It is known that men and women differ in obvious and less obvious ways, as in the prevalence of certain diseases, along with reactions to drugs. But how are these connected to gender? The researchers from the Weizmann Institute of Science seemed to recently uncover thousands of human genes that are expressed differently in the two sexes. According to the findings, harmful mutations that occur in these particular genes tend to accumulate in the population in relatively high frequencies. The detailed map of these genes provides evidence that males and females undergo a separate, but at the same time, interconnected evolution.

Several years ago, two scientists asked why the prevalence of certain human diseases is common. On the basis of this idea, Professor Shmuel Pietrokovski, along with Dr. Moran Gershoni, from the Molecular Genetics Department pertaining to the Weizmann Institute, showed that mutations in genes specific to sperm formation persist precisely because the genes are expressed only in men. A mutation that is problematic for only half of the population is freely passed on to the next generation by the other half.

For the first time in the field of science, the researchers expanded their analyses, identifying these particular genes through the GTEx Project, which is a very large study of the human gene expression recorded in numerous organs and tissues in the bodies of close to 550 adult donors. The project had the ability to provide a comprehensive mapping of human sex-differential genetic architecture.

At the end, 6,500 genes were identified as having a biased activity toward one sex or the other in at least one tissue. Genes were found to be highly expressed in the skin of men relative to that in the women’s skin, as they realized that these were related to the growth of body hair. The gene expression for muscle building is known to be higher in men, while that for fat storage is higher in women.

Other differences were found between the functions of the heart in men and women, along with the capacity of the body to uptake calcium, hence the women are prone to heart disease and osteoporosis as they age. On the other hand, the female genes are more protective with their brains, making them less likely to suffer from Parkinson’s disease, as this has a very high prevalence, along with an earlier onset on men.

Pietrokovski concluded that “The basic genome is nearly the same in all of us, but it is utilized differently across the body and among individuals. Thus, when it comes to the differences between the sexes, we see that evolution often works on the level of gene expression. Paradoxically, sex-linked genes are those in which harmful mutations are more likely to be passed down, to include those impairing fertility. From this vantage point, men and women undergo different selection pressures, and, at least to some extent, human evolution should be viewed as co-evolution. But the study also emphasizes the need for a better understanding of the differences between men and women in the genes that cause disease or respond to treatments.”