The disease is an "autosomal dominant genetic disease." That means it's inherited directly from one parent. Gender doesn't seem to make any difference. One or two of 20,000 people have the disease — probably close to 40,000 in the United States. And estimates say another 160,000 Americans have tested positive for the gene and are presymptomatic or haven't been tested but are at risk.

A child has a 50-50 chance of inheriting the gene from a parent. Each child's risk is completely independent of siblings' risks, so it's not supposed to mean that half the children will inherit the disease. But that happens in many families.

As a genetic counselor explained it to Amy's husband, Robert Bishop, in their family with five children, there's a 1-in-32 chance that no child will get it. And the same chance that all the children will inherit the gene. But there's a 90 percent chance that two or three of the children will carry the Huntington's gene.

Neurologists call it a cruel disease, because it usually doesn't show up until someone is 35 to 40 — after they've already had their children. Occasionally it appears in young children or very old adults.

Not having children seems to be the only effective prevention. But most families have children by the time they know they have the disease. And it begins to claim another generation.

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John's problems were subtle at first. And they were perhaps overlooked for a while because Julie had been leading the family on a wild dance. She'd gone from merely stubborn to outright rebellious, staying out late, getting into trouble, going her own way.

That came crashing down, literally, in 1989, when she was in a serious car accident. She would never again be independent.

But it soon became impossible to ignore what was going on with John. He got in a lot of fender benders. He couldn't seem to focus. His mind "wandered," and he hit an ambulance once.

The day he crashed into a telephone pole while trying to park the car out front, he and Lael decided he probably shouldn't drive. Later, he would come to resent that decision — and Lael.

He'd been having trouble at work. He couldn't concentrate. Tasks went undone. Eventually, he was fired.

In the past, companies were lined up to woo him away from whoever employed him. Suddenly, he couldn't keep a job. He had three employers in three years, including a job The Church of Jesus Christ of Latter-day Saints gave him in "humanitarian services."