The most important new antidiscrimination law in two decades  the Genetic Information Nondiscrimination Act  will take effect in the nation’s workplaces next weekend, prohibiting employers from requesting genetic testing or considering someone’s genetic background in hiring, firing or promotions.

The act also prohibits health insurers and group plans from requiring such testing or using genetic information  like a family history of heart disease  to deny coverage or set premiums or deductibles.

“It doesn’t matter who’s asking for genetic information, if it’s the employer or the insurer, the point is you can’t ask for it,” said John C. Stivarius Jr., a trial lawyer based in Atlanta who advises businesses about the new law.

The biggest change resulting from the law is that it will  except in a few circumstances  prohibit employers and health insurers from asking employees to give their family medical histories. The law also bans group health plans from the common practice of rewarding workers, often with lower premiums or one-time payments, if they give their family medical histories when completing health risk questionnaires.