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Can DNA change in a human lifetime in his own body due to artificial causes? Can a human inherit a different DNA to his/her children? Actually I was talking to my friends about the tribe in Thailand that stretches their necks with rings. I was wondering if they stop putting rings around the women's necks, is it possible that a child will show any residue of this behavior (like being born with a slightly longer neck, etc.) Note: will it matter if the ring thing was not only 50-70 years tradition but say if it was 300 years or 1000 years tradition?

-A curious adult from Israel

March 24, 2006

This is a great question that gets to the heart of genetics. Can what we do affect our DNA in a predictable way?

The quick answer is that behaviors like you describe most likely won't directly affect the DNA of these women or their children. But members of this tribe may end up with more stretchable necks anyway.

Our DNA determines how long or stretchable our necks are. Some people have differences in their DNA that make for a more elastic neck*.

Imagine a member of this tribe for whom the rings don't work very well. Stretching her neck will not change her DNA to look more like someone's with a stretchable neck. Nor will it change her DNA to look like someone's with a longer neck.

Because this is the case, she won't pass down her stretched neck to her kids. But why over time might the tribe end up with more stretchable necks?

This is where natural selection comes in. People with a difference that helps with survival, have more kids. Those with differences that make survival less likely, have fewer kids.

Over time, you end up with a population that has the survival trait. It is important to note that natural selection uses differences that are already there.

Let's see how this might work in your example. Let's say women in the tribe who have the longest necks are the most desirable. This means that they are more likely to have kids.

As women with more elastic necks continue to have more children, this trait becomes more and more common. Soon, the tribe has more stretchable necks than other groups of people.

It looks like the stretching of the neck caused their DNA to change. But what really happened is women with a stretchable neck passed their genes on more successfully than their less stretchable sisters.

I don't know if members of this tribe do have more elastic necks. But one species of animal did end up with a longer neck -- the giraffe.

As food became scarce, long-necked giraffes were able to get leaves from higher up in the trees. These giraffes went on to have calves while the shorter necked giraffes starved. After awhile, you end up with longer necked giraffes.

While this natural selection is going on, it is possible that some giraffe somewhere got a DNA mutation that led to a longer neck. This giraffe would have been even more successful and passed it on, leading to even longer necked giraffes.

This mutation wasn't the result of the need for a long neck. It was a random DNA change that proved useful. Changes to our DNA are happening all the time.

Those changes that are helpful stay in the population. Neutral ones stay or disappear by random chance. And harmful changes become rare.

The giraffe needed a long neck to survive and breed. For the tribe we are talking about, the long neck is needed only to have more kids; it isn't critical for survival.

But this is an important part of natural selection too called sexual selection. Think about a peacock's extravagant plumage. Or moose antlers. Or, maybe, Northern Europeans' blue eyes.

No one can really come up with an obvious benefit for blue eyes. They became possible in a wider population as a side effect of people's lighter skin.

What if women with blue eyes were seen as more desirable 10,000 years ago? These women would have more kids and, over time, would start to dominate the population. Even though blue eyes are recessive!

As you can see, though, actions do not cause specific DNA changes. Needing to walk upright did not change our ancestors' DNA directly. Those who could already walk upright better, survived. Those who couldn't, died.

Of course we are on the brink of being able to change our DNA in a directed way by mucking with our DNA. This isn't the same thing as you asked about but it could have the same result.

Imagine a world where long necks are seen as desirable. And we know the gene changes that can lead to it. And we can change our DNA. My guess is that there will be a lot of long-necked folks running around...

*This hasn't been studied but it is almost certainly true.

By Dr. Barry Starr, Stanford University

Stretching did not create a giraffe's long neck

Blue eyes may have arisen due to sexual selection