(This article concentrates on the typical XX (female) and XY (male), and ignores the sex chromosome abnormalities of loss, damage or addition of one or both sex chromosomes, which cause a number of developmental syndromes.)
In humans, sex is largely determined by chromosomes that carry tightly coiled DNA that carry our genes. The cells of most women possess two X chromosomes, while most men have one X and one Y chromosome. For women, one of the X chromosomes is mainly switched off, and the one that remains active seems to be chosen at random during the first few weeks of pregnancy. So half of a woman's cells generally use the X chromosome inherited from her mother, and the other half the one from her father. If one X has a harmful mutation, cells that use the other X can compensate. This explains why it is rare for women to be colour-blind, while around 8% of men cannot see a difference between red and green, and some are totally colour-blind. Almost all women can see 1m colours, but 5-15% of women have a visual superpower and are able to see 100m colours.
(Men are genetically programmed to have greater muscle power, lift heavier weights and run faster. More female embryos are lost during the early stages of pregnancy but are more likely to survive birth and onwards; more males die at and soon after birth.)
The two X populations cooperate and share not just genetic material with one another but also proteins and enzymes, which give an extra ability to handle disease. Even in tissues where the ratio of the Xs is 50:50, that can change. With a skin injury, one population of X cells might take over the healing process, but the ratio will drop back to 50:50 once healing is complete. If the heart cells carrying paternal X survive the lack of oxygen, it will be that population that survives; that benefit can occur in other organs such as the liver and kidneys.
The X chromosome is rich in immune-related genes. This means that women's bodies can cell population shift to the more effective X for an illness. One important group of genes on X helps cells recognise single-stranded RNA viruses,such as the coronavirus. Woman therefore have twice the options of recognising such a virus. The gene for the ACE2 protein is also on the X.
A further advantage for women is that the male hormone testosterone supresses the immune system, while the female hormone oestrogen makes it more aggressive. Although after the menopause, oestrogen levels decline, female cells still tend to be more critical and powerful in defending against a possible attack.
The downside to XX is that women have higher rates of autoimmune conditions. Female hormones (oestrogens) stimulate the immune system, while male testosterone inhibits it, but even before puberty, autoimmunity is still higher in females.
In both sexes, in the fetus, the thyroid gland immune cells go through an "educational process"; if immune cells recognise other cells from the person's body, the immune cells self-destruct. This happens so that once the baby is born, autoimmunity does not happen. This works well in males, but in females, who have two genetically different cell populations, it is more likely that not all self-recognising cells are destroyed.
On the other hand, having a somewhat autoreactive immune system means XX can more easily detect microbes that resemble human cells. It may also explain why XX have a lower chance of certain cancers; women may be clearing pre-malignant cells before they are detectable medically. This pattern seems to hold true even with cancers that start before puberty, although higher rates of smoking and drinking in males does have an effect.
This sex difference is increasingly recognised but public and health professionals alike are accustomed to attributing the differences to behavioural issues. And it is difficult to understand the difference between biological strength and physical strength, and that a five-foot-three woman has a survival advantage over a six-foot male Olympic weight-lifter. There are also implications for treatments as women (i.e. XX) have different sensitivity to medications.
Moalem believes that having two versions of X explains why women are less vulnerable to certain congenital disorders and better at fighting off infections, including the coronavirus.
- Supercentenarians (over 110) are 95% female.
- More girls than boys reach their first birthday.
- Girls who have asphixia at birth do better cognitively than boys.
- In general, drugs are metabolised slower by XX, and gut transit time is slower, so XX have to wait longer after eating to have an empty stomach.
- Melanoma kills twice as many young men as women, because XX benefit from stronger immune response and female sex hormones, while XY are not necessarily using less sun screen or not going to their doctor.
Source: The stronger sex by Sharon Moalem, in New Scientist, 1 August 2020.
Source: Feature in Good Housekeeping, August 2020.
Source: Feature in Good Housekeeping, August 2020.
