People who know their biological parents and grandparents typically take the information for granted. Some have a difficult time empathizing with the passionate genealogical quests of adoptees and, increasingly, products of anonymous sperm banks and other new technologies where one or both genetic contributors are unknown. In recent years, new legislation has enabled people to search for information about genetic progenitors - even in cases where there had been a signed agreement of nondisclosure. The laserlike focus of that search can be as relentless as Ahab's hunt for the white whale.

Mystery of lineage is the stuff of great literature. Mark Twain made use of it for biting social commentary in his Pudd'nhead Wilson, a story about the mix-up of babies born to a slave and a free person. Sophocles, Shakespeare, Molière, and Dickens built grand tragedy and enduring comedy on the theme. In England in 2002, a white Englishwoman gave birth to mixed-race twins after a mix-up at an in vitro fertilization clinic. Imagine what Shakespeare would have done with that!

If one person's passions can be so riled by such a puzzle, imagine the emotions involved when the uncertainty applies to a whole group - say, of 12 million people. The middle passage did just that to Americans of recent African descent. Names were obliterated from record books, and slaves were typically anointed with a new single first name. Sometimes no names were recorded, just the slaves' numbers, ages, and genders. Some African-Americans have deliberately and actively participated in the erasure, showing no desire to pursue a genealogical trail. For others, fragments of oral history generate a fierce longing to do the detective work.

That is the case among the prominent subjects featured in "African American Lives," a two-night, four-part PBS series scheduled for February 1 and 8. The host and executive co-producer is Henry Louis Gates Jr., chairman of the department of African and African-American studies at Harvard. Gates has assembled eight notably successful African-Americans, among them the media entrepreneur Oprah Winfrey, the legendary music producer Quincy Jones, and the film star Whoopi Goldberg. Each participant, along with Gates, is the subject of some serious professional family-tree tracing. There are surprises for each of them, and the series has undeniable human-interest appeal.

But there are other reasons why it is likely to be a staple for courses on history, family and kinship, and African-American studies for years to come. Who knew that before the Emancipation Proclamation, more than 250,000 free blacks lived below the Mason-Dixon line? We learn that the kinds of fears that preoccupied them in their daily lives were partially mitigated when they bonded in one place, permitting them to vouch for each other's long-term community standing if a white person came and tried to claim them as slaves.

The first three segments are very much driven by traditional genealogical research, the hard work of ferreting through archival materials, birth and death certificates, deeds, trusts, estates and wills, church records, and, inevitably, the sale of slaves. One of the patterns discernible at the outset is the speed of some tales of rags to riches and meteoric ascendancy from modest circumstance to extraordinary accomplishment. The Johns Hopkins neurosurgeon Benjamin Carson, who performed pioneering work in separating twins joined at the head, is the son of a domestic. Winfrey's story is fairly well known - as a child, she was sexually abused and shuttled between homes until finally becoming more settled as a late teenager.

Gates deserves special praise for the way in which he weaves biographies into the larger social and historical context. Reconstruction comes to life in the form of Winfrey's grandfather, Constantine Winfrey, who was illiterate as slavery ended. He taught himself how to read and write, then sponsored a new school, all the while raising a family and tilling the soil. The comedian Chris Tucker's great-grandfather was a beneficent church minister who purchased a large plot of land upon which the sanctuary was built. To keep his congregation together, he sold small plots to members. The Harvard sociologist Sara Lawrence-Lightfoot's ancestors left New England to start a trade school in the South to help the newly freed slaves find employment.

None of the participants knew the rich details of these histories, and the "only in America" element is compelling.

At another level, however, the series performs a disturbing sleight of hand. Conventional wisdom has it that we can choose our friends, but that our families are a given. But with long-term genealogical work, there is a sense in which this can be inverted. We each have two parents, four grandparents, eight great-grandparents, etc. As Gates points out in the fourth segment, current technology permits us to link via DNA analysis to only two specific lines. On the Y chromosome, one's father's father's DNA, going back as far as we can locate the genetic material, can be determined with a high degree of certainty. (That is how Thomas Jefferson - or one of his brothers - was definitively linked to Sally Hemmings's offspring.) On the female side, mitochondrial DNA (mtDNA) can link one's mother's mother's mother going back as far as we can garner the DNA. So, while we have 64 great-great-great-great-grandparents, the technology allows us to locate only two of those 64, if we're going back six generations, as our real legacy and genetic link to the past. But what of the other 62? Those links are equal contributors to our genetic makeup, and we ignore them only because we do not have access to them.

What an arbitrary "choice" of a branch on the family tree!

At one point, upon learning that 50 percent of his ancestry has been traced by DNA analysis to Europe, and that both his maternal and paternal lines are also "European," Gates jokingly asks if he still qualifies to be chairman of African-American studies at Harvard.

But for many, that is no laughing matter. The Black Seminoles are struggling with this very question - whether to use DNA analysis to "authenticate" their relationship to the Seminoles. The reason is straightforward and serious: money. The federal government, pursuant to a land-settlement claim, made an award to Seminole Indians in 1976 and is poised to distribute upward of $60-million.

In 2000 the Seminole Nation of Oklahoma amended its constitution so that members needed to show "one-eighth Seminole blood." The Black Seminoles could use either Y-chromosome analysis or mitochondrial DNA to link themselves through very thin chains back on two edges of the genealogical axis (mother's mother's mother, etc.; or father's father's father, etc.), but that would miss all other grandparents (14 of 16, 30 of 32, 62 of 64).

