In another study, a second team found that people who learned they had the gene were not emotionally scarred by it

Both findings, published in the New England Journal of Medicine, offer new support for genetic tests for Alzheimer's disease, a mind-wasting condition for which there are few treatments and no cure. It affects 26 million people globally.

Several companies -- including Iceland's Decode Genetics' DeCodeME, 23andME backed by Google Inc and privately held Navigenics -- sell tests that allow people to learn if they have inherited the ApoE4 gene variant, which raises the risk of Alzheimer's by more than 50 percent.

A new study by Dr. Richard Caselli of Mayo Clinic Arizona and colleagues found people who had the ApoE4 gene variant showed signs of memory trouble at an earlier age than people who did not have the gene.

Caselli studied 815 healthy people aged 21 to 97 who were grouped according to their gene status.

They found people who had the ApoE4 gene were more likely to develop age-related memory trouble before age 60, and accelerated memory declines were worse in people who inherited the gene from both parents.

Having ApoE4 does not mean a person is doomed to have Alzheimer's, and a team led by Dr. Allen Roses at Duke University in North Carolina reported at an Alzheimer's conference earlier this week that a second gene closely linked to ApoE4 called TOMM40 also significantly raised Alzheimer's risk.

Roses said together, the two genes may account for 85 to 90 percent of inherited forms of Alzheimer's.