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."