of both colors, often appearing as pink. This showcases how both alleles are expressed equally, resulting in a codominant phenotype.

Incomplete Dominance

In contrast to codominance, incomplete dominance occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. In this case, neither allele is completely dominant over the other, resulting in a unique phenotype. A classic example of incomplete dominance is seen in the flower color of certain plants, such as four o'clocks. When a plant with red flowers (RR) is crossed with a plant with white flowers (WW), the resulting offspring (RW) will have pink flowers. Unlike in codominance, where both traits are fully expressed, in incomplete dominance, the traits blend together to create a new phenotype.

Another example of incomplete dominance can be observed in the inheritance of hair texture in certain breeds of dogs. For instance, when a dog with curly hair (CC) is crossed with a dog with straight hair (SS), the resulting offspring (CS) will have wavy hair. Here, the curly and straight alleles do not compete or coexist, but rather blend together to create a new phenotype that is distinct from either parent.

Similarities

While codominance and incomplete dominance have distinct characteristics, they also share some similarities. Firstly, both patterns involve the expression of multiple alleles. In both cases, there are more than two possible alleles for a given gene, and the phenotype is influenced by the combination of these alleles. Secondly, both patterns result in phenotypes that are different from the homozygous dominant and homozygous recessive phenotypes. In codominance, the phenotype shows both traits simultaneously, while in incomplete dominance, the phenotype is an intermediate blend of the two homozygous phenotypes. Lastly, both patterns contribute to the genetic diversity within a population. By allowing for the expression of multiple alleles, codominance and incomplete dominance increase the range of phenotypes observed, promoting genetic variation.

Differences

While codominance and incomplete dominance share similarities, they also have distinct attributes that set them apart. Firstly, in codominance, both alleles are fully expressed, whereas in incomplete dominance, the alleles blend together to create an intermediate phenotype. This fundamental difference in allele expression leads to different phenotypic outcomes. Secondly, in codominance, the heterozygous individual has a phenotype that shows both traits simultaneously, while in incomplete dominance, the heterozygous individual has a unique phenotype that is distinct from either homozygous phenotype. This distinction highlights the different ways in which alleles interact and manifest in the phenotype.

Another difference lies in the genetic ratios observed in offspring. In codominance, the genetic ratio of the offspring from a cross between two heterozygous individuals is 1:2:1, with one homozygous dominant, two heterozygous, and one homozygous recessive offspring. In contrast, in incomplete dominance, the genetic ratio is 1:2:1, similar to codominance. However, the phenotypic ratio is different, with one homozygous dominant, two heterozygous, and one homozygous recessive offspring in codominance, while in incomplete dominance, the phenotypic ratio is 1:2:1, with one homozygous dominant, two heterozygous, and one homozygous recessive offspring.

Conclusion

In conclusion, codominance and incomplete dominance are two patterns of inheritance that involve the expression of multiple alleles. While codominance results in a phenotype that shows both traits simultaneously, incomplete dominance leads to an intermediate blend of the two homozygous phenotypes. Despite their differences, both patterns contribute to genetic diversity and expand the range of phenotypes observed within a population. Understanding these patterns of inheritance is crucial for comprehending the complexity of genetics and the inheritance of traits in various organisms.