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Three most recent questions: 2009-11-19 2009-11-05 Although we can't publish all of the questions we receive, we do our best to share our insights on these issues. If you'd like us to consider a question you have about genetics, please fill out the question form.
Ask a Geneticist by Dr. Barry Starr, Stanford University I understand that lack of sunshine led to Caucasians becoming pale because of lack of vitamin D. But how did it happen? -A curious adult from California
Which wasn’t a problem in Africa, where we all started out. There is so much sunlight there that getting enough vitamin D wasn’t the problem…the sun’s harsh UV light was. This is why Africans tend to have darker skin. But getting enough vitamin D was certainly a problem in Northern Europe. As you know if you’ve ever visited there, it is not a place conducive to getting a lot of sun. The winter days are short and the cold weather causes people to cover a lot of their skin and stay indoors. And it is cloudy an awful lot. It is so hard to get enough sun under these conditions that dark skin is actually a problem. Which is probably why Northern Europeans turned from dark to pale—to get enough vitamin D. This explains the why pretty well but not the how. To understand the how, we need to go over a few basic genetics concepts. Then we’ll see how a trait like fair skin can become common. Bringing Hidden Traits Out As anyone who looks around knows, people all look pretty different. For the most part, these differences are there because we are all different genetically. Now this doesn’t mean we all have different genes. As humans we all share the same set of genes. What makes each of us different is that we can have different versions of these same genes. This explains why we’re all different. But it doesn’t do a very good job of explaining how, for example, two darker skinned parents can have a lighter skinned child. To understand this, we need to remember that we have two copies of each of our genes—one from mom and one from dad. What this means is that the same person can have copies of different gene versions.
People with two dark versions (DD) tend to be much darker than people with two light versions (LL). People with a copy of each (DL) tend to be in between Imagine that two DL parents have a child. The child will get one version of the SLC24A5 gene from mom and one from dad. Which copy the child gets from each parent is totally random. So half the time mom will pass a D and half the time an L. Same with dad. This works out to each child having a 1 in 4 chance of getting two L’s and so being much more fair skinned than his or her parents. That is how Africans can sometimes have a lighter colored child (and, for that matter, a darker one). But now we need to figure out how light colored skin became so common. The answer lies in something called selection. Pale Skin Sweeps Europe Imagine a group of Africans migrates to Northern Europe. This population has lots of different gene versions. In terms of skin color, that means that there is a range of colors from dark to darker. Most of these folks have two dark versions of the SLC24A5 gene—they are DD genetically. But a few might also be DL. These DL people would tend to be on the lighter side of the skin color range. At some point after coming to Europe, darker skinned people started getting sick. Since most of the people had dark skin, this means that most of the people were sick. This was not a good time to be European. Not only were they sick, but their darker skinned children were too. The kids had weak bones and suffered many problems including an awful disease called rickets. Most of these children didn’t survive to have kids of their own. Remember, though, that there were a few lighter skinned people who were DL genetically (they had a light version of SLC24A5). They did better because they could get more vitamin D from the weak European sun. And some of the kids inherited their parents’ less dark skin.
These LL people were much better adapted to their surroundings and did better than even their DL neighbors. These LL folks then went on to have kids who would mostly be fair skinned. Repeat this a few times and voila, a pale race is born. It is important to note here that lack of sun didn’t cause a genetic difference that led to lighter skin. The difference was already there in a few people. It took a pretty brutal selection to rapidly make it the most common skin color gene version in Europeans. Odds and Ends This is a nice, clean story. Of course nothing in genetics is so simple. There are always a few twists to keep things interesting. For example, early results suggest that pale skin swept through Europe between 6,000 and 12,000 years ago (scientists can tell from looking at the DNA around the SLC24A5 gene). But Africans arrived there 30,000-40,000 years ago. So why did it take so long for pale skin to take hold? One idea is diet. Pale skin swept Europe right around the time that agriculture took hold. The idea is a hunter-gatherer can get enough vitamin D from his or her diet and that a farmer can’t. These tens of thousands of years in weak sunlight may also have allowed a random change (or mutation) in the SLC24A5 gene to be passed on. In Africa, having fair skin was bad for you. In hunter-gatherer Europe, it didn’t much matter. So a mutation in the SLC24A5 gene might be better tolerated in Europe. If this were the case, then the original settlers didn’t have a light version of SLC24A5 when they left Africa. It appeared while they were in Europe. Another interesting twist is that SLC24A5 isn’t the only way to end up light skinned. For example, one of the reasons Asians are fair is because of differences in a gene called kitlg. Most likely a few of the settlers of Asia happened to have differences in the kitlg gene. And a few European settlers had differences in the SLC24A5 gene. Both groups were under the same pressure to become light skinned. But they responded in different ways based on the genes already within their populations. More Information |
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