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Genes and Geography

Genes and Geography Photo Credit: NASA

Our early human ancestors began migrating across the globe tens of thousands of years ago. Some left behind archaeological evidence of their travels. But as you'll hear in this Science Update, another record of where we come from and where we've been might be found right in our DNA.


Transcript

Genes and geography. I'm Bob Hirshon and this is Science Update.

People around the world might look different from one another, but inside, we're pretty similar—and that's true even of our genes. That's according to a recent study in the journal Science.

Noah Rosenberg is a research associate at the University of Southern California. Using a computer program, he and his colleagues analyzed the genetic profiles of more than a thousand people from 52 places around the world.

Rosenberg:

One thing we found was that the amount of variation across populations was smaller than we had originally expected and smaller than had been found previously. So the vast majority of the sites in the genome are identical across all populations.

Nevertheless, those sites could be used to predict that person's ancestry solely based on their DNA. That's because certain combinations of genetic types were more common in some regions than in others, and the computer program was powerful enough to tease those out.

Rosenberg:

This is helpful towards trying to figure out the relationships between different populations and the patterns of human migration.

So combined with evidence from fields such as archaeology and linguistics, genetics can help scientists understand human history. For the American Association for the Advancement of Science, I'm Bob Hirshon.


Making Sense of the Research

Although humans throughout history have made a really big deal over differences in populations, whether those differences are based on nationality, ethnicity, or skin color, the fact remains that we're all pretty similar. If you compare any two people to each other—an Eskimo and a North African, a French woman and a Chinese man—you'll find that 99.9% of their DNA is identical. In other words, everything that makes you unique is concentrated in less than one one-thousandth of your genes.

What's more, even within that tiny fraction of DNA that varies between people, the differences between populations aren't as dramatic as the researchers expected. In fact, the overwhelming majority of genetic differences between individuals are just as variable within small populations as they are across the entire world. Comparatively speaking, only a small handful of genetic signatures are more common in some human populations than in others.

Nevetheless, the researchers were able to use these tiny slivers of our genetic code to predict where people came from. They accomplished this by using a powerful computer program that analyzed hundreds of genetic signatures at once. By looking for patterns of "microsatellites"—short strings of DNA that are passed down from generation to generation—the researchers were able to make accurate statistical guesses about people's ancestries.

So what good is this information? Well, for one thing, it could help out archaeologists and anthropologists who study the history of human migrations around the globe. But there's another, more practical use for it. For years, some doctors have been asking people about their ancestries in order to determine if they're genetically pre-disposed to certain diseases. But other doctors have argued that the question is useless because the idea of "ancestry" has no real genetic meaning.

This study suggests that in some cases, where your ancestors came from may in fact have something to do with the kinds of genes you might be carrying. And that knowledge may not only help physicians assess an individual patient's risk for a disease, but also help epidemiologists (scientists who study diseases in populations) understand patterns of disease around the world.

Now try and answer these questions:

  1. About how similar are human beings genetically?
  2. What is the difference between genes that simply vary from person to person, and genes that are distinctive of populations? Are distinctive genes shared by all members of a population?
  3. What factors allowed the researchers to analyze such subtle variations in human genetics?
  4. Can you think of specific situations in which this knowledge may be used? Give hypothetical examples for the following situations:
    • A doctor-patient relationship
    • An anthropologist studying the history of a population
    • An epidemiologist studying a rare genetic illness

For Educators

The Human Genome Project was a joint effort of the Department of Energy and the National Institutes of Health, which identified all 20,000-25,000 genes in human DNA.

The Human Genome Project's page on Genes, Variation and Human History includes two classroom activities on this topic.

The National Institute of General Medical Science has this Human Genetic Variation Fact Sheet about genes and human populations.


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