Obesity, metabolic syndrome, autoimmune disease, Alzheimer’s, even aging differs by sex, Dr. Arnold noted. Twenty years ago, he said, scientists didn’t think that sex chromosomes played any role at all in causing sex differences in these diseases. “But now we know it makes a difference in mice, so we can say: Where does it make a difference in humans?” he said.

A better understanding of the role of sex in disease would eventually enable better treatments, he said. “ That’s kind of the hope — that sex differences are not only important to understanding diseases in men and women, but also to developing a more fundamental understanding of the disease processes, so that you can manipulate them,” Dr. Arnold said.

In most cases, losing a chromosome or having an extra one is lethal, said Jeannie Lee, a geneticist and expert on the X chromosome at Harvard Medical School and Massachusetts General Hospital .

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Most people have 46 chromosomes, with 23 inherited from each parent. A few chromosomes can come with an extra copy, including chromosomes 13, 18 and 21 — which is commonly called Down syndrome. Losing any chromosome other than a second sex chromosome is always lethal to a fetus.

But the sex chromosome is the only one that people can survive with just one copy, Dr. Graves said. “Girls with a single X and no Y suffer few anomalies because the second X is largely inactive anyway. After all, males have only one X,” she said.

People with anomalous numbers of sex chromosomes, such as those with Turner Syndrome, have a range of problems from virtually no issues to infertility, heart problems and cognitive impairment. About one in 2,500 girls is born with Turner Syndrome. It is also possible for people to be intersex, born with reproductive or sexual anatomy that doesn’t fit the typical definitions of female or male, which may but doesn't have to involve sex chromosomes, according to the Intersex Society of North America, an advocacy group.

It is not clear what mechanisms the body has to ensure that most men get only one Y and most women get two X chromosomes, said Karissa Sanbonmatsu, a structural biologist and principal investigator at Los Alamos National Laboratory in New Mexico. In typical females, one X is usually — but not always — turned off, she said, and some research suggests that there is a mechanism that counts how many X chromosomes are present and generally turns off all but one of them.