Difference between incomplete and co-dominance


When you were studying the chapter Genetic Basis of Inheritance (if you didn't understand the concept properly click here which will lead you to a blog post that I wrote, where I have tried to explain in simpler words. I have also provided handwritten notes at the end so don't forget to check it out), you must have noticed that incomplete dominance and co-dominance have slightly the same definition but there is a huge difference between them.

Difference between incomplete and co-dominance:

So, to clear your doubts regarding incomplete dominance and co-dominance here are 7 differences you should know.

Incomplete domincance Co-dominance
1. None of the alleles are dominant Both the alleles are dominant
2. The alleles mix and produce a fine expression. Hence they produce a new trait by combining There is no mixing of alleles. Hence the traits express themselves equally
3. Even though there is mixing taking place only one or two of the traits are noticeable Both the traits express themselves equally
4. The hybrids produce intermediate progeny The alleles are independent of their parental traits
5. The hybrids give rise to new phenotypes meaning new combinations There is no formation of new phenotypes
6. There is presence of quantitative effect There is abscence of quantitative effect
7. Eg, Snapdragon and Four o' clock flower Eg, Roan character in cattles and AB blood group in human

What is incomplete dominance ?

Here, the allelomorphic pairs present in both the genes express themselves only to a limited extent in the progeny obtained. This happens because one gene cannot express the other gene totally so there is no dominant and no recessive traits to be found.

When such type of dominance takes place you get an intermediate expression in the F1 hybrids. For eg, in four o' clock plant (Mirabilis jalapa) the red colour is supposed to be dominant over the white but in since it shows incomplete dominance you get an intermediate expression i.e the colour pink which will be the mixture of the colour red and white.

In F2 generation you get a ratio of 1:2:1 i.e red:pink:white. So we can that the factors which are present don't mix with each other to suppress the other and there is segregation taking place. Other example of incomplete dominance is Snapdragon (Antirrhinum majus).

 Graphical representation of incomplete dominance
Graphical representation of incomplete dominance


What is co-dominance ?

In incomplete dominance the genes express themselves to an extent but in co-dominance they express themselves equally in the F1 hybrids. Therefore, you can say that both can express themselves. 

For eg, the coat colour of a cattle. There are two types of colours which are - red and white. When the red cattle is crossed with white, in the F1 hybrids you would get a roan colour cattle. This is because co-dominance is taking place where they can express themselves equally. 

You get a phenotypic ratio of 1:2:1 i.e red:roan:white. In co-dominance the genotype and the phenotype would both have the same ratios, but in incomplete dominance the ratios won't be the same instead you would get an intermediate progeny.

 Graphical representation of co-dominance
Graphical representation of co-dominance

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