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James Watson's 16 PercentWhat does it mean to say that he's one-sixth African?

By Chris Wilson
Posted Friday, Dec. 14, 2007, at 6:08 PM ET
James Watson. Click image to expand.

James Watson, the co-discoverer of DNA who claimed in October that black Africans are less intelligent than whites, is himself 16 percent African. That claim comes from an Icelandic genomics company, which analyzed Watson's genome (publicly available online). But aren't we all of African descent? How can you tell that someone is exactly 16 percent of one origin?

Check his code for single nucleotide polymorphisms. Watson's genome consists of 6 billion nucleotides, the DNA building blocks. The vast majority of this sequence—99.5 percent—is identical from person to person, no matter who you are or where your ancestors came from. But every now and then, a single nucleotide will vary. These differences—called SNPs—are peppered throughout our genome, and result from the subtle evolution that took place as different populations of humans stumbled out of Africa. In most cases, SNPs fall in the vast stretches of DNA that do not code for traits, though some are associated with physical appearance. By scanning a representative sample of the SNPs in a given genome, researchers can begin to piece together the different fractions of a person's ancestry.

Researchers have identified more than 3 million SNPs, often arranged in groups (known as "haplotypes") of those that happen to be close to one another in the genome sequence. Many of these groups tend to show up in people from a particular place; for example, a certain pattern in a certain location might be most commonly observed in African populations. By scanning hundreds of thousands of SNPs, analysts build a model for the most likely heritage of a person based on which populations have genomes most similar to his. In Watson's case, 16 percent of his genome fits the statistical model of a person of modern African descent. From that, we can conclude about one-sixth of his ancestors came from Africa within the last few hundred generations or so.

Such tests are surrounded in controversy. These ancestral percentages are easily confused with cultural definitions of race and ethnicity. (For example, "African ancestry" does not equal "black.") The accuracy of the tests is also debatable. Some critics charge that the sample populations used to determine which SNPs are associated with which ancestries are too narrow. Sequencing technology continues to improve, but even a small percentage of mistakes can lead to a large number of misread bases when scanning tens of billions of nucleotides. (A genome must be read several times for accuracy.) The company that did the sequencing claims that each base was read an average of 7.4 times, but Kari Stefansson, whose company assessed Watson's heritage, says he found enough errors in the public genome to have doubts about whether the 16 percent figure will hold up. For example, he says there are places where it appears that Watson has two X chromosomes, which would make him a woman.

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Explainer thanks Lisa Brooks of the National Institutes of Health, Michael Egholm of 454 Life Sciences, Arthur Holden of the SNP Consortium, and Kari Stefansson of deCODE.

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Chris Wilson is an editorial assistant at Slate in Washington, D.C.
Photograph of James Watson by Gordon P. Hemsley.
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Remarks from the Fray:

Watson is not a "co-discoverer of DNA". Watson and Crick presented the first accurate model for the structure of DNA. This was an incredibly important contribution. I don't say this to take any credit from Watson, despite the insane and idiotic nature of the things that have lately come out of his mouth. I'm just making a correction to the article.

If anyone discovered DNA, it was Friedrich Miescher in 1869, almost one hundred years before Watson and Crick's groundbreaking paper. The significance of DNA was not appreciated for quite some time, but the narrative is one that I hope any student of molecular biology could accurately recount. The experiments that proved that DNA is the material basis of heredity were performed by several groups that did not include Watson or Crick and preceded their paper on the structure by several years.

So while Watson and Crick's work easily represents one of the most fundamentally important contributions to biology ever, they did not "discover" DNA.

--bluetexan

(To reply, click here.)

The article states that he appears (in some places) to have two X chromosomes; and that if this were true, he would be a woman. That's not strictly true: it's possible to have two X chromosomes and one Y, a condition known as being XXY or having Kleinefelter's Syndrome.

Individuals with Kleinefelter's Syndrome appear male, and the condition may go unnoticed. Most are sterile, but not all.

I'm not saying Watson is XXY, mind you. I'm just saying that with roughly 1 in 500 males having an extra X chromosome (according to NIH), then this is not in itself a reason to discard the data. It's similar to the fact that he has two sons: that strongly suggests that he was fertile, but it only proves that his wife was.

--rentedflowers

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