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As I understand it, co-dominance is when both genes in an allelomorphic pair produce both their effects equally on the organism in question whilst incomplete dominance applies to an instance where a mix of the effects of two alleles are present — i.e. where one allele is not completely masked by the other.

However, I had this notion challenged by the following paragraph I obtained from a reputable online source:

Carnation plants show co-dominance for the anthocyanin gene. There are two alleles:

$F^A$ – allele for anthocyanin pigment (red flowers)

$F^N$ – allele for no anthocyanin pigment (white flowers)


[...] a carnation plant heterozygous for this gene would yield pink flowers [...]

Shouldn't the phenotype of the flower be red and white as opposed to being pink, as the alleles have been said to exhibit co-dominance? Anything I am probably missing out on?


EDIT

I have bountied this question because I think I have a major hole with my understanding on this topic somewhere; to give you some perspective, I am a High-School Sophomore currently on an introductory course in Mendelian Genetics/Inheritance. I read through @swbarnes2's answer where they say:

In the case of flowers, if you have two different alleles for the same pigment gene, you will see a blending, because all the pigment-making cells will produce from both alleles.

In such a case (if this always happens), where do the ideas of codominance and incomplete dominance come in? According to my textbook, the following terms are defined as:

If both genes of an allelomorphic pair produce their effects in an individual (i.e neither allele is dominant to the other) the alleles are said to be co-dominant.

codominance

Incomplete dominance applies to a case where the effect of the recessive allele is not completely masked by the dominant allele (in this case, I assume, a blending would occur)

enter image description here

(I do get that this may not apply to carnation plants, but would it be safe to follow the phenotypes predicted by my textbook definitions for all other cases?)

Hope someone could clear my doubt; I am extremely confused to read (in the comments section) why co-dominance would not mean both colors/phenotypes are shown simultaneously — like being red and white (as opposed to them mixing into pink).

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    $\begingroup$ "from a reputable online source" -- I appreciate your direct quoting of the source, but could you also provide a link to it? $\endgroup$
    – acvill
    Sep 16 at 12:56
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    $\begingroup$ @acvill I mean really, all the notions about dominance are outdated in modern molecular biology; they're good tools for understanding Mendelian genetics where one has access only to lineage information and not any molecular biology, but once you know more they are rarely actually meaningful distinctions. I think that's what's happening here; there's a blending between the understanding of the phenotypes as colors, and the understanding of the phenotypes as enzymatic reactions. I guess that's probably what an answer should explain, I do think I have a better idea now of where OP is coming from. $\endgroup$
    – Bryan Krause
    Sep 16 at 14:35
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    $\begingroup$ @BryanKrause I agree, though someone new to genetics doesn't have that insight, and your first comment came across as dismissive. Maybe this question concerning the molecular basis of incomplete dominance is relevant. $\endgroup$
    – acvill
    Sep 16 at 14:38
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    $\begingroup$ @acvill Thank you for the link; I took a look at it, but there wasn't much in there I could really understand, being a high school student! $\endgroup$
    – Shane
    Sep 17 at 10:38
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    $\begingroup$ Re: "why co-dominance would not mean both colors/phenotypes are shown simultaneously" Not mentioned here is the scope of the phenotype. Is it one flower? One cell in a flower? The whole plant? As Bryan mentioned, often "the notions about dominance are outdated in modern molecular biology". The red and white patches in your flower photo seem to me to be an example of "variegation" rather than co-dominance. $\endgroup$
    – Armand
    Sep 19 at 18:04
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Unfortunately, that nomenclature is kind of old-fashioned; it predates our understanding of what's going on at a molecular level. Don't get hung up on the language. Lots and lots of genes and alleles just don't fit into the paradigm of dominant and recessive, and trying to force them to fit doesn't really work.

In the case of flowers, if you have two different alleles for the same pigment gene, you will see a blending, because all the pigment-making cells will produce from both alleles. There really is no such thing as "white" pigment; either it's causing a color outside the visual spectrum, or it's a broken version of a gene in the pathway that should be making a color-absorbing molecule. We would usually more properly refer to this as "haploinsufficiency".

If you had blue and red alleles, it would likely make a purple flower, not one that was blue in some patches, red in others.

For something like blood type, AB blood cells make both A and B molecules on their surface (O is broken), but AB is just as good as A in terms of causing an immune reaction; there is no alteration in phenotype due to the lower dose of each allele.

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Friend..you got the meaning of co dominance and incomplete dominance correct... but let me make co dominance easy... its just like mixing colors on a color plate...think it just like that.... otherwise in real sense both colors are indivisually expressed but being expressed in equal amount and with uniformity they appear as pink... other example could be ABO Blood group in humans....as for refrence https://www.genome.gov/genetics-glossary/Codominance hope it helps..

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  • $\begingroup$ Welcome to Biology.SE. This looks like it could become a good answer with work, but answers are much more likely to receive a favorable response if they include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion and thus inappropriate for this site. A good example of how to format references. In addition, it is best to leave poor questions unanswered. ——— Please take the tour and consult the help center pages for additional advice on How to Answer and How to Ask effectively on this site. $\endgroup$
    – tyersome
    Sep 16 at 20:41

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