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EIGHTEEN YEARS ago, a fertility technique called intra-cytoplasmic sperm injection, or ICSI, revolutionized the treatment of male infertility by joining sperm and egg in an unprecedented manner. No longer did the sperm have to fight its way into an egg; instead, a technician’s skilled hands did all the hard work by injecting it. The technique was, essentially, a way to circumvent the process of fertilization.

It was a technical tour-de-force that allowed men with severe sperm production problems, considered sterile at the time, to father children of their own. Today, ICSI has become one of the most widespread fertility techniques, used in over half of the in vitro fertilization treatments done in the United States. About one million babies worldwide have been born via ICSI, according to G. David Adamson, director of the Fertility Physicians of Northern California and a member of the International Committee for Monitoring Assisted Reproductive Technology.

But as the first generation of ICSI children enters adolescence, one side effect of the technology has become clear: it enables the passing down of genetic defects from parent to...