ALBUQUERQUE— In 1598, Joyce Gonzalez’s great- great- great- great -great -great -great -great -great -great -grandfather followed the famous conquistador Juan de Oñate from Spain to Mexico, then north on the Camino Real, the Royal Road to Santa Fe.

In the 1800s, one of Mary Ann Chavez’s distant relatives, possibly a French fur trapper and trader from Quebec, also made his way into northern New Mexico.

Mrs. Chavez and Mrs. Gonzalez, though not related, share a Hispanic heritage and a fascination with genealogy. They also share the burden of having forebears with genetic diseases that, like the remote mountain villages in this region, have remained largely hidden from medical diagnosis and treatment. Now, thanks to the efforts of patient advocates and the work of a clinic here at the University of New Mexico Medical School, these illnesses are finally being confronted and studied.

“We call it the family curse,” said Mrs. Chavez, 73, “and you don’t know you’ve got it until you’re 40 or 50 when your eyelids start to droop, and you begin to have trouble swallowing and get muscle weakness.”

The illness is called oculopharyngeal muscular dystrophy, or OPMD, and the largest group of Americans affected are Hispanics living in northern New Mexico. They are descendants of the wandering French-Canadian or, perhaps, early Spanish colonists. Mrs. Chavez’s son, her brother and innumerable aunts, uncles and cousins have all inherited the disease.

OPMD is caused by a genetic mutation first identified by Canadian researchers in Quebec, where the largest population of affected individuals in North America is found. The defective gene is thought to have been introduced by three French sisters who came to Canada in 1648. OPMD has now been identified in 29 countries, including Spain, France, Britain and Israel.

Mrs. Gonzalez’s disease is cavernous angioma, also called C.C.M., for cerebral cavernous malformation. It is caused by abnormal blood vessels that form raspberrylike clusters in the brain and spinal cord. If these angiomas bleed or press against structures in the central nervous system, they can produce seizures, neurological deficits, hemorrhages and headaches.

Cavernous angiomas occur sporadically in the general population, but 20 percent are inheritable, and the disease is found at a much higher rate in Mexican-American families, particularly in northern New Mexico. Like OPMD, it is an autosomal dominant disease, meaning that each child of an affected parent has a 50 percent chance of inheriting it.

“I had just put my left arm down on the kitchen table when I experienced this intense pain from the tips of my fingers to my elbow,” said Mrs. Gonzalez, 50. “It felt like my whole arm was burning.” Her pain spread to her right arm and lasted on and off for 15 years until an M.R.I. found an angioma in her cervical spinal cord. It was surgically removed three years ago and she is largely free of symptoms.

Mrs. Gonzalez and others with the familial condition often have multiple angiomas — and as she casually points to her temple, she says she has five more, including three in her brain stem that are surgically inaccessible.

“My greatest gift,” she said, “has been knowing that my two children have been tested, and miraculously they’re both negative” for the gene that causes the malformation.

That gene, called CCM1, seems to be found uniquely in Mexican-Americans, and this same genetic mutation caused the recent cerebral hemorrhagic death of Mrs. Gonzalez’s 9-year-old second cousin. Many relatives in her mother’s family have had seizures, and Mrs. Gonzalez has persuaded nine relatives to undergo genetic testing. Five of the nine were positive for the CCM1 gene, and all have angiomas in their brainstems.

“The Hispanic families that have lived here for 300 or 400 years — we’re practically all cousins,” she said. That led her to trace the genealogies of four other Hispanic families with histories of cerebral cavernous malformation. She held a complex genealogical chart whose five converging family trees pointed to Gerónimo Márquez, the 16th-century patriarch of her family. “He could be the guy that brought it for all of us,” she said.

Dr. Connie Lee, director of the Angioma Alliance, a volunteer support group, estimates that tens of thousands of Hispanics carry the common CCM1 gene and do not know it. The Spanish cyclist Alberto Contador is one of them: he collapsed with seizures during a race in 2004. Surgeons found an angioma in his brain and repaired it. This summer, he won the Tour de France.

Recently, Charlie Abeyta, his wife and two close relatives jammed into an exam room with Dr. Leslie Morrison, an assistant professor of neurology at the University of New Mexico School of Medicine who is director of its OPMD and cavernous malformation clinics.

Mr. Abeyta, a 74-year-old carpenter with OPMD, had driven from his home in southern Colorado to see Dr. Morrison. As she watched him climb onto the exam table, she asked, “Trouble getting out of chairs?” Mr. Abeyta acknowledged a little difficulty, but added that he was still able to work. His real problem, he said as his family nodded in affirmation, was that it took him 40 minutes to eat a hamburger. Still, his eyelids were working fairly well after three operations.

Dr. Morrison sketched a genetic pedigree as she listened to the Abeyta family discuss whether various aunts, uncles and cousins had symptoms of ptosis (eyelid drop), dysphagia (trouble swallowing) or hip and shoulder weakness, the primary hallmarks of OPMD. Genetic testing had already confirmed that two of Mr. Abeyta’s three sisters, and four of their nine children, were positive. “We’re always on the lookout for people tilting their heads back to see better, because their eyelids fall down,” Dr. Morrison said.

Her clinic helps coordinate referrals for so-called sling procedures, surgery in which the eyelid is drawn up to a normal position and suspended from a muscle in the forehead. Swallowing difficulties may be treated with esophageal dilation, or even Botox injections to relax the swallowing muscles.

“Patients with OPMD have a normal life expectancy,” Dr. Morrison said, “but patients with more severe OPMD may end up in wheelchairs with feeding tubes. There’s a lot of variability in how the disease is expressed, even within the same family. “

Dr. Morrison also sees Mrs. Gonzalez, her relatives and more than 50 other families in the cavernous malformation clinic. Genetic testing and magnetic resonance imaging have greatly improved the diagnosis and monitoring of C.C.M. patients; treatments include seizure and headache medication, and neurosurgery to treat surgically accessible angiomas that have recently bled or that threaten vital functions.

“Mexican-Americans now live all over the country, and the genetic implications — that you can pass these two diseases on to half your kids — mean that health practitioners and patients, particularly those in the Southwest, need to be aware of the symptoms,” Dr. Morrison said. “If you have seizures, recurrent headaches or strokelike symptoms, tell your doctor about angiomas or cavernous malformation.

“And if you always thought that droopy eyelids and trouble swallowing was a family trait, think again,” she continued. “Think about OPMD.”