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Genetics Sexual-Reproductive

The Shrinking Y Chromosome

2 years, 9 months ago

12495  0
Posted on Aug 14, 2020, 1 p.m.

The Y chromosome is a symbol of masculinity, however, it is becoming clear that it is not strong and enduring. This chromosome carries the master switch gene that determines whether an embryo will develop as a male (XY) or female (XX), it contains very few other genes, and is the only chromosome not necessary for life. 

This chromosome appears to be degenerating rapidly, leaving females with two perfectly normal X chromosomes but males have an X and shriveled Y. If this rate of degeneration continues on this course it could vanish as it is estimated to have around 4.6m years before it is completely gone. This may sound like a lot of time, but when you look at the full picture life has only existed on Earth for around 3.5 billion years. 

The Y chromosome wasn’t always like this, originally it was the same size as the X chromosome and contained all of the same genes. But unlike other chromosomes the Y has the fundamental flaw of only ever being present as a single copy which is passed on from fathers to their sons. This means that genes on the Y chromosomes can’t undergo genetic recombination of genes that occur in each generation to help eliminate damaging gene mutation. The Y chromosomal genes have been degenerating over time due to being deprived of the benefits of recombination, and are eventually going to be lost from the genome. 

Recent research suggests that the Y chromosome has developed some mechanisms that might slow down the rate of gene loss to a possible standstill. A study published in PLoS Genetics sequenced portions of the Y chromosome from 62 men and found that it is prone to large scale structural rearrangements allowing for gene amplification. The study also suggests that the chromosome has developed unusual palindrome structures which should protect it from further degradation. The researchers recorded a high rate of gene conversion events within these palindromic sequences that basically allowed damaged genes to be repaired using an undamaged copy as a template. 

A growing body of evidence suggests that Y-chromosome gene amplification might be a general principle across the board when looking into other species; and these amplified genes play important roles in sperm production and in regulating offspring sex ratio. A study published in Molecular Biology and Evolution suggested that this increase in gene copy number in mice is a result of natural selection. 

The scientific community appears to be divided on the question of whether this chromosome will eventually vanish, some say that the defense mechanisms are doing a good job and have rescued the Y chromosome, while others say that all this is doing is buying the chromosome more time before it eventually vanishes. This is an ongoing debate that can understandably get fairly heated. 

Proponents of the vanish argument claim that over the long term perspective the Y chromosome is inevitably doomed, even if they manage to stay around a bit longer than expected. Japanese spiny rats and mole voles have already lost their Y chromosomes entirely. Some argue that the process of genes being lost/created on the Y chromosome inevitably lead to fertility issues, which in turn can drive the formation of entirely new species. 

Should the Y chromosome disappear in humans, this does not necessarily mean that males are on their way out. Even if the chromosome is lost completely both males and females are still necessary for reproduction. In this case the master switch gene that determines genetic maleness has moved to a different chromosome. The catch is that the new sex determining chromosome that SRY moves to should start the process of degeneration all over aging due to the same lack of recombination that created the problem to begin with. 

In humans while the Y is needed for normal reproduction many of the genes it carries are not necessary when using assisted reproduction techniques, meaning that genetic engineering may be able to replace the gene function of the Y chromosome. Even if all humans are able to conceive this way it is unlikely that all fertile humans will want to stop reproducing naturally. 

This does make for interesting conversation in the area of genetic research, the case of the disappearing Y chromosome is still up for debate as scientists do not know for certain if it will disappear or not. Even if it does, men will likely still be needed so that normal reproduction can continue, in any event there will most certainly be more pressing concerns over the next 4.6 million years. 

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