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Other Genetic Principles

I have heard that alcoholism skips a generation. Is this true? And if it's true, why do they skip a generation?

-A curious adult from California

May 12, 2006

Certainly some traits like alcoholism can skip a generation. But they don't have to. As you'll see, whether or not a trait skips a generation depends on both parents' genes.

Let's look at an easier example than alcoholism -- red hair. In my wife's family, having red hair seems to have skipped her generation.

My mother in law and my son both have red hair. But neither my wife nor any of her siblings are redheads. So how did red hair disappear and then reappear a generation later?

The reason it skipped is because of my father in law's genes. If my mother in law had married a different man, she might have had some or all red haired kids. And if my wife had married a different man, she would not have had any redheads.

We'll use my family as an example to figure out how all of this works. We can get down to the actual DNA changes involved because of some testing my family recently had done. But before launching into this, we need to get down to the nitty gritty of how red hair works.

A common way to get red hair is if both copies of a gene called MC1R aren't working properly. (Remember, we have two copies of most of our genes, one from mom and one from dad.)

A gene is a set of instructions for making a protein. The MC1R gene makes the MC1R protein.

One of the things the MC1R protein does is converts red pigment to brown pigment. So if someone doesn't have any working MC1R, they have a build up of red pigment and end up with red hair.

Why would a gene not work? By having mistakes or DNA changes that affect its instructions.

A gene is written in a language that has only four letters -- A, C, G, and T. And all the words are only 3 letters long. It has always been amazing to me that all of the complexity around us can come from such a simple code!

For a gene to work properly, the "words" need to form the right instructions. If one of the 3 letter words doesn't make sense anymore, then you may get the wrong instructions and end up with a protein that doesn't work.

And if that gene is MC1R, then you can get red hair (as long as both copies have the wrong instructions). So how does this help explain how a trait can skip a generation?

Well, to have red hair, both copies of the MC1R gene need not to work*. If only one copy is broken, then the working copy can still change most or all of the red pigment to brown.

In other words, people with one working MC1R gene do not have red hair. But they'll pass the other broken copy down to their kids half the time (they are carriers).

So here is how red hair skipped a generation in my family. My mother in law has two copies of the MC1R gene that don't work (which explains her red hair). My father in law has two copies that do work.

The only possible result here is kids with one working copy and one broken copy of the MC1R gene. (They got one nonworking copy from mom and a working one from dad.) Out of nine brothers and sisters, only one had a redheaded child. Why? Because that one married a carrier like herself.

Now, what is cool is we can now trace exactly why our son is a redhead. Recently my family figured out all of the letters of our MC1R genes. What we found was that my wife and I both had a single letter difference at letter number 452 in one copy of our gene.

This change caused one of the three letter words to change from GCT to CCT. And my son got this nonworking MC1R gene from my wife and myself. So he has two nonworking copies of the MC1R gene which gave him red hair.

It is again amazing that two different letters have such a dramatic effect on my son's hair color. And that because I only have one, I don't have red hair.

So this is one way for a trait to skip a generation. When a redhead marries someone with two functioning copies of the MC1R gene, they have carriers for kids. These kids will not have red hair but can pass it down to their kids.

But unless the spouse brings a red hair gene to the party as well, then there won't be any redheads in that generation either. This may be what happened in my wife's family.

Of course, there are other ways for traits to skip generations too. For example, when two carriers have kids, the chances for a redhead are only 1 in 4 anyway. The carriers may not end up with a redhead just because of statistics. You can also click here for another more complicated way that traits like cleft chin can skip generations.

And alcoholism is even more complicated than these. As we talk about here, alcoholism involves lots of genes and environmental factors too. Yikes! But the same sorts of ideas apply.

Traits do not have to skip generations. Both parents' genes affect whether or not a trait will be seen in a child.

Imagine my mother in law had married a redhead herself. Then most likely all of her kids would have had red hair. In other words, red hair would not have skipped a generation.

*Red hair is called a recessive trait -- you need two copies of a certain version of the gene to end up with red hair.

By Dr. Barry Starr, Stanford University

Red hair can sometimes skip a generation.

Genes are written in a language with 4 letters.