He was the Arnold Schwarzenegger of quarter horses, a muscle-bound show stallion from Oklahoma whose foals boasted his extraordinary heftiness.

But what appeared desirable could kill.

In 1992 HYPP was linked to the illustrious quarter horse sire Impressive.

Impressive's heavy musculature came not from robust health and conformation but from disease: hyperkalemic periodic paralysis, an ailment that fires muscles into uncontrollable contractions and causes them to beef up as if they had been exercised isometrically.

Impressive was born in 1969, and in short order he passed the dominant HYPP gene on to thousands within his breed, a domino effect of disease rushed along by artificial insemination.

The disease stands out because its spread was hastened and, in effect, promoted by breeders. But it is also notable because the identity of the Impressive gene in 1992 was one early milestone in the quest for a map of genetic makeup of the horse.

The project began two years ago, when 70 scientists from 23 countries met in Lexington and agreed to collaborate.

The map is in its infancy, but when it gets further along it will be a boon to horse breeders.

[First draft of horse genome sequenced February 8, 2007]

It could help prevent the spread of diseases like Impressive's, and it could help owners of racehorses not only breed faster animals, but sounder ones. New vaccines also could develop from the research, said Ernest Bailey, a leading equine geneticist at UK's Gluck Equine Research Center.

What breeders sometimes overlook in their pursuit of performance is good health. Poor bones, bad temperament, allergies and disease are passed down from one generation to the next.

The frequency of some problems is shocking: Up to 25 percent of horses suffer developmental bone diseases, according to a 1996 study.

The gene map could keep such problems to a minimum by alerting breeders to the horses that could pass on defects, and how likely they would be to do so.

Genetic markers are signposts on chromosomes that are created by links between genes. In effect, proximity breeds marriage on chromosomes; a gene for a disease, for instance, tends to replicate alongside a gene next to it, creating a sequence that can be recognized in a lab.

Gene maps are already well along for humans and the animals we eat or share our homes with: cattle, sheep, pigs, chickens, dogs and cats.

The horse was left behind, in part because its lack of culinary appeal and usefulness to Americans kept them out of the money for genetic mapping research projects.

The equine gene project also could help the horses' human masters with their own health: One research project at the Gluck Equine Research Center focuses on equine infectious anemia, a disease that is similar to AIDS. Recognizing the possible link, the people-oriented National Institutes of Health funded the equine research project.

The quarter-horse disease, HYPP, is also suffered by humans - but instead of causing bigger muscles, it causes weakness.

Humans and horses - and other mammals - are not vastly different when you compare their chromosomes. Human chromosome No. 8, for instance, appears to be the same as horse chromosome No. 9.