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Elusive Genetics


Lynn Jorde is a professor in the human genetics department at the University of Utah School of Medicine. GT's Jeanene Swanson caught up with him to discuss the use of genetic markers to define race and whether genetic tests can accurately correlate disease with ancestry.

Genome Technology: What are genetic markers of race?

LJ: There probably aren't any real genetic markers of race. If we look at a lot of markers, a lot of polymorphisms, we can learn something about the ancestries of individuals — in other words, we can learn something about the geographic origins of someone's genes. But oftentimes, those origins are complex. In an individual, some genes may originate from Africa, some from Asia, some from Europe. In that way, genetics tells us that traditional concepts of race are really too simplistic. Terms like African-American or Hispanic [are] misleading when we try to then use those terms to understand someone's predisposition to disease or even their genetic makeup. There's a lot of room for variation within those categories, [and] there's a lot of overlap among the categories.

GT: What types of genomic tools help you find these markers?

LJ: There are several microarray platforms that people are using in these kinds of analyses. SNP chips give you a lot of information for just a few hundred dollars. People are looking at population differences in gene expression; they are using expression arrays and asking, do we see differences in expression patterns of a population? To some extent [we] do. So it's not just a difference at the DNA level, but also a difference in terms of expressed RNA. That's interesting because you're getting closer to functional differences.

As we're looking at different populations what we see is differences in frequencies of these polymorphisms. But there are very few of these polymorphisms that are completely absent in one population and always present in another. In that way, there are probably few, if any, absolute markers of population difference. That is because of two things: one is that humans derived from a small population not so long ago — 100,000 years or so — so we haven't had a lot of time to accumulate differences; and we've moved around and exchanged DNA a lot.

GT: How can these markers be applied clinically?

LJ: We can take collections of individuals, and ask the question, which people tend to be more similar to each other, which people tend to be more different from each other? And there are a number of ways of doing that now with these large data sets of half a million SNPs or a million SNPs.

What you find is, not so surprisingly, individuals that are from the same part of the world tend to look more similar to each other, individuals from different parts of the world tend to look different from each other. It requires hundreds or thousands of these DNA variants to be able to figure this out. If you just used a few, you wouldn't see much of a pattern. That's because most of the variation is so extensively shared among populations.

Sometimes people want to use population affiliation as an indication of whether someone is going to have a particular disease-causing gene, but it's not a very good predictor. There is some information there; we know, for example, that sickle cell disease is more common in people of African descent than in people of northern European or eastern Asian descent — that's a fact. But you still see sickle cell disease occasionally in Europeans. So just knowing someone's population affiliation doesn't necessarily predict particular disease genes.

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