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Those in search of certifying their identity or behavioral quirks using DNA tests may find disappointment as much as illumination.
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I was bemused by the headlined stories over the past few weeks touting new genetic genealogy services, including the start-up, 23andME, launched by the wife of the founder of Google, and AfricanDNA, the brainchild of Harvard scholar Henry Louis Gates, Jr. According to 23andME, its service will "shed new light on your distant ancestors, your close family and most of all, yourself."

But what do these services really offer? I have more than a passing interest in this question because my recently published book, Abraham's Children: Race, Identity and the DNA of the Chosen People, is the first of a new genre: using DNA to help us connect with our common ancestors--our ethnic identities. DNA can do that. What it can't do, at least not yet, is provide the average identity seeker with anything but a vague snapshot of our ancestry and personal genetic profile.

Remember a few years ago when Adrian Targett, an English schoolteacher, was linked by DNA to a 9,000-year-old skeleton found in a cave near where he lived in Cheddar, a tourist town famed for the distinctive cheese made there. "Cheddar Man Found Alive in South of England," touted one headline. Well, not really. The genetic genealogical test--identical to some of the tests sold to the public today by commercial firms--indicated only that the two were linked to the same female ancestor. In kinship terms, they were cousins approximately 450 times removed. But they share only a few bases out of more than 3 billion nucleotide pairs in the human genome. Applying exponential math, Targett has inherited the DNA from many trillions of other relatives as well. Their relationship is certainly fascinating and important for researchers, but it's genealogically tenuous, to say the least.

Going back ten generations we each have about 1,000 ancestors, which means we share about one millionth of a random neighbor's DNA by direct descent. Twenty-five generations ago, about the time when Columbus happened on the shores of the Americas, the number of our ancestral cousins swells to an astronomical 30 million, almost all of the world's population at that time. Yet, we have only one continuous male and female lineage. The male and female lines have become popular markers of identity because of the historical importance of surnames and the fact that these two DNA lineages are so easy to track. Yet, they are only two of millions of ancestral lines that we each carry. Many millions and perhaps billions of people living today are genetically linked to Cheddar Man, although that extremely distant relationship might not show up in the few sections of the DNA that current technology can track.

This vast majority of the human genome, which amounts to more than 99 percent of our DNA, remains beyond the reach of commercial DNA tests. Services such 23andME do provide glimpses of this genetic bounty but the information is virtually meaningless. Yes, it's reports may indicate that we are have a gene that is linked to weight gain or wax ear build up, but even these seemingly simple traits are rarely the result of individual genes. They are activated--"expressed" is the term used by geneticists--by gene-gene and gene-environmental interactions.

Make no mistake about it. Genetic genealogy is fascinating and it provides scientists with greater insight into our distant ancestors. But for the vast majority of us, it's little more than expensive entertainment, and potentially misleading. Those in search of certifying their identity or behavioral quirks using DNA tests may find disappointment as much as illumination. For example, in my book, I mention the case of Lisa Black, a systems administrator in Oakland who is African American. She was shocked to find that the DNA lineage that current technology can track, her female ancestors, were Native Americans, Chinese, and Sardinians. It came as a major blow to a veteran of the Black Power movement of the 1960s. "For me to have a whole half of my identity to come back and say, 'Sorry, no African here.' It just negates it all. ... What does this mean? Who am I then?"