Bryan Sykes, professor of human genetics at Oxford, confesses that when he began this book he was influenced by Easy Rider, which he had seen again for the first time in years, and was drawn to the aimless wandering of its three male characters. Sykes, too, wanders about a huge terrain: nothing less than “a genetic portrait of America.” In the last third of DNA USA, he tells the story of a train trip from Boston to San Francisco and back, with side excursions by car, complete with Tocqueville–like observations on Yankee oddities such as slow trains, political advertising, and guns openly on sale.

Sykes, who owns the firm Oxford Ancestors and has much to say about the commercial side of genetic testing, carries little kits that allow him to take samples quickly. The size and diversity of the population of the United States rule out a systematic survey. Instead, he discusses samples taken from volunteers: at a meeting of the New England Historical Genealogy Society in Boston, for example, or in chance encounters in a San Francisco hotel lobby. He nearly wangles a sample from the father of Ray McDonald, the San Francisco 49er in town for a home game, as he was curious to know if they were related to the Scottish clan.

The serendipity belies the fact that Sykes is one of the world’s most eminent geneticists. In the 1980s, he and an Oxford colleague, Robert E. Hedges, developed methods for extracting DNA from fossilized bones, and Sykes’s 2001 bestseller, The Seven Daughters of Eve, zeroed in on seven women by tracking the DNA of mitochondria, which is passed down through mothers virtually unaltered except for a harmless mutation every few thousand years, and which becomes a telltale sign of genetic relationship. In Adam’s Curse: A Future Without Men (2004), he described the deterioration of the Y chromosome as “a graveyard of rotting genes.’’

This road trip begins—and ends—with the idea that the genes of Africa, Asia, and Europe converge in America. Sykes doesn’t offer a grand concept this time; but for patient readers, a multitude of ironies and surprises make the important point that genes rarely back up our assumptions about race or ethnic identity. Current genetic evidence (as Sykes describes it) seems far too unpredictable to serve a eugenicist agenda. What, for example, defines a black man as black? Sykes meets with an African-American Stanford psychiatrist who turns out to have a Y chromosome typical of Ashkenazi Jews, as well as other white genes that he suspects came from a long-ago rape. Sykes reports that a third of the dark-skinned male customers of an American gene-testing company turn out to carry a European Y chromosome. About half of these men are so angry they demand their money back.

They know the history—that white men had sex with black slaves and servants—but “they look black and they certainly feel black.” Sykes doesn’t say so, but many must think that white genes would show up in people with lighter skin. In fact, people who have very little DNA from African ancestors may be dark-skinned if that DNA happens to include the pigment genes. (On the other hand, people with fair skin may have almost entirely African DNA.) Typically, the company repeats the test and the results are the same. By then, the customers have “quieted down. Maybe they have asked their grandmother on their father’s side, and she may have said that there was talk of a white ancestor in the family long ago. Always ask a woman about these things,” Sykes observes. “Women always know more than the men.”

Sometimes not only pride, but money, is at stake. After tribal coffers filled with profits from casinos and reparations for historic land seizures, applications for Indian tribal membership mushroomed. Native-American nations make their own rules as to who qualifies: In 2000, the Seminole expelled its existing 2,000 “black” members, descendants of freed slaves who often married the children of their former masters. When 95 descendants of freedmen were tested, signs of Native-American ancestry ranging from zero to 30 percent, with an average of 6 percent, were found. To a hushed audience of freedmen, it was announced that these results matched the average of African Americans from Baltimore or New York. Many of the enrolled Seminoles, Sykes comments, may have even less genetic evidence of Native-American ancestry.

Indian tribes distrust genetic testing, in any case. They tell origin stories that begin on this continent, although scientists inform us that genetic and archaeological evidence indicates that the first Americans arrived from Asia. (As Sykes explains, every Native American tested has had some genetic code associated with the Chinese and Japanese.) Sykes concludes that mammoth hunters from Siberia arrived in Alaska by land or sea, and that another group of Asians traveled by boat to Central and South America. More recent genes come from Chinese immigrant railroad workers.

Such theories have made Native Americans deeply suspicious of white men who come wanting cheek swabs. When, after much debate, the Seaconke Wampanoag of Rhode Island did volunteer for genetic testing, the results astonished even Sykes. The tribe is famous for sharing Thanksgiving dinner in 1621 with the Mayflower settlers. But the tests showed that, at least on the maternal side, there was no evidence of Native American genes at all. Sykes leaves us with the mystery: “No race or ethnic group can ever be accurately defined by the genes they carry,” he states unambiguously, and the odd results with the Seaconke are a vivid illustration of that point.

Jews, who have their own 16-page chapter here, had every reason to fear eugenics, but in America they embraced genetic testing in order to fight Tay-Sachs disease, now virtually eliminated in this country. Sykes comments that while Native Americans have been on the “receiving end of gung-ho and invasive academic projects run by other people, Jewish-Americans have looked into their genetics themselves.” Such empathy is characteristic of Sykes’s style, and feeds his lyrical passages. Describing the train ride to Massachusetts, he writes:

Our driver was now applying the whistle with increasing vigor. At the approach to every crossing, no matter how small the road, the same melancholy blast rippled through the sleeping woods. Occasional homes interrupted the monotony of the trees, gray-slatted timber in plots with the abandoned swings of children long gone that parents didn’t have the heart to dismantle. On one lawn an old rusting jalopy had collapsed at its final resting place, the grass carefully mowed around it as if it were a grave. These were most definitely homes, with all the paraphernalia of the living, not the pristine empty house-tombs of Cape Cod.

The pace speeds and slows. We’re treated to textbook-style dashes into ancient history, details about the technology of genetic testing, personal snapshots of sea lions and General Custer’s tombstone, and technical explanations that seem written in genetic code. In its meandering and scattered way, DNA USA reflects its subject. Genetic code is messy indeed, but not too messy to teach, as we make our way in a “world that mocks the artificial divisions we have created for ourselves.”

Temma Ehrenfeld is a writer in New York.

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