DNA can reveal ancestors' lies and secrets

"Why would you expose the next fetus to the risk of amniocentesis?" Grody asked.

In the growing world of direct-to-consumer DNA testing, customers are usually on their own to discover and digest non-paternity.

The industry has ballooned to more than three dozen companies from its inception about nine years ago.

There is a wide range, from those that offer basic ancestry testing to a few that scan several hundred thousand genes looking for susceptibility to certain diseases.

Scans, which require a cheek swab or a vial of saliva, cost $100 to $1,000.

23andme, a year-old company that mines genomes for medical information, warns customers in a consent form that testing could reveal that "your father is not genetically your father."

The company allows customers to display vast swaths of their DNA next to that of relatives -- with everyone's consent -- making cases of non-paternity easy to spot, geneticists say.

Rachel Cohen, a spokeswoman, said she is not aware of any such cases, though the company does not look for them or necessarily hear about them from customers.

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Y chromosomes

In ancestry testing, non-paternity shows up most often in comparisons of Y chromosomes, passed from father to son. (Some companies also test mitochondrial DNA, which everybody inherits from their mothers, but it is less useful in genealogy because surnames are usually passed down the male line. Women often submit the Y-DNA of a close male relative.)

All the males in a single bloodline have the same Y-chromosome, except for the tiny mutations that accumulate over generations.

If a father and son have vastly different Y-chromosomes, they are not related.

If cousins have dramatically different Y-chromosomes, it is safe to conclude that somewhere along the line that joins them, someone is keeping a secret.

And if the Y-chromosome looks nothing like those of the other people with the same last name who have posted theirs on the Internet, it is fair to wonder whether somebody is hiding something. Of the 147 people in the Kincaid project, most fall into four main groups of Y-DNA.

But about 10 Kincaids didn't match up with anybody else.

"You can let your mind run wild," said Bob Kinkaid, 68, from Star Tannery, Va., who didn't find any Kincaids with Y-DNA similar to his.

"You never know when a male child may have been adopted," he offered.