DNA Basics

Does DNA account for only physical attributes of a human being, or can it also carry an emotional history from one generation to the next? I am wondering if the psychological damage of slavery for African Americans, for example, is carried forward into current generations on not only a psychological basis, but also a physical one.

-A curious adult from Washington

August 3, 2004

My first inclination was to answer no but then I got to thinking...

The reason my gut reaction answer was no is because there was a big debate early on in genetics about traits and how they are passed on.

One school thought that things we do and things done to us can be passed on. The other school argued that it is all about selecting the most fit individuals in a population who then pass their traits on.

One of the best examples I've heard to distinguish these two is to see how they would explain a giraffe. A giraffe's ancestors didn't have long necks but now they do. How did this happen?

One idea was that as food became scarce, the giraffe's ancestors had to stretch their necks to reach food. They then passed on their stretched necks to their kids, who did the same until finally you get a giraffe.

The other idea is that as food became scarce, the animals with longer necks did better and they bred while their short-necked brethren starved. After awhile, only giraffes that passed on their long neck genes survived.

Of the two models, the second one is the one that explains almost everything in genetics. The first one was discredited early on (although the Soviet Union tried to use these ideas to improve agriculture with disastrous results).

Notice I said that the second one explains almost everything in genetics. There are examples where two animals can have the exact same A, G, C, and T's but look different. A recent example is from a mouse.

In this mouse, the babies were either blonde or brown depending on what mom ate while they were inside of her. The brown mice were also less likely to suffer from obesity, diabetes, and cancer than their blonde brothers and sisters.

How did diet affect these mice so profoundly? The different food didn't change the sequence of their DNA -- as I said above, all the letters stayed the same. A different part of the DNA was changed, however.

What happened was the DNA was changed so that a gene was permanently turned off. Remember, you can think of DNA as a collection of recipes. Each recipe in the collection is a gene. What the different diet did was make at least one gene, agouti, unreadable so that the mice were brown instead of blonde.

There are human examples too. In a Swedish study, researchers found that men who had lots of food as adolescents were more likely to father diabetic kids than brothers who had little to eat as adolescents. In another example, babies born to mothers who were depressed during pregnancy were cranky, irritable, and had trouble sleeping. When researchers did tests they found that the babies had brainwave patterns and blood of a depressed adult.

So, while it is possible for a parent's experiences to affect their children down to the level of DNA, I'm not sure your example would be true. First, all of the former slaves' DNA is different. While they share genes for dark skin, etc., former slaves can have less in common genetically with each other than their former masters; race is mostly a cultural construct. (See link below for a more complete answer on this topic.)

Also, there was no single "slave experience." Each slave lived under very different conditions. Taken together, these two facts probably make it unlikely that there is some sort of genetic memory of slavery in all of African Americans.

By Dr. Barry Starr, Stanford University