Other Genetic Principles

What are the traits called when they skip 2 generations and appear on the 2nd? Like for example my grandparents have it and it skips my parents and comes up in me. What's that called? Is there such a thing as a dead trait that only appears once and is gone forever...

-A middle school student from Israel

February 20, 2009

We've all seen people who look more like one of their grandparents than either of their parents. Sometimes it is because the grandparent and the grandchild both share the same color eyes or hair. Or maybe they share a cleft chin.

These kinds of traits can skip generations for different reasons. Which is why there is no single name for traits like these.

For example, red hair and blue eyes can both skip a generation because they are recessive. A cleft chin can skip generations because of something called incomplete dominance.

We've dealt with both of these pretty completely in the past so I won't go over them here. (Click here for why recessive traits can skip a generation and here for why incomplete dominance can cause a trait to skip a generation.) What I'll tackle instead is the dead trait question.

A trait can definitely be around and then disappear. It can disappear because of chance (genetic drift). Or it can be lost because the environment has changed so that it is now a disadvantage for humans to have (natural selection). Either way we need to dig a bit deeper into how traits happen to understand how traits can go missing.

Different Versions of Genes can Cause Traits

Traits happen because of genes. So it makes sense that different traits happen because of different genes. What you may not know is that different versions of the same genes can cause different traits too. It is this last kind that can go missing so I'll focus on them.

Let's look at my favorite trait, red hair. The gene involved in red hair is called MC1R. There are lots of different versions of this gene that we can put into two groups -- red and not red.

Remember that we have two copies of most of our genes -- one from mom and one from dad. So people can either have two red MC1R versions, two not-red versions, or one of each. Only people with two red versions will end up with red hair.

Of course all of this means that mom and dad have two copies of all of their genes too. Which copy they pass down to their kids is totally random. It is like flipping a coin -- heads their child gets one copy, tails the other.

This is all the information we need to understand how traits might disappear.

Gene Versions Can Disappear

Imagine that a man and a woman meet. He is a redhead and she is not. A quick look at their genes shows that he has two red versions of the MC1R gene and she has two not-red versions.

OK, it is starting to get old writing "red version of the MC1R gene" all the time. So instead I'll do what geneticists do. I'll call the not-red version "R" and the red version "r." (Not red gets to be capitalized because it is dominant -- it wins when someone has a red and a not-red version.)

Back to our story. As you can see below, the red haired man is rr (both MC1R gene copies are the red version) and the woman is RR (she could also be Rr which is why we had to look at her genes).

When they have kids, they will each pass one copy of their MC1R genes to each child. Remember, which copy they pass is totally random.

Of course in this case that doesn't matter. Since she has two R's and he has two r's, each child ends up with one of each. Each child is Rr. They each do not have red hair but carry the red version of the MC1R gene*. Imagine the couple has two kids like this:

When the kids grow up, they each marry someone who does not have the red version of the MC1R gene. In other words, the spouses are both RR so can only pass an R to the kids.

But the Rr spouses can pass either an R or an r and which gene version gets passed is totally random. This means that each child has a 50-50 shot at getting an R and a 50-50 shot at getting an r. This also means that none of these kids could end up with an r. Here is a possible way for the genes to be passed down:

Again, think of this as flipping a coin. If you flip a coin three times and get heads each time, what are the odds that the next flip will be tails? Still 50-50. What happened before does not affect the next flip.

Same thing with passing down genes. Just because the first three kids didn't get an r, that doesn't mean the 4th one is more likely to. The 4th child still has a 50-50 shot of getting R.

In fact the odds of all four kids being RR are around 1 in 16. About the same as all four kids ending up boys. So, if none of the kids gets an r, then the red hair version of the MC1R gene would die out in this family.

Of course, they could just marry a redhead and it would be back in the family. But there could be situations where the red hair version would pass away forever.

Bye Bye Trait

There had to be a first redhead at some point. In other words, in our distant past, someone had a DNA change in his or her MC1R gene so it became a red hair version. What if that person died before passing on his or her new mutation? Or what if they passed it on but all the kids they passed it onto died?

Another way a gene version can be lost is if the whole species dies out. Scientists have found that at least some Neanderthals had red hair. But their red hair version of the MC1R gene was different than any of the ones that are around right now. So that gene version was lost when we wiped out the Neanderthals.

A less severe example of this is when a small group goes off to colonize a new place. If they didn't bring the red hair version with them, it would be lost if everyone else in the world died.

I don't know of any examples like this but there are groups of people that were founded by a small group that are missing some gene versions. For example, there are some Amish communities that no longer have the version of the CFTR gene that leads to cystic fibrosis.

Humans have over the years undoubtedly lost many gene versions. A whole lot of them probably went missing about 75,000 years ago. Scientists think this was when most humans were wiped out.

As long as no new catastrophe wipes most of us out, we'll probably get some of these versions back through random DNA change. I wonder what humans would look like today if we would have kept all of those different gene versions.

*Note that there are no redheads in this generation. But if either child marries another Rr, then they could have a red haired child. The red hair skipped a generation.

By Dr. Barry Starr, Stanford University

More Information

Different traits can come from different versions of the same genes.

This is what's left of a volcano that nearly wiped out humans 75,000 years ago.