Although it has been known that different genes determine different characteristics in male and females, new research suggests that differences in gene expression has a much larger impact than previously thought.

Many diseases and traits show difference between the sexes. For example, autism is more common in males than females, but multiple sclerosis is more common in females. Males species are nearly always larger and taller than their female counterparts. Previously it was thought that these differences were due to the different genes male specific Y chromosome and the female specific X chromosome. However, new research has revealed that it might be due to the same genes on autosomal chromosomes being differently expressed between the sexes.

Researchers studied RNA to determine the genome wide gene expression of females and males of four mammal species; mouse, rat, dog and macaque. The species shared a common ancestor 100 million years ago and represent mammals at different stages of the evolutionary tree, so scientists could investigate if sex expression changed with evolution.

It was found that some genes were the same in both males and females but were expressed differently, known as sex bias. The scientists estimated that 12% of the height difference between men and women was determined by the differently expressed genes. The research reveals that differently expressed genes can result in different phenotypes in males and females.

The evolutionary analysis shows that most of the genes evolved their sex bias recently. The genes that showed sex bias in each species were mostly specific to that species.

However, researchers stress that even if sex expressions varies with mammal species, human tests would be needed to confirm sex expression differences in humans. This research could help understand genetic drivers for certain diseases and ensure treatments that are more targeted to males and females.

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