No matter where you travel around the world, men in any given human population tend to be taller than women. Now researchers have uncovered a key genetic mechanism behind this anatomical contrast.
Combing through three large public health databases, a team led by scientists from the Geisinger College of Health Sciences in Pennsylvania found 1,225 adults with unusual chromosome combinations.
The combinations were statistically modeled against each adult’s height, revealing an intriguing difference in the influence of a sequence found on both the X and the Y chromosomes known as the SHOX (short-stature homeobox) gene.
Unlike the X and Y chromosomes in a typical male cell, one of the two X chromosomes (known as the ‘inactive’ X chromosome, or Xi for short) in a typical female cell tends to run at a reduced capacity to avoid complications.
The data showed that the Y chromosome delivers more of a SHOX ‘effect’ than the inactive X chromosome, contributing an extra 3.1 centimeters (1.2 inches) height on average.
We’ve known for a while that the SHOX gene might be playing some role. However, this study specifically quantifies the difference between how busy it is on the Y chromosome compared to the partially muted X chromosome.
“This finding is consistent with the hypothesis that reduced SHOX expression in females results in a net difference in height between the sexes,” write the researchers in their published paper.
The researchers calculated that this difference accounts for 22.6 percent of the average height difference between XY chromosome males and XX chromosome females, alongside other factors (including the wealth of the country you live in).
Across the three databases of health data used in this study (one from the UK and two from the US), the average height difference between men and women came out to between 12.85 and 13.72 centimeters.
“These results are consistent with the hypothesis that elevated SHOX expression on the Y chromosome relative to the Xi chromosome results in taller stature among males than females, largely explaining human sexual dimorphism for height,” write the researchers.
Height is only partly down to genetics, with taller parents being more likely to have taller kids. Hormone levels, including testosterone (much more abundant in men than women), are also thought to play a part in determining how tall we get.
This research could be a useful starting point for all kinds of other future studies into differences in phenotypes (observable characteristics) between the sexes, as well as diseases and disorders that affect the sexes differently.
Conditions such as Alzheimer’s disease are more common in women, for example, and there are likely to be a complex number of related reasons why.
As with height, distinguishing between genetics and hormones would offer some helpful insights.
“Disentangling the effects of sex-related hormones from genomic variation could provide new insights into the pathogenesis of medical conditions with observed sex discrepancies, including autoimmune, cardiovascular, and neurodevelopmental psychiatric disorders,” write the researchers.
The research has been published in PNAS.
