Veterinary geneticists Gregory Acland, left, and Gustavo Aguirre test the eyes of a Portuguese water dog, one of the breeds afflicted with an inherited disease that also blinds humans.Photo courtesy of Cornell Veterinary Image Lab

The young Portuguese water dog didn't know rods and cones from tennis balls, so at first she didn't know what she was missing. She suspected she was missing the neighbor's cat, the one she once could spot in the dark with night vision nearly as keen as any cat's. But gradually the cat had become an indistinct smudge in the distance, and now she could only hear it.

In fact what the 3-year-old dog was missing were her rods, the sensory structures in the retinas of the eyes that normally transmit dim light. In progressive retinal atrophy, or PRA, the canine form of a similar inherited disease in humans called retinitis pigmentosa, the rods, and thus night vision, are always the first to go.

As with human retinitis pigmentosa, at that stage the dog's only clear view was through the centers of the retinas because progressive degeneration of the cones, the daylight sensory structures, was stealing her peripheral vision. Next, cataracts formed on the lenses, precisely over the last living section of the retinas. Removal of the cataracts unshuttered a tiny window of vision for a few more months, until the relentless destruction of the retinal cones was complete. In the vigorous body of a 4-year-old were the useless eyes of an old blind dog.

"I remember when I got my first dog. I was 10 years old and it was a beautiful English cocker spaniel, and someone said, Ôwhen dogs get old they go blind,'" recalls Cornell geneticist and veterinarian Gregory M. Acland. That's not quite right, he now realizes. Acland is part of a team at the James A. Baker Institute for Animal Health, mapping the canine genome and discovering markers for defective genes that cause inherited diseases such as PRA. Some funding for the studies comes from the American Kennel Club and other dog-breeder organizations that are anxious to eliminate the defective genes from the lines of purebred dogs.

But human health organizations, such as the National Institutes of Health, also are supporting the dog gene studies at Cornell. The reason is that so much of the human and canine genome are similar and so many inherited diseases are shared by both species that discoveries in the canine system could lead to breakthroughs in human medicine.

One immediate benefit to dog owners is the development of a screening technique that detects defective genes in young animals before they breed and pass the defect to future generations. The canine genome project would not be economically feasible without a parallel and better-funded human genome project, according to Gustavo Aguirre, head of the Inherited Eye Disease Studies Unit at the Baker Institute. Techniques developed in the course of the human genome project are quickly adopted by researchers studying genes of dogs and other animal species, he said.

As with other new human therapies, dogs will be among the first to benefit from clinical trials of new retinal treatments, Aguirre predicts. The many diseases that dogs "share" with humans, from inherited eye diseases to degenerative arthritis, make dogs an ideal model system. Or as Acland puts it, "Fruit flies do not get arthritis, and normally mice don't get it either."

The youthful owner of the cocker spaniel grew up to become a veterinarian and then a veterinary geneticist, and he learned the truth about old dogs: In fact, many dogs live healthfully to a ripe old age without going blind. The difference is in the genes. The challenge for Acland, Aguirre and other veterinary geneticists is to sort out the differences.