Scientists Use 'The X Files' to Reveal Why Women Are Shorter Than Men

Scientists from the University of Helsinki have revealed a theory behind the different in height between men and women.

Researchers analysed a commonly occurring genetic variation in chromosome X, one of the two sex-determining chromosomes, in almost 25,000 northern European individuals.

The study, which was published in PLOS Genetics, was designed to find genetic factors that could explain individual differences in several traits, including height, BMI, blood pressure and lipid levels.

They also investigated whether the X chromosome would contribute to some of the well-known differences between men and women in certain traits.

According to the team, the X chromosome has been neglected in most of the previous studies where genetic variants have already been identified.

Dr. Taru Tukiainen, who is currently working at the Massachusetts General Hospital in Boston, said: "Studying the X chromosome has some particular challenges. The fact that women have two copies of this chromosome and men only one has to be taken into account in the analysis.

"We nevertheless wanted to take up the challenge since we had a strong belief that opening 'the X files' for research would reveal new, interesting biological insights."

The results revealed that a genetic variant close to ITM2A, a gene that has a role in cartilage development, is more common among people who are below average height.

Present in over a third of Europeans, the variant also increases the expression of ITM2A, suggesting that the more the gene is expressed the shorter the person will be. The effect of this variant on height was shown to be much more prominent in women.

Professor Samuli Ripatti, who led the study, said: "The double dose of X-chromosomal genes in women could cause problems during the development. To prevent this, there is a process by which one of the two copies of the X chromosome present in the cell is silenced.

"When we realised that the height associated variant we identified was nearby a gene that is able to escape the silencing, we were particularly excited."

According to Ripatti, the gene is expressed in higher levels in women because both copies of ITM2A remain active. He said: "Identifying associations in regions like this where X-chromosomal gene doses are not balanced between men and women can be particularly valuable in helping us to understand why some characteristics differ between sexes.

"Based on our calculations, this variant accounts for a significant, though small proportion, 1-2% of the current difference in mean height between men and women in the Finnish population."