Thanks to inbreeding, dogs are more like us than ever before. Take the golden retriever. In the past few years, the breed has begun to suffer from one of a cluster of rare diseases that also afflicts humans, maladies that cause the skin to form scaly patches and that can sometimes be fatal. A new study of golden retriever DNA shows that the same gene is mutated in the dogs and in some people, offering a much-needed clue to the disease’s origins.

Mating closely related dogs helps keep the breed pure, but it can also cause trouble. “When dogs are bred for qualities like size, temperament, or color, the selected genes may be physically close to other genes that cause disease,” explains geneticist Catherine Andre, who heads the Canine Genetics Group at the University of Rennes 1 in France. “The mutated copy of the gene can rapidly spread in a given breed.” And that can lead to an explosion of once-rare disorders such as hip dysplasia, epilepsy and syringomyelia (in which a too-small skull forces the brain against the spinal cord).

Also cropping up lately in golden retrievers, Jack Russell terriers, and Norfolk terriers, are forms of a series of skin disorders collectively known as ichthyosis. Named after the Greek word for fish, the disorders are so rare in people that even the most common forms affect only about one in 2,500 individuals. Severe types such as Harlequin ichthyosis (which breaks the skin up into diamond-shaped plates outlined by deep cracks and is usually fatal in the first few days of life) occur so seldom that researchers can’t even make estimates of their prevalence. So it was hard to track down the gene or genes responsible. But a clue has emerged — now that dogs have started coming down with the same condition.

To determine whether canine and human ichthyosis have a common basis, Andre and colleagues first checked for telltale mutations in golden retrievers. The investigators screened the entire genome with 50,000 genetic markers, ultimately finding mutations in a gene called PNPLA1.

For the next step, directed by co-author and geneticist Judith Fischer of the University of Freiburg in Germany, the researchers tested whether human patients had mutations in the same gene. Mirroring the results in dogs, family members with ichthyosis had mutations in both copies of the gene.