how mixed race people have a skin tone that is in between their parents’ skin tone, if Mendel’s law of dominance is true?


how mixed race people have a skin tone that is in between their parents’ skin tone, if Mendel’s law of dominance is true?

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Mendel’s law of dominance is incomplete. Not all traits are controlled by simple dominance, to which Mendel’s law would apply.

You can have incomplete dominance as well as co-dominance, which do not obey Mendel’s law.

There are multiple genes regulating skin tone. Mixed race people inherit some genes from each of their parents. For this reason their skin tone is something in between.

Mendel’s law of dominance is a quick rule-of-thumb – not an absolute “law” (as, for example, the law of gravity is [relatively] absolute)

Mendel’s Law is meant as a predictive tool. Inside that relative predicative norm are a range of variations.

Mendel used his observation of specific physical traits (aka phenotypes) of pea plants as a basis for developing his law. He correctly developed the pundit square and law of dominance through these observations, but that doesn’t mean all physical traits are “mendelian.”

We have access to the source code of life now, DNA. We can identify when phenotypes (physical characteristics) are controlled by single or multiple genes, and through large studies of heritable traits (and disease), determine that not all genes have “complete penetrance.” So even if a gene it’s directly responsible for a trait, that gene does not always result in that trait in offspring.

TLDR; Mendel made amazing leaps in logic with very limited data, but not all physical traits are inherited in simple mendelian fashion, skin tone and eye color are two such traits in humans.

A single gene can have several different copies with varying levels of dominance rather than just two with a simple dominant-recessive pattern. Instead of just A and a, there’s a1, a2, etc. These are called alleles.

You can also have several different genes that control the same thing. Instead of just A, there’s B, C, etc. These are called loci, and this is called a “polygenetic trait” – poly as in many and genetic as in genes (loci). Skin color is a polygenetic trait.

In the case of skin color, I believe there are 8 known loci. Each one adds or doesn’t add a little melanin depending on the copy it has, leading to a wide variety of skin colors.

Let’s say a dark-skinned parent and a light-skinned parent have a kid. The dark-skinned parent has 7 “dark skin” alleles + 1 “light skinned” alleles. The light-skinned parent has 7 “light skinned” alleles + 1 “dark skinned” allele. Any combination between these two is possible in the child.

Maybe they kid’ll have 6 “dark skinned” alleles and 2 “light skinned” alleles, resulting in skin only a little lighter than the dark-skinned parent. Or the kid could inherit 4 “dark skinned” alleles and 4 “light skinned” alleles and have mid-toned skin between light and dark.

Picture a row of paintbrushes. An individual paintbrush can either have paint on it (which is dominant) or no paint on it (which is recessive). You take each paintbrush in turn and rub it onto the same paper. You need a lot of layers to make it vibrant/dark. So the more paintbrushes don’t have paint, the lighter the result is. There’s 8 paint brushes. They can’t be dominant over each other; all they do is rub the paper and either add paint or not.

TL;DR: Alleles are dominant over other alleles, although not always completely, as other comments have pointed out. However, in the case of skin color specifically, the main reason we see so much variety is not because of alleles (dominance within a gene) but loci (multiple genes) creating a polygenetic trait (the genes are working together).

Sometimes loci can hide other loci, but this does not work the same as dominance. Dominance ONLY applies to alleles.

[all the edits are fixing typos bc it’s 7am and I have not slept.]