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Ok let me start with the definitions of incomplete dominance and codominance.

incomplete dominance - The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.

codominance - The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways.

It is the standard textbook example of incomplete dominance to show a cross of red and white Snapdragons of pure bread. Which yields the phenotypic ratio of 1 red : 2 pink : 1 white. The standard text example of codominance is AB blood type where the A glycoprotein and B glycoprotein together produce a distinguishable phenotype apart from AA or BB.

I really don't understand the distinction between intermediate phenotypes and distinguished phenotypes. How is pink not distinguished from white and red. Consider a hypothetical example of some insect that interprets red, white and pink as distinguishable signals where perhaps red is a safe flower, white is ignored, and pink is dangerous. Say these signal recognitions have evolved based on the insects contrast against the flower pigment and the probability of being eaten by a predator because of increased exposure while getting nectar from the flower. I suppose I have a misunderstanding but wouldn't the red, white, and pink phenotypes be distinguishable rather than intermediates in that case?

So I'm probably just being an idiot but how is incomplete dominance different from codominance?

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  • $\begingroup$ "Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete or semi-dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example in Co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots." - from the wiki on dominance $\endgroup$
    – rg255
    Nov 27, 2014 at 21:58
  • $\begingroup$ But aren't pink flowers just red and white spots at a molecular level? $\endgroup$
    – vajra78
    Nov 27, 2014 at 22:09
  • $\begingroup$ I think you may find the answer you are looking for in this post $\endgroup$
    – Remi.b
    Nov 28, 2014 at 0:51
  • $\begingroup$ I ran across this reference for a break down of dominance in "Human Molecular Genetics 3" by Strachan & Read. ncbi.nlm.nih.gov/pubmed?cmd=historysearch&querykey=2 $\endgroup$
    – vajra78
    Dec 2, 2014 at 4:43

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Certainly if you are talking about signals, red, white, and pink phenotypes are distinguishable rather than intermediates...

But inheriting a character is far away from the concept of distinguishing signals. As genotype of an organism decides its phenotype, you should know that in a hybrid both the paternal and maternal traits would exist. If one is dominant then it is expressed and the recessive is not.

However if the dominant gene is unable to express itself independently in presence of the recessive and if the recessive is partially expressed, then both the dominant and recessive give rise to an intermediate phenotype which "only one of them cannot produce"

This is what we call blending. In case of Snapdragons, pink is a phenotype that allele responsible for red or white alone could not have produced.

In case of co dominance you can notice multiple dominance. i.e., no recessive character is there. All alleles are expressed "independently of one another" in an organism.

Eg:- AB blood group.

Also consider cross between black coat coloured cat and white cat. The offspring produced has black and white patched coat colour. i.e., Black and white didnt blend to give rise to grey. I mean, alleles responsible for these traits are independently expressed.

Hope that clears your doubt. :-)

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incomplete dominance are more likely mean blend, like you make juice contain apple and pear by blender, you can not distinguish them.

codominance can be think as mix, like you a salad that contain apple and pear.

like the example of incomplete dominance: pink flower, pink is differ from white and red, but it related to the two color,

as the example of codominance ABO blood type, blood type AB means it contain A antigen and B antigen, we can distinguish them

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AB blood group is the best example one can give about co dominance. What matters is both A and B are dominant and are expressed independently of one another. The quantity they are produced is not important here as our genetic make up cannot do relatively very large difference between synthesis of the two.

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    $\begingroup$ You can edit your original answer, you know. . . $\endgroup$
    – HDE 226868
    Nov 29, 2014 at 20:01
  • $\begingroup$ But aren't pink flowers just red and white spots on a flower petal cell at a molecular level, just like A and B proteins on a blood cell? I don't know the mechanism, I suppose if the red and white proteins on a flower petal interacted to form another protein then that would be blending. Otherwise, wouldn't the pink just be a pointalistic illusion. $\endgroup$
    – vajra78
    Dec 1, 2014 at 6:42
  • $\begingroup$ I'm pretty sure the mechanism is that the pink flowers don't make as much functional pigment as the red ones, because that "white" allele for that gene in the pigment process is just broken. $\endgroup$
    – swbarnes2
    Oct 13, 2015 at 21:18
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incomplete dominance is when one allele is partially dominant over the other allele of the same trait.both these alleles are dominant but one is more powerful than the other .for examples a mating between a white female and black male in humans and have a colored child.

co-dominance is when two alleles are equally expressed.

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  • $\begingroup$ I doubt your example is correct. Human skin color is polygenic inheritance. $\endgroup$
    – Dexter
    Oct 14, 2015 at 4:43
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Remember, the vast majority of traits don't conform to the Mendelian paradigm, even when its stretched a bit like this. So this really is a very academic question, and neither term is applicable to a huge number of real-life situations.

To use the examples of snapdragons and blood types, an AA, AO or AB blood type all show no reaction at all to the A antigen. If AA was protective towards A blood, but AO and AB showed weak reactivity, we'd likely call them incompletely dominant.

For the snapdragons, it really doesn't make any sense to call either allele dominant. What's really going on there is that the "white" allele is a broken gene in the process that makes red pigment, and flowers with only one working allele don't make as much red pigment as flowers with two working copies. Whereas one working copy of the ABO gene (that is, the A or B alleles) is enough to teach the immune system not to react.

So I'd say that incomplete dominance is usually a results of haploinsuffieicny; one allele is broken, and the other by itself doesn't do as good a job as two working copies, and co-dominance is usually two working alleles both doing different things.

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