“The fear is potent in our society that insurance companies are asking,” Dr. Green said. “The No. 1, 2 and 3 issue that subjects are concerned about is, will they be discriminated against if this is in their medical record?”

Just three states — California, Oregon and Vermont — have broad regulations prohibiting the use of genetic information in life, long-term-care and disability insurance.

At least one insurer, the Northwestern Mutual Life Insurance Company, asks potential customers in Massachusetts about genetic testing — and stipulates that refusing to share results could lead to a declined application or an extra premium. Jean Towell, a spokeswoman, says applicants are told “out of fairness” that insurers have the right to decline coverage if any medical information is omitted.

“We think it’s best to have it all spelled out in black and white so buyers can make a well-informed decision,” she said.

At least for now, 12 other companies ask no explicit questions about genetic testing. But when Dr. Green asked company executives why not, he said, “at least one of them has told me, ‘We would do this, but we don’t want to be the first.' ”

Still, he added, “you can imagine a world where millions of people have this information, and that would reach a tipping point that the insurance companies can no longer ignore.”

Even if most insurers are not asking now, they do seek out medical records and can use genetic test results listed there. By contrast, under the federal law, an employer who asks for an employee’s records must tell the provider to withhold any genetic information.