Designer babies aren’t futuristic. They’re already here.

At first, Matthew assumed the weakness in his knee was the sort of orthopedic nuisance that happens when you turn 30. It was weeks before he consulted a doctor, and months before it occurred to him that there could be a connection between his worsening limp and a cousin’s shoulder problem when they were kids. DNA testing confirmed it: Matthew, like his cousin, had a genetic form of dystonia, a condition where muscles contract uncontrollably. Their grandfather most likely had dystonia as well.

I’d met Matthew only a few months earlier, when he’d married my friend’s daughter, Olivia, in one of those hip old New York hotels with an elegant downtown vibe. Since I was the only genetic counselor of their acquaintance, they brought their questions to me. With their permission, I am sharing their story. I have changed their names to preserve their privacy.

Matthew was lucky. His was a mild version of DYT1 dystonia, and injections of Botox in his knee helped. But the genetic mutation can cause severe symptoms: contractures in joints or deformities in the spine...