The Y chromosome may be a symbol of masculinity, but it is becoming increasingly clear that it is anything but strong and enduring. After 12 weeks after conception, the master genes switch on others that regulate the development of the testis. The embryo makes male hormones that ensure the baby develops as a boy.
Although it carries the “master switch” gene, SRY, that determines whether an embryo will develop as male (XY) or female (XX), it contains very few other genes and is the only chromosome not necessary for life. Women, after all, manage just fine without one.
What’s more, the Y chromosome has degenerated rapidly, leaving females with two perfectly normal X chromosomes, but males with an X and a shriveled Y. If the same rate of degeneration continues, the Y chromosome has just 4.6 million years left before it disappears completely.
If we go back 166 million years ago, to the very first mammals, the story was completely different. The early “proto-Y” chromosome was originally the same size as the X chromosome and contained all the same genes. However, Y chromosomes have a fundamental flaw.
Unlike all other chromosomes, which we have two copies of in each of our cells, Y chromosomes are only ever present as a single copy, passed from fathers to their sons. This means that genes on the Y chromosome cannot undergo genetic recombination, the “shuffling” of genes that occurs in each generation which helps to eliminate damaging gene mutations. Deprived of the benefits of recombination, Y chromosomal genes degenerate over time and are eventually lost from the genome. Despite this, recent research has shown that the Y chromosome has developed some pretty convincing mechanisms to “put the brakes on”, slowing the rate of gene loss to a possible standstill.
There is division in the scientific community on whether the Y chromosome will completely disappear. Some argue that its defense mechanisms do a great job and have rescued the Y chromosome. But others say that all they are doing is allowing the Y chromosome to cling on by its fingernails, before eventually dropping off the cliff. The debate continues.
To reproduce we need sperm and men, which means that the end of the Y chromosome could herald the extinction of the human race. The new finding supports an alternative possibility – that humans could develop a new sex-determining gene.
However, the evolution of a new sex-determining gene carries risks. What if more than one new system develops in different parts of the world? A “war” of the sex genes could lead to the separation of new species, which is exactly what happened with mole voles and spiny rats. So if someone were to visit Earth 11 million years from now, they wouldn’t be able to find humans — or different human species, kept separate by their different sexing systems.
Footnotes
- PLOS Genetics is a peer-reviewed open-access scientific journal established in 2005 and published by the Public Library of Science. The founding editor-in-chief was Wayne N. Frankel (Columbia University Medical Center). The current editors-in-chief are Gregory S. Barsh (HudsonAlpha Institute of Biotechnology and Stanford University School of Medicine) and Gregory P. Copenhaver (The University of North Carolina at Chapel Hill). The journal covers research on all aspects of genetics and genomics. [Back]
- The northern mole vole (Ellobius talpinus) is a species of rodent in the family Cricetidae. It is distributed over large parts of Eastern Europe and Asia. This vole is found in Kazakhstan, Turkmenistan, Kyrgyzstan, Uzbekistan, and Ukraine, the southern parts of Russia, western Siberia, northern Afghanistan, Mongolia, and northern China. The northern mole vole is a small mammal about 5.1 in long with a short tail, weighing up to 0.1543236 lbs. The females are slightly larger than the males. The body is wedge-shaped, the head flat, the neck short and the musculature of the forelimbs strongly developed. It has short, dense, brownish fur somewhat paler on the underparts. The feet are naked and pink. It is adapted to live underground, digging its burrows with the help of its large incisors. [Back]
- The Okinawa spiny rat, also known as Muennik’s spiny rat, is an extremely rare species found in temperate forests on the island of Okinawa, Japan.01 It has grooved spines protruding from its short, thick body fur and shares its genus Tokudaia with two other species that have lost their Y chromosome and SRY gene due to abnormally large sex chromosomes. [Back]
Further Reading
Sources
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