Ask Marilyn

You once wrote about a woman with a 50 percent chance of inheriting a rare terminal illness. She can be tested to learn whether she has the gene. If the test is positive, and she has the gene, she doesn’t want to know. But if the test is negative, she does want to know. You said this can be accomplished. Can you explain? —Donnelle Macho, Woodbridge, Va.

The woman would need to arrange with a doctor to take the test with these conditions:

After the test, the doctor would privately flip a coin marked “positive” and “negative.” If both the test and the coin flip are negative, the doctor would tell her the test result.

But if either (or both) result is positive, the woman would not be told anything. So she wouldn’t know whether the test was positive or if it was the coin flip that was positive.

If the woman wished, she could ask the doctor to flip the coin a second time. Again, if both results are negative, she would hear the test result. But if either (or both) result is positive, she would hear nothing.

As the years go by, the woman can ask the doctor to flip the coin as many times as she wishes. If the test result was negative, she will be likely to hear it before long. But if she wants to stop at three or four flips, she can. Regardless, if the test result was positive, she will never know for sure.