In the distant future, does everyone have blond hair?

Answer

Brilliant reasoning, but it shows how easy it is in science to deceive yourself.

It’s true that two blond people will almost always have blond children, and a little bit of everything can happen with dark-haired parents – including a blond child. It is dangerous to deduce from this that more and more blond people are coming. After all, couples in which both partners are blonde are and remain a minority. If one of their blond children later marries a dark-haired one (they are in the majority, so the chances are high), there is a good chance that their children won’t have a blond…

In the end it is a lottery, in which everyone has two tickets: blond or dark. If one of your lots is “dark”, you yourself are dark. To be a blonde yourself, you must have already received two “blonde” tickets. Children each receive one ticket from mom and dad. Blonde parents, who had two blonde tickets themselves, can only pass one blonde ticket to their children. So they end up with two blond lots and are also blond themselves.

Dark-haired people can have two dark tickets, or one blond and one dark. In both cases they have dark hair. If both parents only have dark lots, all their children will inherit doubly dark as well. If the parents jointly offer three dark and one blond tickets, all their children will inherit at least one dark ticket, so they are all dark. One child in four has been given one blond ticket, which can pass it on to his children, even if it is dark itself.

If both parents have a dark and a blond ticket, and are therefore dark themselves, the family lottery ensures that one child in four inherits two dark tickets, two in four a dark and a blond ticket, and one child in four inherits two blond tickets. Only the last one looks blond.

Watch out, these are averages. A chance of one in four does not mean that in such a family with four children there must necessarily be a blond child. Just like you don’t necessarily have all the numbers on the dice after throwing six times, even though they all have a chance of one in six. But at the level of an entire population, the distribution will be correct.

This story is a simplification, with each parent inheriting only two tickets. In practice, there are probably dozens, all of which make the hair color a bit darker or blonder. But the basic logic remains: it’s enough to inherit a dark lot from one side to see its effect, while the blondes only show up when you get two of them. In the family lottery, the blondes are on the losing side.

What can help though is when blondes are more sexy, making it easier to find a mate and ultimately have more offspring than darker types. For example, every generation more and more blond lottery tickets are entered into the lottery. But in this age of cheap hair dye, we’re all as sexy as we want to be (in terms of hair color, anyway), and that effect no longer plays.