Ask a Geneticist

by Dr. Barry Starr, Stanford University

Can blue-eyed parents have a baby with brown eyes? Or green eyes? Can two parents without a cleft chin have a child with one? Can two curly-haired parents have a child with straight hair?

-Curious children and adults from around the world

The answer to all these questions is yes. They are exceptions to the rule but do happen often enough that we get these questions a lot.

Life is not as simple as it is painted in school and on TV. Neither are inheritance and genetics.

To make things easier to understand, schools and especially TV tend to oversimplify genes and genetics. This is fine until people start believing that this is the way genetics really works.

Then, when two blue-eyed parents have a brown-eyed child, they sometimes begin to suspect things they don't need to. What is important for these two parents to realize is that the genetics they have been taught is too simple to explain everything.

We'll talk about two ways these things can come about. They both deal with the fact that genes can turn on and off between parents and children.

Genes can and do change. Genetics is complicated by the fact that genes don't always stay the same. What we learned in biology class would be true if genes were set in stone. However, genes can and do change from generation to generation (and even within the same person).

But how can one gene turn into another gene? For example, how can a blue eye gene turn into a brown eye gene?

A small change can turn a
brown eye into a blue eye

They can because genes for things like blue and brown eyes are very, very similar. In fact, they are really just different versions of the same gene.

Remember, genes are written in a code made up of 4 letters, A, G, C, and T. Changing one letter can be the difference between a blue and a brown eye version of a gene. So, to turn a blue eye gene into a brown eye gene, you may only need to change a single letter.

How can this happen? Lots of ways. We'll only talk briefly about two.

A letter can get changed when our DNA gets copied. As we grow, develop, and maintain ourselves, our DNA gets copied over and over and over. (Remember, for example, we all started out as a single cell and now are made up of 50 or 100 trillion cells.)

Our DNA copying machinery is nearly perfect but it still will make an occasional mistake. If that mistake happens in sperm or egg cells, it will get passed on. And if the change is in the right place in the blue eye gene, blue-eyed parents can now have a brown-eyed child.

Another way genes can change is something that happens when our sperm and eggs get made. Remember, we have two copies of each of our chromosomes (except males who have a single X and a single Y chromosome).

Just before the sperm and eggs get made, some of the DNA between the two copies gets swapped. This process, called recombination, means that your DNA is different from your mom and dad's...it is a mix of the two.

And sometimes this mixing up fixes or breaks a gene. We go into more detail about this at http://www.thetech.org/genetics/ask.php?id=29.

Genes need to be read to work. What all of this points to is that genes need to work to have their effect. A gene is really just a set of instructions. When a gene is on, that means its instructions are being read in the cell.

Sometimes a gene can be read in one person but is unreadable in another. What happens if a gene is unreadable in a parent but a child's cells can read it? That's right, a blue-eyed parent can have a brown-eyed child.

But how would this work? By changes to the DNA that don't involve the A, G, C, or T's. These changes essentially shut off a gene in one person but the gene can turn back on when passed to another person.

Let's give a couple of ways this might happen.

Believe it or not, sometimes what your mom eats while she is pregnant can affect your hair color. Well, if you're a mouse, anyway...

Scientists did an experiment where they fed a mouse one food and her pups were black. A different food resulted in white pups. And all of the A, G, C, or T's were the same between the pups. What happened?

The food ended up attaching little chemical groups called methyls to the DNA. These methyls made the gene unreadable. So even though genetics would predict the same color pups, the environment changed the outcome.

Sometimes one gene
isn't enough

What's another way for a gene to become unreadable? Sometimes you need more than one gene to see an effect. A cleft chin is the classic example here.

You can inherit a working copy of the cleft chin gene and still not have a cleft chin. For the cell to be able to read the gene, a second gene is needed.

Scientists don't know what this particular second gene does but I can give an example of how it might work. For a gene to be read by a cell, the cell has to be able to recognize where that gene starts.

All our genes are arranged on a chromosome sort of like recipes in a cookbook. To make spaghetti, you couldn't start in the middle of the souffle recipe that came before it in the book. You would make an awful mess instead of the spaghetti.

The same is true for a gene. You can't start at the wrong place or you end up with a mess too.

We know where the beginning of the recipe is because of punctuation. It starts on a new page, with a title and a capital letter at the start. Genes have punctuation too.

Not titles and capital letters, of course. But the start of a gene is marked. An important part of that punctuation is proteins that sit on the DNA at the beginning of the gene. These proteins act like the title telling the cell where the start of a new gene is.

So imagine this. You have a perfectly good cleft chin gene but the gene that makes the punctuation protein is broken. So the cell has no way to make the cleft chin protein because it can't tell where to start.

Now imagine this unclefted parent has a child with someone with a broken cleft chin gene but a working punctuation gene. A possible result would be a child with both working genes and a cleft chin!

Well, I've probably gone on long enough. But I've really only begun to scratch the surface of genetics. Bottom line is that genes are way more complicated than we are taught.

Genetics can be thought of as telling us what is most likely, not what will or will not happen. So two blue-eyed parents will most likely have blue-eyed kids, two parents without a cleft chin will most likely have a kid without a cleft chin, etc.

Then again, they may not.

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