For the first time, the Food and Drug Administration said it would allow a company to sell genetic tests for disease risk directly to consumers, providing people with information about the likelihood that they could develop various conditions, including Parkinson’s and Alzheimer’s.

The move on Thursday is a turnaround for the agency, which had imposed a moratorium in 2013 on disease tests sold by the company, 23andMe, which is based in Mountain View, Calif. The decision is expected to open the floodgates for more direct-to-consumer tests for disease risks, drawing a road map for other companies to do the same thing.

23andMe will now be reporting telltale markers for 10 diseases. Most, like factor XI deficiency, a blood clotting disorder, and Gaucher disease type 1, an organ and tissue illness, and celiac disease are rare. Anyone who buys the $199 Ancestry and Health test from the company will automatically learn if they have mutations increasing their risk for those diseases.

Risk genes for Alzheimer’s and Parkinson’s are treated separately. Customers will have to specifically say that they want that information. The company’s website offers links to genetic counselors for those who are weighing whether to be tested. If they want those results they will be included at no extra charge, though patients have to pay for the counseling separately.

Until now, the only way for people to get such genetic tests was to see a medical professional who would order a test and later deliver the results to patients. Often, patients were required to see a genetic counselor before getting a test.

The process for customers is simple. A customer spits into a tube and then mails it to 23andMe. The company’s lab extracts DNA from the saliva cells and tests it with probes that find genetic markers using a special chip for genotyping. In about six to eight weeks the company sends the customer an email saying the results are in. By logging onto an online account, the customer can see the report and its interpretation. It will include not just the genes for disease risk, but also reports of ancestry and results on things like how much the person is likely to weigh and whether alcohol will elicit flushing of the face, shoulders and neck, or even the entire body.

This is not the first foray by 23andMe into direct-to-consumer disease risk testing; several years ago it marketed tests to consumers claiming it could provide genetic information for a variety of diseases from a simple saliva sample. But when the F.D.A. shut that endeavor down in 2013, it told the company it had to prove that its tests were accurate, and that customers understood their results. The F.D.A. established on Thursday what it called “special controls” to permit the company to sell the tests, and future ones, if it met those requirements. The company did that and Tara Goodin, a spokeswoman for the F.D.A., said the agency now intends to issue a new exemption allowing other companies to market similar tests under the same conditions.

 While some applaud the F.D.A.’s move, saying people do not always need the intervention of medical professionals and genetic counselors to learn their risk for certain diseases, others worry that it sets a dangerous precedent.

People often need genetic counseling before deciding they want to know if they are at risk for Parkinson’s disease, said James Beck, chief scientific officer of the Parkinson’s Foundation.

“Once you get the test and read the results, there is no going back,” Dr. Beck said.

But Dr. Robert Green, a genetics professor at Harvard Medical School, said that most people who want to get such information on their own are able to handle it just fine. He has studied tests for Alzheimer’s risk in rigorous studies, asking if patients who simply got their results, without counseling, understood what they were doing and were able to handle the information. The vast majority were, he said.

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