A Genetic Clue to Why Men Are Taller Than Women

Men are taller than women, by an average of about five inches. But why? It’s not a genetic inevitability — there are many species in the tree of life where females outclass males.

A new study, published on Monday in the Proceedings of the National Academy of Sciences that involved genetic data from a million people, has found a partial explanation.

It involves a gene called SHOX, which is known to be associated with height. SHOX is present on both the X chromosome — females have two X chromosomes — and the Y chromosome; males have one X and one Y.

The researchers suspected that SHOX might explain differences in male and female height, but there was a problem with that hypothesis. Since SHOX is on both the X and Y chromosomes, it would need to have a different effect on each chromosome.

Does it, the researchers asked?

To investigate the hypothesis, they asked if an extra Y chromosome boosted a person’s height more than an extra X chromosome.

There are rare conditions in which people are born with an extra X or an extra Y, or have a missing X or Y. To find people with these conditions, researchers plumbed data from three biobanks, or repositories of deidentified genetic and medical data from individuals. One biobank was from Britain, and the other two were from the United States.

With nearly a million individuals’ data in the biobanks, the group was able to find 1,225 people with either missing or extra X or Y chromosomes. Some of these conditions, like in people with one X and no Y, were known to be associated with health issues — as well as, in this case, short stature.

And, they found, an extra Y did provide more height than an extra X. Their hypothesis was borne out.

The biochemistry of the SHOX gene may be the reason.

Matthew Oetjens, a genetics researcher at Geisinger College of Health Sciences in Danville, Pa., and senior author of the study, explained: The placement of the SHOX gene, he said, is near the end of the sex chromosomes. In females, most genes on one of the two Xs are silenced, or inactive. But one region where the genes remain active is at the very tip of the X. The SHOX gene is close enough to the tip that it is not quite silenced.

In men, the X, with its SHOX, is fully active. So is the Y.

This means that a woman, with her two X chromosomes, will have a slightly lower dose of the SHOX gene than a man, with an X and a Y.

As a result, males get a slightly bigger SHOX gene effect. That, the researchers calculated, accounts for nearly a quarter of the average difference in height between men and women. Dr. Oetjens said that other features of male sex hormones account for most of the rest of the difference, and other genetic factors are thought to play a role.

The work is “definitely cool,” said Eric Schadt, a professor in the department of genetics and genomic science at Mount Sinai School of Medicine in New York City.

“It is a great use of these biobanks to uncover what is still somewhat of a mystery,” he said. “Even though the effect is modest, it does explain 20 percent or so of the height difference.”