Difference between Incomplete dominance and Co-dominance – Incomplete dominance vs Co-dominance

Incomplete dominance definition

Incomplete dominance is one of the mechanisms for dominance among heterozygotes in which the dominant allele does not fully overpower the phenotypic expressions of the recessive one, and there is an intermediate phenotype within the heterozygote.

Incomplete dominance can also be referred to as semi-dominance or partial dominance because the phenotype that results of the genotype can be described as a mix of recessive and dominant alleles. A good example of this can be seen in the flowers of which dominating alleles are red and the recessive allele is white. The heterozygous flowers with these alleles could appear pink due to insufficient dominance. In cases of incomplete dominance the dominant allele can’t fully dominate the recessive one because of that the resulting genotype is a mixture of both.


Incomplete dominance is significant because it is the reason for two different alleles not mentioned in the work of Mendel in his study. Mendel clarified his Law of dominance to indicate that one of the two alleles is dominant because it is always dominant over the recessive character. Mendel could not analyze incomplete dominance because the pea plant that he used to test didn’t exhibit an incomplete dominance. But, his model can still be used to analyze the outcomes of crosses between alleles that have incomplete dominance. According to his model the resultant F1 generation will have the ratio 1:2:1 while the phenotypic ratio will be pink: red white.

This means that alleles can still be inheritable according to Mendel’s rules even though they have a partial dominance. In quantitative genetics, when the heterozygous phenotype is exactly the same as (numerically) those of two homozygotes, it is considered to be there is no dominance. For dominance to take place the heterozygotic phenotype must be closer than one homozygotic.


Examples of incomplete dominance

Wavy hair in humans

The curly hair type is the most dominant characteristic of humans, whereas straight hair is a recessive characteristic. In heterozygous species the resultant phenotype is wavy, that is a mix of curly and straight. Therefore, wavy hair is a result from an incomplete dominance, and the characteristic is due to the mix of the two characteristics. Wavy hairtherefore is a distinct phenotype from curly or straight hair. Children born from two parents who have homozygous genotypes have an inherited ratio of 1:2 according to the phenotypic proportion or curly: straight: wavy.

Pink flowers in Mirabilis jalapa

Incomplete dominance can also be seen when a pure line , or homozygous to the red flower (C1C1) can be crossed with form a pure line of white petals (C2C2) The F1 progeny does not have pink petals instead of red (C1C2).


When an F2 is born from its offspring, its descendants show the following traits :

  • 1/4 red petals 1 C1C1
  • 1/2 pink petals – 2 C1C2
  • 1/4 white petals 1 C2C2

The dominant characteristic for the color of flowers is red. However, recessive traits are white. Therefore, the phenotype that results from insufficient dominance is pink. The phenotype that is resulting is an original one that was not present in the family with homozygous parents.


Co-dominance definition

Co-dominance is the method of dominance in some alleles in which both alleles of one gene within heterozygotes lack the recessive and dominant relationship and every allele is capable of a certain degree of expression phenotypic.

Co-dominance may be considered to be not dominance in the sense that the heterozygote exhibits the characteristics from both homozygotes. This is why heterozygote genotypes give the phenotype that is different from any of the homozygous genotypes. For codominant alleles every upper case base symbol that have different superscripts are utilized. The upper case letters show that every allele has the ability to be expressed in a certain degree even in the presence of an alternative allele.


Co-dominance is a common occurrence. It can be seen in plants where dominating phenotypes are red while the recessive phenotype appears white. The heterozygote is characterized by flowers that have spots of white and pink. As with incomplete dominance, the phenomenon of co-dominance wasn’t explained by Mendel because the model he used did not explicitly express co-dominance.

But, his model can be utilized to analyze the outcomes of crosses between alleles through co-complete dominance. According to his model the resultant F1 generation will have the ratio 1:2:1 while the phenotypic ratio will be red spot white. Co-dominance is usually recognized in animals and plants with two distinct colors, but it can also occur in other less well-known characteristics like blood type. This is why co-dominance differs from incomplete dominance because when both alleles exist, but in a separate way. when dominance is not complete, the genotype is likely to be a mixture of both alleles.


Examples of co-dominance

Blood type in humans

The blood type of humans is determined by the expression of the gene that encodes the proteins found at the surface of blood cells. The variants include A, B and O in which A and B are two distinct proteins. O is an absence of proteins. The presence of the B and A proteins, similar to two different colors of flowers are able to occur in tandem because of co-dominance. In other words, if both proteins B and A are passed down to offspring as well as both expressed, AB blood type might be present in offspring. But O is the type of blood. O indicates a dominant or regressive relationship in which if both genes have been expressed O does not get expressed.


Different animals have distinct shades of their skin and feathers because of co-dominance. When a bird with white feathers breeds an animal that has black feathers offspring are both black and white feathers because of co-dominance. When co-dominance is present, both characteristics are expressed in a way that is independent of one another. Similar phenomena are seen in cows. The breeding of white and black cows produces cows with the appearance of black and white. Because of co-dominance, both characteristics are manifested independently of one another. The mix of hues of hair on cattle is known as roan. This is also the consequence of co-dominance.

Difference between Incomplete dominance and Co-dominance – Incomplete dominance vs Co-dominance

Basis for ComparisonIncomplete dominanceCo-dominance
DefinitionIncomplete dominance is one of the mechanisms that is used to determine dominance for heterozygotes where the dominant allele can not fully overpower the phenotypic expressions of the recessive one, and there is an intermediate phenotype within the heterozygote.Co-dominance is a mechanism for dominance in certain alleles, where both alleles from one gene within heterozygote do not share the recessive and dominant relationship and every allele is capable of a certain degree of the phenotypic expression.
Phenotype of the HybridThe resulting phenotype for the hybrid results in a mixture, blend of two homozygotes.Both homozygous phenotypes can be manifested in a different way.
Expression of allelesIn the absence of dominance result that one is a result of two different alleles can be noticeable over the others.
The effect of two parental alleles creates an intermediate in the offspring.
In co-dominance, the impact of both alleles is very apparent.
Both alleles of the parent produces their effects in a way that is independent of each other.
PhenotypeThe resulting phenotype becomes a unique one due to the mixing of the two phenotypes of both parents.
The phenotypes of both parents could be seen within the hybrid.
The new phenotype doesn’t possess its own variant.
The resulting phenotype isn’t an original one since the two phenotypes of parents are not mixed together.
Both phenotypes from the parent are evident when examining the hybrid.
The new phenotype manifests as an amalgamation of two phenotypes derived from two alleles.
DominanceThe one of two alleles is not predominant over another.Each of the alleles is not acting in a dominant or recessive way over the other.
Quantitative methodIncomplete dominance permits quantitative analysis of two insufficiently dominant alleles.Co-dominance allows only the qualitative analysis of gene expression.
ExamplesExamples of partial dominance include flower petals of pink Mirabilis jalapaand the hair structure of human beings.Examples of co-dominance are the blood type of animals as well as the colored feathers or hair.

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