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I understand how codominance works. We have two (or more) alleles for one gene, neither having dominance over the other. They are both expressed independently, which means that different phenotypes are visible simultaneously. However, I'm not sure how it really works when we take, say, coat color. If both alleles are active, assuming each one produces a different pigment for each color, that means there are both pigments in a cell. What I see explained is that, if you look closely, each hair has a different color, and when you see it from far away, it gives the impression that the coat is made of one single color. But how can one hair have the same color all throughout? How can the different pigments be aggregated like this? As in, how is it that they do aggregate like this?

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  • $\begingroup$ Why do some cells produce hair, and others fingernails, or insulin? FTM, why do some animals have spots or stripes? $\endgroup$ – jamesqf Jan 1 '18 at 3:08
  • $\begingroup$ This paper may help get you started, how spots and stripes form . ncbi.nlm.nih.gov/pmc/articles/PMC4380182, how simple rules/interactions can create complex patterns is quite interesting. $\endgroup$ – John Jan 1 '18 at 6:36
  • $\begingroup$ @jamesqf, why does a strand of hair have one color only if every one of the cells it's made of produces both pigments? That's what I mean. I was looking for something in broad terms, but it seems to be impossible to generalize. $\endgroup$ – ixjf Jan 1 '18 at 10:32
  • $\begingroup$ The point I was trying to make is that (AFAIK - I'm not a biologist) each cell does not produce both pigments. Indeed, hairs are not produced by single cells, but by rather complex organs called follicles. At some point in their development, they've differentiated to produce different colors and types of hair. So you get coat patterns in animals, differences between head & body hair in humans, &c. $\endgroup$ – jamesqf Jan 1 '18 at 18:26
  • $\begingroup$ Right! That's closer to what I was looking for. So there's a kind of centralization and differentiation at work here. I'm only in 12th grade and was curious about the details of how this works. The book we use doesn't go into any length about this, and I'm pretty sure asking my teacher would be pointless. $\endgroup$ – ixjf Jan 1 '18 at 19:16
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The terms dominance/recessivity, additivity, co-dominance, incomplete dominance, etc... express a relationship between genotypes and phenotypes but the mechanism behind this relationship is not being defined by these terms.

There might be a large number of mechanisms associating genotypes at a given locus with phenotypes. There is, in general, no reason to expect a mechanism of relationship of dominance at a given locus to be similar to that at another locus. In other words, there is no general mechanism leading to dominance/recessivity, additivity, co-dominance, incomplete dominance, etc...

On top of that, the actual mechanism in specific cases leading to particular genotypes - phenotype association are often unknown and I don't think we have good knowledge about what mechanisms could be more common or even how to categorize such mechanisms.

You might want to have a look at the posts

or some papers such as

In general, it appears suggested that one needs to know the specifics of the biological pathway to be able to predict dominance relationships and gene interaction network (see Llaurens et al. (2009)).

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  • $\begingroup$ I do realize there isn't a general mechanism for this. That's why I asked the question specifically for coat hair (need not even be generalized, just a light), hoping that would narrow it down. The question still intrigues me. I was hoping something that I would think is a given would have been understood by now! $\endgroup$ – ixjf Dec 31 '17 at 23:50
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    $\begingroup$ You will need to be even more specific than coat color. For example, in female cats, X inactivation (dosage compensation) has an important role in determining coat colour. In human, 378 markers affecting skin color have been identified! You might want to narrow the question to a very specific study system. $\endgroup$ – Remi.b Jan 1 '18 at 0:12
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    $\begingroup$ I would recommend cherry picking your system of interest so that you have best chance that an answer exists. Consider the Mc1r gene in mice (Hoekstra et al. 2006) for example. Also, as the current post has already received an answer, I would recommend asking this new (narrowed down) question on a new post. $\endgroup$ – Remi.b Jan 1 '18 at 0:12

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