One attempt to fill in the blanks is the use of a technology called admixture mapping through ancestry-informative markers, or AIM's. Unlike Y DNA or mtDNA tests, this technology examines groups' relative sharedness of genetic markers found on the autosomes - the nonsex chromosomes inherited from both parents.

In the last segment of the series, each of the nine subjects, including Gates, is given information using molecular genetics and computer-assisted analysis of all three kinds of DNA markers. Each of the subjects accepts the ostensibly scientific news of his or her percentage ancestry, deduced by AIM's - that is, African or European or Native American - as if it were of the same certainty as a clerk's entry of a birth date on a certificate. Oprah is crestfallen when she is told that she is not Zulu.

Gates has no match to Africa at all using the conventional tests - so he deploys Mark D. Shriver, a Pennsylvania State University geneticist at the forefront of admixture mapping, to conduct a special test for him. Gates's autosomes are compared to the small set of African samples Shriver has in his database, from no more than six West African regions. When compared against those few, Gates is closest to the Mende people of Sierra Leone.

Shriver himself seems wary of these results. He surely knows the clusters of DNA are at best crude approximations completely contingent on available samples. Africa has over 700 million inhabitants, and among them it has the greatest amount of human genetic variation found on any of the seven continents. Depending on methods, some regions will be completely missed, while others will be oversampled. The scientists who do the analysis will freely admit that when pressed, but the seekers' eagerness to know spurs a willingness to accept as definitive these artifacts of sampling contingencies.

Ancestry-informative markers (with one exception) are shared across all human groups. It is therefore not their presence or absence, but their rate of incidence, or frequency, that is being analyzed. When taken together, these markers appear to yield certain patterns in people and populations tested. A specific pattern of alleles - corresponding genes on each of a set of chromosomes - that have a high frequency in the "Native Americans" sampled then become established as a "Native American" ancestry result. The problem is that millions of people around the globe will have a similar pattern - that is, they'll share similar base-pair changes at the genomic points under scrutiny. This means that someone from Hungary whose ancestors go back to the 15th century could map as partly "Native American," although no direct ancestry is responsible for the shared genetic material. AIM's, however, arbitrarily reduce all such possibilities of shared genotypes to "inherited direct ancestry." In so doing, the process relies excessively on the idea of 100-percent purity, a condition that could never have existed in human populations.

To make claims about how a test subject's patterns of genetic variation map to continents of origin and to populations where particular genetic variants arose, the researchers need reference populations. The public needs to understand that these reference populations comprise relatively small groups of contemporary people. Moreover, researchers must make many untested assumptions in using these contemporary groups to stand in for populations from centuries ago representing a continent or an ethnic or tribal group. To construct tractable mathematical models and computer programs, researchers make many assumptions about ancient migrations, reproductive practices, and the demographic effects of historical events such as plagues and famines. Furthermore, in many cases, genetic variants cannot distinguish among tribes or national groups because the groups are too similar, so geneticists are on thin ice telling people that they do or don't have ancestors from a particular people.

Instead of asserting that someone has no Native American ancestry, the most truthful statement would be: "It is possible that while the Native American groups we sampled did not share your pattern of markers, others might since these markers do not exclusively belong to any one group of our existing racial, ethnic, linguistic, or tribal typologies." But computer-generated data provide an appearance of precision that is dangerously seductive.

There is a yet more ominous and troubling element of the reliance upon DNA analysis to determine who we are in terms of lineage, identity, and identification. The very technology that tells us what proportion of our ancestry can be linked, proportionately, to sub-Saharan Africa (ancestry-informative markers) is the same being offered to police stations around the country to "predict" or "estimate" whether the DNA left at a crime scene belongs to a white or black person. This "ethnic estimation" using DNA relies on a social definition of the phenotype. That is, in order to say that someone is 85 percent African, we must know who is 100 percent African. Any molecular, population, or behavioral geneticist is obliged to disclose that this "purity" is a statistical artifact that begins not with the DNA, but with a researcher's adopting the folk categories of race and ethnicity. With the demonstrable skew of the incarcerated population over the last few decades along social categories of race, African-Americans need to be particularly sensitive to the use of phenotype as the starting point for understanding genotype.

The fourth part of "African American Lives" would have benefited from a lot more scientific humility about just how much we can know about our "percentage ancestry." Oprah may have some Zulu (among the "other 62") in her lineage that current technology can neither tap nor exclude. And since nothing in the current state of scientific knowledge can rule that out, we should be so informed by an otherwise enlightening series. The Bantu migration entailed massive movements of people across the African continent. So it is possible that as a "West African," Oprah could indeed have a Bantu link somewhere in the ancestral pedigree. That this possible link might not be called Zulu is more a function of social definition and historical effect.

So, since the jury is still out, don't resign your post, Professor Gates. And nervous jokes aside, let's all recognize that scientific imprecision on matters of identity and identification have implications far graver than the undermining of a TV program's entertainment value.

Troy Duster is past president of the American Sociological Association and director of the Institute for the History of the Production of Knowledge at New York University, where he is a professor of sociology. He is also a chancellor's professor at the University of California at Berkeley. His books include Backdoor to Eugenics (Routledge, 2003).