Can two parents with fair skin and blonde hair have a brown haired child with darker skin?
-A curious adult from Wisconsin
January 22, 2010
Yes that is definitely possible. Not that common, but it does happen.
Unfortunately we don't have a good enough handle on human coloring yet to explain how this happens in every situation. But we do know enough to explain at least one way it could happen.
That one way involves one of my favorite genes, MC1R. This gene is involved in both skin and hair color. Some versions of this gene can lead to pale skin, freckles, and/or lighter hair. And in some circumstances, they can even cause your hair to be red.
Let's imagine a situation where both parents have a fair version of the MC1R gene. If neither parent passes it to their child, then that child may very well end up with darker hair and skin than either parent.
Next we'll dig a little deeper to see how coloring works. From there we'll be able to get a good idea about how likely it is for these parents to have a darker child.
Humans get their skin, hair, and eye colors from pigments called melanins. We have two major pigments -- eumelanin and pheomelanin.
The more common of these two is eumelanin. It is responsible for most hair, skin and all eye colors. The less common pheomelanin is responsible for red hair and freckles and, to a lesser extent, blonde hair.
There are lots of genes involved in making these pigments. When any one of them isn't working quite right, you can end up with different coloration.
For example, if your OCA1 gene isn't working properly, you can end up with albinism. This is an almost complete lack of pigment. But this form of albinism only happens if both of your copies of OCA1 aren't working well.
See, we have two copies of most of our genes -- one from mom and one from dad. In most cases of albinism, if one copy of the affected gene works, then you still make plenty of pigment. And so don't have albinism.
The working copy of OCA1 can make enough melanin to keep the albinism at bay. The version that can lead to albinism is said to be recessive to the working version. And people who have one copy of the recessive gene version are called carriers.
The MC1R gene is involved with melanins too. But it isn't directly involved in making pigment. Instead, one of its jobs is to change pheomelanin into eumelanin.
This is why some MC1R versions cause red hair. Basically the eumelanin doesn't get converted and so pheomelanin builds up.
The MC1R gene is different from many other pigmentation genes in that having only one working copy can have an effect too. In other words, carriers of the fair version of MC1R sometimes have specific traits themselves. They often have pale skin, lighter hair, and freckles.
OK, so now we know enough to figure out the chances that two lighter parents who carry a fair version of the MC1R gene won't pass it on to their child. But because coloring is more complicated than just MC1R, we can't really figure out the exact chances for these parents having a darker child.
From Light Comes Dark
Imagine that our two parents are carriers of the fair version of the MC1R gene. They have lighter skin and hair than they would without this gene version. Let's say they look like the image to the right.
They have light skin and blonde hair. The M's underneath each parent represent his or her MC1R gene copies. They each have one working copy (M) and one non-working (or fair) copy (m). Geneticists often make the recessive copy a lower case letter.
Now let's think about the kids they might have. In my crude drawing, I have made the MC1R genes from dad blue and the ones from mom pink.
Which version of the MC1R gene that gets passed down is completely random. So each parent has a 50% chance of passing an M and a 50% chance of passing an m. The image to the left shows the four possible combinations that their kids might have.
In each of these four cases, one MC1R gene copy came from mom and the other from dad. Each child has a 50% chance of getting the same MC1R gene combination as their parents. Each child also has a 25% chance of having red hair.
And finally, each child will have a 25% chance of not getting the fair version of the MC1R gene from either parent. This child has a good chance of ending up darker than his or her parents.
Unfortunately coloring is too complicated to say that each of their kids has a 25% chance of being darker than them. This is because having a fair version of the MC1R gene doesn't always lead to lighter hair and/or skin. There are undoubtedly other genes that we don't know about that are involved too.
MC1R is Just One Gene
So far we've only talked in detail about one gene involved in human coloring -- MC1R. The reason we talked about this one is that it is fairly simple and we understand it pretty well. But there are lots of other genes too.
As we begin to figure out how they work, we'll be able to discover other ways for a child to end up darker than his or her parents. Or lighter for that matter.
So the bottom line is that yes, parents with lighter hair and skin can have a darker child. We know one way it can happen and scientists are working on figuring out the other ways.
By Dr. Barry Starr, Stanford University