Science Sundays: Human Pigmentation

By: Meradeth Houston on 11:41 AM

Hi everyone! Welcome back to Science Sundays. Before I launch into this, I should point out that I'm hanging out at A Bibliophiles Thoughts on Books today, talking about scary books :) Pop on by and tell me the scariest book you've ever read!

Okay, this post requires a small disclaimer: I know human pigmentation (and by that I mean skin color, hair color, eye color), can be a bit of a touchy subjects. I'm going to do my best to explain it from a scientific point of view, and note how the arguments that have been used for discrimination are totally unfounded.

So, to kick this off, I need to talk about some background info, namely Continuous Traits. Like the name suggests, these are traits that are continuous in nature--they don't have separate, discrete categories that are distinct from one another. A good trait to use as an example is height: in humans there are short people, tall people, and every possible height in between. It's impossible to segregate people into tall/medium/short without a whole lot of people falling into the "grey" area in between. On the other extreme, a human discontinuous trait would be attached and unattached earlobes (even though there is some discussion on this point...).

The thing with continuous traits is that they are coded for by lots and lots of genes, and there are many variants of each of these genes (called alleles). The reason why we see so much variability in continuous traits is that there are so many different combinations of these many genes and alleles. A person of medium height might have several gene allele's for tall height, and several alleles for short height, so they end up right in the middle. People who are taller might have more alleles for greater height, and the opposite for short people. I hope that makes sense :)

Now, the same thing works for human pigmentation. The amount of melanin (the color-conferring protein the body produces) that is produced is subject to many different genes (nine main ones, and a lot more that we don't know about completely yet). Depending on how many alleles for more melanin production, or less melanin production, we end up with the wide range of human coloration we see. There are of course other elements to coloration too: tanning, the amount of blood vessels just under the skin, and the thickness of the top layer of skin cells (along with many others) that also lead to added variation.

To get a little more detailed, there are two different types of melanin: eumelanin and phaeomelanin. The former is a darker brown color, and the latter confers a more reddish color. So, a person who has a specific mutation in their MC1R gene actually doesn't produce much (if any) eumelanin, leading to the very pale skin, red hair, and often green eye coloration we see.

The genes that lead to pigmentation production effects both eye and hair color, too, however there are several different genes that play into both of those areas. In eye color alone, there are at least 15 genes that play a role, and probably more. Some of these genes are in common with skin color (like MC1R), and some are not. So eye color is partly linked to skin color, but not completely.

So, while there is a general trend for people with dark hair to have dark eyes and hair, the different genes, and different alleles they carry for each gene, influence the hair/eye/skin coloration differently. This is how we end up with those individuals with dark hair and/or skin and strikingly light eyes (this is so one of my favorite combos--it's strikingly beautiful!). This is also how one can have offspring with coloring combinations that are a bit different from their parents--it all depends on which alleles get passed on, and the combinations they make!

Thanks a million to Juliana Neumann for suggesting this topic! Shout out questions in the comments and I'll do my best to answer them :)