From always, the size differences between men and women have struck by their constancy and their magnitude through human populations. Far from being dictated only by hormones or the environment, these deviations also find their origin in a more discreet but formidably effective genetic mechanics. At the center of this architecture, a gene involved in bone growth reveals how the same DNA sequence, present in both sexes, can produce deeply different effects depending on whether it is expressed on an X or Y chromosome.
This gene is found in the pseudo-autosomal region of sex chromosomes x and y-an area that both sexes share. However, its expression is not equivalent. In women, one of the two X chromosomes is partially deactivated, which reduces the effect of the Shox gene. In men, on the other hand, the gene is fully expressed both on the X and on the Y.
This genetic expression imbalance, although subtle, produces visible consequences. By compiling the data of nearly a million individuals from three major biobanks (British and American), a team led by Matthew Oetjens, the Geisinger College of Health Sciences, has shown that this genetic asymmetry explains up to 22% of the average size of the sexes, as reported by the New York Times.
The differences of size between men and women seen through chromosomal anomalies
To understand the influence of sexual chromosomes on stature, the researchers turned to rare but revealing cases: anomalies in the number of X or Y chromosomes, called sexual aneuploidies. These natural variations, although infrequent (1 in 450 people according to estimates), offer a life -size biological laboratory.
The study published in PNAS compared the sizes of 1,225 adults with these genetic variations. Some had an additional X or Y chromosome, others had one less. A significant result, a chromosome in addition increased the size more significantly than an additional X chromosome. On average, an additional chromosome saved 3.1 cm, a more marked effect than that of a second X chromosome, even active.
Syndromes like Turner (45, X) or Klinefelter (47, XXY) illustrate these differentiated effects. The first, linked to a missing X, usually leads to a smaller size. The second, which combines two x and one y, induces a high stature, but often less than in those with two Y.
The researchers observed that the Shox gene, expressed without restriction on chromosome, unlike partially inactivated X, plays a major role. But that doesn't stop there.
A partial map of the organic puzzle
If Shox weighs heavily in the scale, it is not alone in determining the size. The study underlines this. Male sex hormones, especially testosterone, also influence bone growth, especially during puberty. These hormonal effects, combined with the genetic imprint of the Y, would explain together the majority of size differences between men and women.
Other elements remain to be explored. Some genes located on sex chromosomes partially escape inactivation, such as Shox, but their exact role remains uncertain. The team of researchers also offers future investigations on the non -linear effects of the total number of sex chromosomes. After a certain threshold, additional copies no longer increase the size, even reduce it.
Finally, this research opens the way to comparisons between sexes in other medical areas. Many disorders, such as autoimmune diseases or certain neuropsychiatric disorders, have men-female disparities whose genetic origins are still poorly understood. The study of size dimorphism could well become a model.
