Is it true that only blonds have blue eyes?

Editor’s note (2/13/2021). Some information in this article is outdated:

1. It describes eye color and hair color as each being determined by a single gene. We now know that both of these are influenced by many different genes.
2. It cites the reason for blonde + blue traveling together is that the “two genes” are on the same chromosome. This is also mostly untrue. Four of the most important genes are located on entirely different chromosomes. For example KITLG is on Chr12, OCA2 is on Chr15, MC1R is on Chr16, and SLC24A4 is on Chr14.

It would be more accurate to say that some genes affect both traits. And that a person’s ancestry affects the chance of inheriting both traits, as described in this article.

No, it isn't true. My dad has black hair and blue eyes and so do lots of famous people like Gabriel Byrne, Lucy Lawless, and Hugh Grant. So what was the geneticist talking about?

He may have been talking about a couple of things. The first is that blue eyes and blonde hair both come from having less of a pigment called eumelanin. This is just something the two traits have in common -- they are not because of the same gene.

The genes for each of these traits determines where and how much of the pigment will be made, not if it will be made anywhere in the body. In other words, the hair color gene determines how much eumelanin will be in your hair and the eye color gene how much will be in the front of your eye.

The geneticist may have also been referring to the fact that the two traits usually do travel together. Black hair and blue eyes is a much more rare combination than is blonde hair and blue eyes.

The reason why these two traits are linked is that the genes responsible for hair and eye color happen to be close together on the same chromosomes. When genes like these are close together, the traits tend to end up coming in pairs (blonde hair/blue eyes, etc.). Why would it work that way?

To answer this question, we need to go back a step or two. As you might remember, we all have 46 chromosomes that come in pairs. Eggs and sperm each have 23 single chromosomes so that when they combine, you get 23 pairs again.

You might also remember that we all get half our DNA from our mom and half from our dad. A lot of people conclude from this that we each get 23 chromosomes from each parent that are exactly the same as one of the ones our parents had.

If this were the case, then of course we'd get the eye and hair color genes together if they're on the same chromosome -- our chromosomes would look exactly like one of our mom's and one of our dad's. For example, for chromosome 15, one would be exactly the same as one of mom's and the other would be exactly the same as one of dad's.

People often conclude that all the different people we see around us come from the random assignment of parent's chromosomes to their children. In fact, reality is a lot more complicated than this. We are all so different because the DNA on our chromosomes gets mixed.

What really happens is that before getting split up to go into a sperm or egg cell, the chromosomes of each pair swap DNA like mad (except for the Y chromosome in men which has no partner). This means that the chromosomes you get from your parents are really mixtures of both of their chromosomes. To use our example above, the chromosome 15 you get from your mom is really made up of parts of both of her chromosomes.

OK, OK, well and good but what does that have to do with anything? Well, the way the DNA gets mixed up is that big bunches all go at once. What this means is that the closer two genes are to one another, the more likely they will travel as a pair when DNA is swapped.

Why this is true is just a matter of statistics. For genes to get separated, one end of the DNA that is being swapped has to fall between the two genes; otherwise they would travel together on the piece of swapped DNA.

To make this point clearer, try this. On a piece of paper, draw two lines. On one line, put two little boxes right next to each other and on the other, put them at each end of the line like this:

Once you've drawn these, shut your eyes and put your pencil down on the paper at random. You should find that the points are almost always between the two faraway genes and rarely between the close genes.

This mimics what happens when two genes are linked. The occasional dark hair/blue-eyed person comes from one of these rare DNA swaps that take place between the genes. The swap either took place in one of your ancestors (as would be true for Gabriel Byrne's kids) or in one of your kids.

I hope this explanation was clearer than that of the other geneticist you were listening to!

Read More:

• Medline: Eye color genetics

• Medline: Hair color genetics