The Better Half: On the Genetic Superiority of Women is by Dr Sharon Moalem (male); a neuroscientist and evolutionary biologist. Its a fascinating, unexpected and thought-provoking argument that the simple fact of having two X chromosomes, instead of one X chromosome and one Y chromosome, is the secret to womens underappreciated success in the game of life.

Dr Sharon Moalem is a science author.

A quick refresher on chromosomes: among the 23 pairs of chromosomes X-shaped twists of DNA that are the encyclopaedia of us found in every human cell, are two sex chromosomes. In genetic females, these two sex chromosomes are both an X chromosome. In genetic males, one is an X chromosome and one is a Y chromosome.

We inherit one sex chromosome from our father and one from our mother; genetic females inherit one X sex chromosome from each parent, and genetic males inherit an X sex chromosome from the mother, and a Y sex chromosome from their father.

The X chromosome is the genetic powerhouse of the sex chromosomes, containing more than 1000 genes that orchestrate a huge number of vital cellular processes. In contrast, the Y chromosome is a stunted thing that only carries about 70 genes, most of which are involved in the production of sperm.

In genetic females, only one of their two X chromosomes is needed, so the second X chromosome is deactivated or silenced when that person is merely a bundle of cells in the uterus. The silenced X chromosome gets condensed down into a bit of cellular debris called a Barr body.

For a long time, that second, silenced X chromosome was assumed to be dead. But it turns out that second X chromosome in the cells of genetic females is actually a genetic back-up plan, helping the cell and the person to survive by throwing a genetic lifeline when things get tough. Far from being inert, about 23 per cent of those thousand or so genes on the silenced X chromosome are still active.

Dr Sharon Moalem on women having an extra X chromosome: Its like having two toolboxes. One toolbox may have a broken hammer, so you use the hammer from the second box."Credit:Getty Images

Moalem argues that this back-up set of genes gives women a significant survival advantage, as evidenced by the fact that women consistently outlive men, even in times of hardship.

Having the use of two X chromosomes makes females more genetically diverse, and the ability to rely on that diverse genetic knowledge is why females always come out on top, he writes.

This advantage is particularly evident with the immune system. Moalem recalls his time tending to HIV-positive children at an orphanage in Bangkok, and his observation that the HIV-positive boys were consistently more likely to get sick with opportunistic infections than the HIV-positive girls.

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He goes on to note that HIV-positive men are also more likely than HIV-positive women to develop tuberculosis and pneumonia, while HIV-positive women tend to have higher immune-cell counts a sign of immunological strength in the early stages of HIV infection than men do.

The X chromosome carries a large number of genes involved in immune system functioning. Moalem argues that because women have two copies of the X chromosome, they are able to produce a more diverse and effective population of immune cells than if they relied on the immune genes of only one X chromosome, as men do.

But there is a price for that more aggressive immune response; sometimes it goes overboard and starts overreacting to benign things, such as our own cells. This is the phenomenon of autoimmunity, and it disproportionally affects women.

If a microbe is the wolf, and its dressing up like Grandma, better trying to kill Grandma every once in a while than to risk being fooled by a wolf dressed like Grandma, he explains.

Having two X chromosomes also offers an unparalleled advantage if it happens that a gene on one of those chromosomes is dangerously mutated.

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Say you inherit a malfunctioning gene on the X chromosome from your mother that might be associated with developmental problems. If you also have inherited an X chromosome from your father that carries a functional copy of that gene, you have a back-up, an understudy, for that faulty gene. But if you inherit a Y chromosome from your father, youre stuck with the faulty one.

This is why so-called X-linked intellectual disabilities almost entirely affect genetic males; more than 100 genes associated with intellectual disabilities have been found on the X chromosome.

Moalem also highlights a problem that numerous female authors before him have also drawn attention to: that medical science and medicine still view women as being biologically the same as men. That persistent ignorance one might even call it wilful denialism has had some devastating consequences.

Women with autoimmune conditions have long had their symptoms dismissed or trivialised by the medical establishment, which was working on the assumption that these diseases were equally prevalent in men and women.

Not that that lack of understanding has slowed females down too much. As Moalem points out, theres only one way to judge the winner in the genetic battle of the sexes: The real test of ones mettle is being able to survive the challenges of life, he writes. So, who is left standing at the end of life?

Thats right. Women.

Bianca Nogrady is the editor of The Best Australian Science Writing 2019 (NewSouth).

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Immunity and our DNA: Why women are the stronger sex - The Age