Hit a wall in your efforts to construct your family tree? Can’t get past the garbled last name that authorities at Ellis Island conferred on great-grandpa Maurizio? It’s DNA to the rescue—or, as critics say, yet another example of questionable “recreational genetics.”

Tomorrow, Ancestry.com, the Website where 15 million people have been accessing census and other records to build their family trees since the company’s founding in 1997, is rolling out its latest genealogy resource. Called DNA Ancestry, it starts by having you take a cheek-swab sample and mail it to the company, which will compare it to DNA samples in its database and tell you if it gets a hit—that is, someone to whom you are even distantly related. If it finds someone, you can contact him or her through an anonymous email and piggy-back on their own genealogy research. “If you don’t know your family history, you can match your DNA profile to one in our database and connect to other people who are related to you and might have broken through the wall” of historical records needed to construct a family tree, says Ancestry.com vice-president Brett Folkman.

Both women and men can have a DNA analysis of their mitochondrial DNA for $179. mtDNA, which is inherited by sons and daughters from their mothers, has become a standard way for a number of online sites to trace ancestry, as Family Tree DNA and Ancestry By DNA, among others, do. Men can also have their Y chromosome, which is inherited by sons (but not daughters) from their fathers virtually unchanged, analyzed for $149 or $199, depending on whether you want 33 or 46 genetic markers included. Unlike other DNA-based ancestry sites, which focus on telling you where in the world—sometimes down to the village—your family roots are sunk and even when your “family” migrated out of Africa tens of thousands of years ago, DNA Ancestry aims to link you to specific individuals. It can’t tell you your exact relationship to someone else, only that a relationship exists. For example, a Y-DNA test could verify that you’re related to a co-worker, but not that you both share the same great-grandfather.

This will work as the company says—linking you to someone who has mined federal census data from 1790 to 1930 and the 100 million names in passenger ship records from 1820 to 1960 in Ancestry’s database, among other sources, to piece together a family tree—only if Ancestry.com has lots of DNA profiles in its database. Within six months, that should be about 50,000, says Megan Smolenyak, the company’s chief family historian and co-author of the 2004 book "Trace Your Roots with DNA." “As more people add their results,” she says, “the DNA Ancestry database becomes a powerful asset for users to make connections and discover their family tree.”

The question is whether even 50,000 is enough. The service might tell you that you and another user share a great-great-grandfather—that is, four generations back. Everything more recent than that would diverge, so although genealogical research that this long-lost cousin of yours has done might fill in the distant parts of your family tree, it won’t help much with the branches that include your grandfather’s and father’s generations.

Overselling the value of DNA for ancestry searches is causing more and more scientists to scorn what they are calling “recreational genetics.” Since 2000, a study finds, some 460,000 people have bought DNA tests from some two dozen companies that trace ancestry, and some users are doubtless being misled. One problem is that by testing only a limited number of genetic markers, the services miss many relatives: while they tell you that your family roots are sunk in, say, the Piedmont, they overlook that you have just as many roots in, say, Nova Scotia, or this African village as well as that one. More problematic, in terms of identifying an individual to whom you are related, is that it can make the connection sound more notable than it is. Yes, you and Sam might have the same great-great-great-grandfather but, under standard assumptions about reproduction and survival, so do (on average) 500 other people living today. But hey, you might just get lucky and find that one of them is a demon genealogist who has done all the census-digging and Ellis-Island sleuthing for you.