They said it was their family curse: a rare congenital deformity called syndactyly, in which the thumb and index finger are fused together on one or both hands. Ten members of the extended clan were affected, and with each new birth, they told Stefan Mundlos of the Max Planck Institute for Molecular Genetics, the first question was always: “How are the baby’s hands? Are they normal?”

Afflicted relatives described feeling like outcasts in their village, convinced that their “strange fingers” repulsed everybody they knew — including their unaffected kin. “One woman told me that she never received a hug from her father,” Dr. Mundlos said. “He avoided her.”

The family, under promise of anonymity, is taking part in a study by Dr. Mundlos and his colleagues of the origin and development of limb malformations. And while the researchers cannot yet offer a way to prevent syndactyly, or to entirely correct it through surgery, Dr. Mundlos has sought to replace the notion of a family curse with “a rational answer for their condition,” he said — and maybe a touch of pioneers’ pride.

The scientists have traced the family’s limb anomaly to a novel class of genetic defects unlike any seen before, a finding with profound implications for understanding a raft of heretofore mysterious diseases.