Investigating the reasons why certain physical traits between men and women differ, researchers at the University of Helsinki in Finland turned their attention to the sex chromosome shared by men and women - chromosome X - and how it might influence height.
Chromosomes are DNA-carrying structures in the nuclei of human cells. The sex chromosomes - X and Y - determine whether an embryo becomes male or female.
The mother contributes an X chromosome to the child, and it is the chromosome from the father - which can be an X or a Y - that determines whether the child is a girl or a boy.
In each human cell, men have one X chromosome and one Y chromosome, while women have two X chromosomes. The X chromosome includes about 155 million DNA building blocks and represents 5% of the total DNA in cells.
"Studying the X chromosome has some particular challenges," admits study author Dr. Taru Tukiainen.
"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," she adds.
'Silent' X chromosomes
Dr. Tukiainen and colleagues found that a genetic variant with a role in cartridge development is more common among people who are shorter than average. The researchers also found that the effect of this gene was much stronger in women.
But why was this gene - which is associated with a gene called "ITM2A" - more powerful in women? Here's where the X chromosome comes in.
ITM2A is a chromosome X gene. Normally, one of the X chromosomes in female cells is randomly and permanently inactivated - except in egg cells. But the researchers found that ITM2A could escape this "silencing" of one of the X chromosomes.
This means that two copies of the gene could remain active in each cell in women - unlike men, who, with just one X chromosome, would only have one copy of the gene per cell.
Co-author Prof. Samuli Ripatti explains:
"We were particularly excited. Because both copies of ITM2A remain active, the gene is expressed in higher levels in women. 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," he adds, "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."
Because the X chromosome is such an important source of DNA, the researchers hope that their work, which is published in the journal PLOS Genetics, provides motivation for future researchers to further examine this chromosome and how it influences other biological differences between men and women.
They think that such research will give scientists a better understanding of why some diseases are more prevalent in one sex than the other.
Last year, Medical News Today reported on a study that found the female version of the X chromosome has evolved to play a role in sperm production.