Y chromosome

The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. Along with the X chromosome, it is part of the XY sex-determination system, in which the Y is the sex-determining because it is the presence or absence of Y chromosome that determines the male or female sex of offspring produced in sexual reproduction. In mammals, the Y chromosome contains the SRY gene, which triggers development of male gonads. The Y chromosome is passed only from male parents to male offspring.

Alleged Degeneration

There have been widespread claims in the media that the Y chromosome will be gone in 5-10 million years and with it, human males. This is just another form of casual misandry.

By one estimate, the human Y chromosome has lost 1,393 of its 1,438 original genes over the course of its existence, and linear extrapolation of this 1,393-gene loss over 300 million years gives a rate of genetic loss of 4.6 genes per million years. Continued loss of genes at the rate of 4.6 genes per million years would result in a Y chromosome with no functional genes – that is the Y chromosome would lose complete function – within the next 5-10 million years. Comparative genomic analysis reveals that many mammalian species are experiencing a similar loss of function in their heterozygous sex chromosome. Degeneration may simply be the fate of all non-recombining sex chromosomes, due to three common evolutionary forces: high mutation rate, inefficient selection, and genetic drift.

With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest-evolving parts of the human genome. However, these changes have been limited to non-coding sequences and comparisons of the human and chimpanzee Y chromosomes (first published in 2005) show that the human Y chromosome has not lost any genes since the divergence of humans and chimpanzees between 6–7 million years ago. Additionally, a scientific report in 2012 stated that only one gene had been lost since humans diverged from the rhesus macaque 25 million years ago. These facts provide direct evidence that the linear extrapolation model is flawed and suggest that the current human Y chromosome is either no longer shrinking or is shrinking at a much slower rate than the 4.6 genes per million years estimated by the linear extrapolation model.

In any case, even if the Y chromosome were to lose all gene function and disappear, sex selection in humans could occur as a result of some other mechanism. Perhaps another pair of chromosomes would differentiate to replace X and Y.

Even the misandric BBC noted:

"It's a very nice piece of work, showing that gene loss in the male-specific region of the Y chromosome proceeds rapidly at first - exponentially in fact - but then reaches a point at which purifying selection brings this process to a halt."

Health

Mosaic Loss

Males can lose the Y chromosome in a subset of cells, known as mosaic loss. Mosaic loss is strongly associated with age, and smoking is another important risk factor for mosaic loss.

Mosaic loss may be related to health outcomes, indicating that the Y chromosome plays important roles outside of sex determination. Males with a higher percentage of hematopoietic stem cells lacking the Y chromosome have a higher risk of certain cancers and have a shorter life expectancy. In many cases, a cause and effect relationship between the Y chromosome and health outcomes has not been determined, and some propose loss of the Y chromosome could be a "neutral karyotype related to normal aging". However, a 2022 study showed that mosaic loss of the Y chromosome causally contributes to fibrosis, heart risks, and mortality.

Further studies are needed to understand how mosaic Y chromosome loss may contribute to other sex differences in health outcomes, such as how male smokers have between 1.5 and 2 times the risk of non-respiratory cancers as female smokers. Potential countermeasures identified so far include not smoking or stopping smoking and at least one potential drug that "may help counteract the harmful effects of the chromosome loss" is under investigation.

XX Male Syndrome

XX male syndrome occurs due to a genetic recombination in the formation of the male gametes, causing the SRY portion of the Y chromosome to move to the X chromosome.[8] When such an X chromosome is present in a zygote, male gonads develop because of the SRY gene.[8]

Brain Function

Research is currently investigating whether male-pattern neural development is a direct consequence of Y-chromosome-related gene expression or an indirect result of Y-chromosome-related androgenic hormone production.

Women

In 1974, male chromosomes were discovered in fetal cells in the blood circulation of women.

In 1996, it was found that male fetal progenitor cells could persist postpartum in the maternal blood stream for as long as 27 years.

A 2004 study at the Fred Hutchinson Cancer Research Center, Seattle, investigated the origin of male chromosomes found in the peripheral blood of women who had not had male progeny. A total of 120 subjects (women who had never had sons) were investigated, and it was found that 21% of them had male DNA. The subjects were categorised into four groups based on their case histories:

• Group A (8%) had had only female progeny
• Group B (22%) had a history of one or more miscarriages
• Group C (57%) had their pregnancies medically terminated
• Group D (10%) had never knowingly been pregnant before

The study suggests that possible reasons for occurrence of male chromosome microchimerism could be one or more of the following:

• Miscarriages
• Pregnancies
• Vanished male twins
• Sexual intercourse

A 2012 study at the same institute has detected cells with the Y chromosome in multiple areas of the brains of deceased women.

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