The reason some genetic disorders are relatively common while most are extremely rare has proved to be almost ridiculously obvious. The bigger the gene, the greater the chance that something will go wrong with part of it. In many cases, it seems as simple as that.

Sometimes rather subtle differences in the defects of a single gene can make a profound difference in a patient's fate, as Louis Kunkel of the HHMI unit at Children's Hospital, Boston learned after he and his team discovered the gene for Duchenne muscular dystrophy (DMD) in 1986. Major flaws in that huge gene result in the presently incurable DMD, a muscle-wasting disease that leaves young boys wheelchair-bound by age 12 and generally kills them by their late twenties, when the muscles that control their breathing fail. By contrast, lesser defects in that same gene produce a much more benign disease, Becker's muscular dystrophy.

— Harold Schmeck, Jr.

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Louis Kunkel and his team found the genetic flaws that cause two forms of muscular dystrophy.

Photo: Burt Glinn/Magnum

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