They swab the cheeks of strangers and pluck hairs from corpses. They travel hundreds of miles to entice their suspects with an old photograph, or sometimes a free drink. Cooperation is preferred, but not necessarily required to achieve their ends.

Derrell Teat, determined in her research, once waited outside a restaurant with a test kit hoping to capture a reluctant would-be relative’s DNA on a coffee cup.

If the amateur genealogists of the DNA era bear a certain resemblance to members of a “CSI” team, they make no apologies. Prompted by the advent of inexpensive genetic testing, they are tracing their family trees with a vengeance heretofore unknown...

By next year, close to half a million people will have taken a DNA genealogy test, according to estimates from companies that provide them. The tests detect genetic markers that distinguish the descendants of an individual and reveal if two people share a recent common ancestor.