-A curious adult from Maryland
March 12, 2009
This story made quite a splash in the news awhile back. There are lots of reasons why you might think something like this is unlikely.
For example, red hair isn't that common -- only 1-2% of people in the world have it. And the odds of having two sets of twins are something like 1 in 10,000.
But if the dad in this case carries the red hair gene, then the odds of having two redheads and two dark haired kids aren't at all unlikely. In fact they are around 37.5%.
And if we ignore the odds of having two sets of twins and just focus on hair color, then the odds of the particular mix of kids they had are something like 25%. About the same as getting two heads in a row on a coin flip. To see why, let's talk a little about what causes different hair color in the first place.
Hair Color and our Genes
We get our hair color from special pigments called melanins. You can think of these pigments as dyes that color our hair.
People with black or brown hair have more of a dark melanin called eumelanin. People with red hair don't have much eumelanin. Instead, they have more of another dye called pheomelanin.
There are many genes that determine how much of each pigment our cells make. A key gene in this process is called MC1R.
One of the things the MC1R gene is responsible for is turning pheomelanin into eumelanin. Some people have versions of MC1R that are not very good at doing this. If both of someone's MC1R genes are like this, then that person will have a build up of pheomelanin and so have red hair.
Remember, we have two copies of each of our genes -- one from mom and one from dad. So to get red hair, a person needs to get the red hair version of the MC1R gene from each parent. Which is what almost certainly happened in the case you're asking about.
The Genetics of Red Hair
Since people have two copies of each of their genes, this means each of us has three possible combinations for MC1R. A person might have two copies of the not-red version, two copies of the red version, or one copy of each.
Now, it's pretty obvious what happens if you have two not-red versions -- you have a hair color other than red. And it's obvious what happens with two red versions -- you have red hair. But what if you have one copy of each?
In the case of MC1R, it means you don't have red hair. This is because the not-red version is dominant over the red version.
If a gene version is dominant, then it will always be the one that we see (it "hides" the recessive version). So this means that if you have one copy of the not-red and one copy of the red version of MC1R, you'll always have a hair color other than red.
This is why you need to have two copies of the red hair MC1R to end up with red hair -- because it's recessive.
How Could this Couple Have 2 Sets of Twins with Different Hair Colors?
The mother of the twins you mentioned above has red hair, which means that she has two copies of the red MC1R gene version. So it would make sense that her children could be red haired too.
But the father is dark-haired. So wouldn't all of his kids also be dark-haired, since not-red hair is dominant over red hair? Not necessarily.
Remember we said that even if you have one copy of the not-red MC1R version and one copy of the red one, you'll always have a hair color other than red? In genetics, a person who has only one recessive copy of a gene is called a carrier.
This would have to be the case in the couple you mentioned. For a dark-haired man to have red-haired children, he would have to be a carrier for the recessive red MC1R gene version. Let's see why.
First off we'll call the recessive, red hair copy of MC1R, r, and the dominant, not-red copy of MC1R, R. This is something geneticists do to make explaining all of this much simpler.
So mom has two copies of r and so is rr (she has to have 2 recessive copies to get her red hair). The father is dark haired, so he has at least one dominant copy. So he is either RR or Rr. Both of these would end up in him having a hair color other than red.
In this case, we know that the dad must carry a recessive MC1R gene version since he had red haired kids. So we know he is Rr.
Now we're going to use a genetics tool called a Punnett square to calculate the possible combinations of children this couple can have. Each parent contributes one of their two copies of the MC1R gene to their kids. The copy they pass is totally random.
So to figure out what the possible combinations are, let's make a Punnett square. In our Punnett square, let's put mom's two MC1R copies on the left side and dad's two on the right like this:
Now we fill in the boxes to give each possible combination. The first box looks like this:
Let's fill in the rest:
As we can see, the kids can end up having rr, which means they will have red hair or they can end up as Rr -- dark-haired!
How likely is this?
A Punnett square is useful because you can also figure out how likely something is. From our Punnett square, you can see that each child has a 50% chance of having red hair (rr), and a 50% chance of not having red hair (Rr). So it's not surprising that this couple had at least one child with red hair! In fact, this couple had around a 94% chance of at least 1 of their 4 kids being a redhead.
Let's crunch a few more interesting numbers. Say this couple just had one set of twins. What's the chance of them both having red hair? Since each kid has a 50% chance of having red hair that would be:
0.5 x 0.5 = 0.25, or 25%
This is the same chance as both kids NOT having red hair.
And the chance that one of them would have red hair and the other one dark? Well, the first kid is going to have either red or dark hair, and then the second has a 50% chance of being whatever the first one isn't:
1 x 0.5 = 0.5
So the twins have a 50% chance of ending up the mix they did.
But we know that our couple had 2 sets of twins. This set has the same odds (50%) of being one red and one dark-haired. So the chances of having TWO sets of twins with this combination is:
0.5 x 0.5 = 0.25
So to answer your question, this combination of children isn't so rare! In fact, having two redheads is the most likely outcome for these parents.
The couple even had a reasonably good shot of having ALL of their children be red-haired! If each twin set has a 25% probability of being both red-haired, that's:
0.25 x 0.25 = 0.0625, or 6.25%
Of course I have only focused on red hair so far. I haven't factored in that they had two sets of twins nor any of their other traits. Each trait would need to undergo a similar kind of analysis to figure out how likely each child was to get a certain trait. This number is bound to be higher.
I saw was that the odds of these parents having a second set of twins similar to the first was 1 in 500,000. These odds include the chances of having a second set of twins.