When we consider examples of incomplete dominance we take that of FOUR O CLOCK plant -

We say that alleles for red and white colour of flower are not completely dominant against each other. Hence in hybrid progeny we Say (as in standard textbooks) that pink colour appears due to "expression of single gene for pigmented flower which produces only pink colour (quantitative inheritance )"

In codominance concept we take example of SHORT HORNED CATTLE which again appears as same case white and red cattle produces roan coloured hybrid progeny but due to "juxtaposition of small patches of red and white colour hair"

The same principle can be applied to four o clock plant too. Suppose pink colour of flower is too due to juxtaposition of red and white coloured elements.

Then where is the difference between the two concepts?

Further the thing which puzzled me more was that - "initially cattle example was considered to be in complete dominance. Later it was CORRECTED to codominance " Also incomplete dominance is also known as "mosaic dominance" but actually mosaic happens in codominance. Kindly clarify my doubts


1 Answer 1


Mosaic dominance

I think your confusion comes from the statement

incomplete dominance is also known as "mosaic dominance"

I had never heard of mosaic dominance before and I could not find much on this concept. I found a few papers but they either referred to mosaicism (see below) or they did use this term only a few times without defining it. There is an entry for mosaic inheritence in the online medical dictionnary though (here). The definition they give is

inheritance in which the paternal influence is dominant in one group of cells and the maternal in another.

It is a little vague to me as 1) the phrasing is a little unusual and 2) it does not specify the underlying mechanism. If anything, it feels like a synonym of codominance and not of incomplete dominance.

Mosaicism is a thing in genetics but it is mainly unrelated to the concept of dominance as it refers to genetic polymorphism among cells within an organism.

Where did you read about mosaic dominance? Can you please give a direct quote?

If anything, mosaic dominance sounds like a synonym of codominance and not a synonym of incomplete dominance.

Codominance vs incomplete dominance

Outside the above issue, I think you pretty much define these two terms through your examples. You might want to have a look at the post (although it is a post who did not get much attention)

In short, codominance implies that the specific phenotypic effect of each allele is being visible in the heterozygote, while incomplete dominance implies that the heterozygote has a phenotype that is somewhat intermediate between the two homozygote phenotypes.

  • $\begingroup$ Well thank you very much for the answer. There is no direct quote on mosaicism but it is written like this-"incomplete (partial or mosaic) dominance is the phenomenon where none of the two contrasting alleles ir factors is dominant. " $\endgroup$ Jul 22, 2017 at 14:54
  • $\begingroup$ But there is still one doubt does pink appears due to expression by single gene for pigmented flower, if yes how did that gene form? Or it is mixed effect of red and white colouring proteins as written above , if this is true then why it is not included in codominance $\endgroup$ Jul 22, 2017 at 15:03

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