The genes within our chromosomes contain the code that builds our bodies. The Y chromosome, with only about 70 genes, is a specialist that fashions the male reproductive system. But the X chromosome, with nearly 1,000 genes, does much more.

“The X chromosome has the genes that go into making the brain and the immune system, the two crucial things you need to survive as a human being,” Moalem said.

Both of a female’s X chromosomes are present in all her cells. But within each cell, only one of the X’s calls the shots. Half of a woman’s cells are dominated by the X chromosome that came from her mother, half by the X contributed by her father.

“That genetic diversity is really valuable,” Moalem explained. “One of the immune system’s most important weapons is the ability to recognize a virus. Well, genes on the X chromosome are involved in viral recognition. Right away, women have two different populations of immune cells that are best at spotting invaders.