It's one of the darkest secrets of the Warren Jeffs polygamist community.

An especially severe form of birth defect is on the rise and may mushroom in coming generations.

"I don't want to describe it in too much detail," said Isaac Wyler, who was related by marriage to some of the victims. "It's not a real pretty sight."

According to experts and former Jeffs followers, the cause of the birth defect is clear: Intermarriage among close relatives is producing children who have two copies of a recessive gene for a debilitating condition called Fumarase Deficiency.

They predict the scale of the problem will increase dramatically in the future. Wyler, who has lived in the polygamist community most of his life, said he expects residents to continue marrying close relatives.

"Around here," Wyler said, "you're pretty much related to everybody."

Fumarase Deficiency is an enzyme irregularity that causes severe mental retardation, epileptic seizures and other cruel effects that leave children nearly helpless and unable to take care of themselves.

Dr. Theodore Tarby has treated many of the children at clinics in Arizona under contracts with the state. All are retarded. "In the severe category of mental retardation," the neurologist said, "which means an IQ down there around 25 or so."

Until a few years ago, scientists knew of only 13 cases of Fumarase Deficiency in the entire world. Tarby said he's now aware of 20 more victims, all within a few blocks of each other on the Utah-Arizona border.

The children live in the polygamist community once known as Short Creek that is now incorporated as the twin towns of Hildale, Utah, and Colorado City, Ariz. Tarby believes the recessive gene for Fumarase Deficiency was introduced to the community by one of its early polygamist founders.

According to community historian Ben Bistline, most of the community's 8,000 residents are in two major families descended from a handful of founders who settled there in the 1930s to live a polygamist lifestyle.

"Ninety percent of the community is related to one side or the other," Bistline said.

For many years, Bistline was a member of the Fundamentalist Church of Jesus Christ of Latter-Day Saints (FLDS), which today reveres fugitive polygamist Warren Jeffs as a prophet.

"They claim to be the chosen people, the chosen few," Bistline said. "And their claim is they marry closely to preserve the royal bloodline, so to speak."

Wyler, who says Jeffs kicked him out of the FLDS group two years ago, has observed some of the "Fumarase children" in their home environment.

"I've seen some children that can talk and communicate a little," Wyler said. "And I've seen others that are totally laid out. They have no movement. They can't do anything by themselves. Literally, if they're 8 years old, it's like taking care of a baby."

Tarby saw the first "Fumarase child" in the community 15 years ago. He said the oldest victim is now about 20 years old. In March 2000, Tarby co-authored an article in the medical journal "Annals of Neurology" describing eight new cases of Fumarase Deficiency in the Southwest. It has now grown to 20 known cases in the polygamist community on the Utah-Arizona border.

Tarby said children suffering from Fumarase Deficiency have unusual facial features and frequent "grand mal" epileptic seizures. The children require constant care from parents and close relatives. "In some ways, they are really kind of remarkable people," Tarby said. "They do treat these kids pretty well."

Wyler agreed that the parents and close relatives are loving caregivers. He said it's partly because they believe it's a calling from God. "They would just assume they've been given a test and they need to pass this test," Wyler said. "And it's their lot in life to take care of a child like this. And they'll give it everything they've got. And they'll do a good job. Very good job."

Tarby said the early founder who brought the recessive gene into the community had numerous children, so copies of the gene were passed on to children and grandchildren. When cousins or other close relatives marry, two copies of the gene can be passed on to a single child, triggering the disease.

In the FLDS community, marriages with cousins and even closer relatives are common, according to Bistline. "There are people that have married their nieces," Bistline said. "People who have married their aunts."

It's all part of the community's religious system, according to Wyler. "Well, around here, of course, when you get married, you're told who to marry and when to get married and things like that. So, that's really not going to change, I don't believe."

"As long as they've got the leadership they've got," Bistline said, "they'll never change."

It's believed that more than half the residents carry the recessive gene. That means the number of cases will likely grow. Tarby said there could be hundreds of victims in coming generations. "No, it wouldn't surprise me," Tarby said. "Wouldn't surprise me."

Wyler hopes FLDS leaders will change their marriage practices. "Now that they know there's a problem," Wyler said, "they need to quit sweeping it under the rug and pretend there's not a problem. And (they should) say, 'OK, now you know when you cross these certain lines together, then this happens.' And they need fresh blood."

Tarby has suggested to community residents that they undergo genetic screening before marriage. They've ignored the suggestion, Tarby said. "I really doubt that if we could tell them, you know, 'This male has the condition and this female has the condition; you shouldn't mate,' that wouldn't stop them."

On one occasion at an Arizona clinic, Tarby explained to one of the fathers the reason he had a Fumarase child. "You and your wife are related," Tarby said he told the man.

The father replied, "Up there we're all related." Tarby said he was not sure if the man meant "up there in Colorado City or up there in heaven."

Tarby said the children are a financial burden on taxpayers, although he's not sure how much. In Arizona, the children frequently receive medical services at state expense, Tarby said. He believes some Fumarase children live on the north side of the border and receive some of their medical care in Utah, presumably at taxpayer expense. Officials in both states say they can't reveal data because of privacy laws.

When asked if he considered the situation wrong, Tarby said, "Wrong? I've given up trying to sort those things out. I don't think they're going to change much."

In the course of investigating the problem, KSL-TV learned the names of some victims and their parents but chose not to reveal them. Through intermediaries, KSL offered parents a chance to speak, but they did not respond.

E-mail: hollenhorst@ksl.com