In humans, the main characters that play a role in distinguishing individual genders are the sex chromosomes: the X and Y chromosomes. Humans are diploid organisms, and their chromosomes exist in pairs, each pair of chromosomes being twin sisters with almost no differences in shape. But the X and Y pair of sex chromosomes is a bit of a departure: according to the pattern of chromosome numbering from largest to smallest, the X chromosome should sit in the 8th chair of the 23 pairs of chromosome members, but its partner, the Y chromosome, is only a third of its size, and even a little smaller than the smallest, chromosome 22. If you are a female and your sex chromosome is a pair of XX, then everything is normal and in the replication and reproduction of female cells, the X chromosome behaves essentially the same as the other 22 pairs of autosomes: the same two complementary pairings, recombination and exchange, you in me and me in you, acting in a common direction towards the elimination of differences between each other. But once the Y chromosome joins the camp of genetic information, and the sex chromosomes become a pair of XY, everything changes. The Y chromosome is too short to make a complete pairing and recombination swap with its partner, the X chromosome, so it has to bend down to form a circle, and then make a small pairing and recombination swap with the small Y chromosome at the top. Such an exchange is not enough to maintain the long-term stability of the Y chromosome, because in the law of chromosome inheritance, if no recombination and interchange occurs, it means the road to extinction. So how does the Y chromosome maintain its long-term stability? The secret was not uncovered until 2003, when scientists discovered that the Y chromosome is unique in its ability to undergo recombination and swapping on its own. In addition to differences in appearance and behavior, there are also huge differences in the types and numbers of genes carried. For example, the Y chromosome carries the SRY gene, a key gene that initiates male development, but not the X chromosome, which can carry 2,000 to 3,000 genes, while the poor Y chromosome can only carry 20 to 30 genes. Moreover, the nucleotide sequences on the Y chromosome simply appear to be a mountain of meaningless junk, making it difficult to find genetic treasures. Such characteristics once stalled the human genome sequencing project. Could such huge differences in sex chromosomes between female and male cells cause genetic disorders? Life is never a highly balanced body, and female cells select one of the X chromosomes to curl up in an ordinary living cell in order to maintain the same dose of genes on the X chromosome as in males, like putting unused clothes into a closet and putting a lock on it to seal it. This highly curled up and inactive X chromosome is known as a “Barr’s vesicle”. A painful evolutionary journey The Y chromosome did not exist from the beginning, but was the product of a genetic mutation. One day, 300 million years ago, a gene called SOX3 on one of the original sex chromosomes mutated and became a gene called SRY. The SRY gene is the key gene on the modern Y chromosome that determines male sex. How did scientists deduce that this event occurred 300 million years ago? Animal taxonomy has made a major contribution to this. The sex of reptiles is not determined by sex chromosomes; they are usually determined by their environment. For example, turtles and crocodiles develop as males on sunny water beaches; they develop as females in cool, backward environments. A little more advanced than reptiles are the monotremes of mammals, such as the famous platypus and echidna. Monotremes are the oldest animals with Y chromosomes, and their sex is no longer simply determined by their environment. And the branching evolution of mammals and reptiles occurred 300 million years ago, so scientists inferred that the Y chromosome appeared in that era. Then applying the molecular evolutionary clock in molecular biology to estimate, it was found that the SOX3 and SRY genes on the X and Y chromosomes also had a 300 million year history of branching. After the gene of SOX3 mutated into the gene of SRY, another chromosome inversion event occurred, so that the SOX3 gene (at the bottom of the X chromosome) and the SRY gene (at the top of the Y chromosome), which were originally from the same root, were separated by a river and died. Chromosomal inversions are a more common way in which reorganization of internal segments of chromosomes occurs, and this behavior often results in a rearrangement of the location of genes on the chromosome. Unlike the X chromosome, inverted genes on the Y chromosome can be repaired by twin sisters in female cells, so these inverted segments are often removed by the Y chromosome itself, and as a result, with no backup to compensate, the poor Y chromosome loses more and more genes and becomes more and more shrunken, to the point where today it is only a third of the size of the X chromosome. Today, it is only one-third of the X chromosome. After the emergence of the Y chromosome in our platypus ancestors, the Y chromosome, which insists on walking a one-way street, has been subject to various genetic inversions and deletions, making the genes on the Y chromosome increasingly distinct. In human males, only the two ends of the Y chromosome retain the ability to recombine with the X chromosome, whereas in females, the two genetically homogeneous X chromosomes remain fully recombined at all sites. Thus, after more than 300 million years of evolution from primitive sex chromosomes with no differences, the Y chromosome has undergone a dramatic structural change and has become more functionally specialized as the trigger that initiates male development. A long conflict Interestingly, the Y chromosome and the X chromosome, created by genetic mutation, are not a match made in heaven, but more like a pair of mutually incompatible rivals. The root of the trouble between X and Y was first planted 300 million years ago. At that time, a mutation in one gene prevented the exchange of material between the same size X and Y chromosomes. The more mutations they each had, the greater the difference due to the loss of exchange, as if one gradually became a wet southern water country while the other became a vast northern dryland due to dryness. In this case, a gene with the same function on the Y chromosome of the “water country” can be used to create a hard and developed male head and horns with calcium, while on the X chromosome of the “dry land” it is used to create nutritious milk. This is how sex differences begin to form, and in order to further enhance their respective sexes, the X and Y chromosomes recruit more genes that favor sex accentuation, eventually forming a genetic home base for the maintenance of sexual characteristics. Genes on the Y chromosome that are beneficial to males are often detrimental to females; conversely, genes on the X chromosome that are beneficial to females always attempt to destroy male traits. For example, a recently discovered gene on the X chromosome, DAX, and the Y chromosome, SRY, an important switch gene for male sexual organ development and sperm production, are feuding rivals. In normal male cells, one DAX gene gives way to one SRY gene, and male characteristics appear normally, but in a very small number of genetically abnormal male cells, they have two DAX genes on the X chromosome, and then the DAX gene attacks the SRY gene, and easily defeats the SRY gene, so that the person looks exactly like a woman even though the cells are still XY type. In living organisms, the war machine starts as soon as it is created. The struggle between sex chromosomes is sometimes simply to put each other to death, never to be tolerated. Because females have two X chromosomes and males have one X and one Y, when the two pairs of chromosomes meet, three quarters are X and only one quarter are Y. Thus, the X chromosome is three times more likely to evolve the ability to attack the Y chromosome than the Y chromosome is to evolve the ability to attack the X. All genes on the Y chromosome can be attacked by an endless stream of X genes, and eventually the Y The Y chromosome loses its armor and has to “turn off” or remove most of the genes that have been targeted. This is the deeper reason why the Y chromosome is getting shorter and shorter and more and more “silent”! This prolonged war has made the Y chromosome so shrunken and so genetically inactive that biologists were once worried about its seemingly inevitable demise. The world’s leading expert on the Y chromosome, Weil Weigel, was a great success. The world’s leading expert on the Y chromosome, David Page, wrote the words “Save the male” on his teacup in those foreboding days. However, what scientists didn’t expect was that the Y chromosome has an extremely resilient side. The Y chromosome is indeed the shortest of all human chromosome “books”, but its contents are not easily grasped by humans. In the early days of the human genome sequencing project, scientists found that the middle section of the Y chromosome had too many duplicate genes and inert genes without any physiological function, and it was a maddening task to identify a specific gene from the piece. A scientist working on the Y chromosome said graphically that analyzing the genes on the Y chromosome is like walking into a room made of mirrors, with an identical mirror image everywhere, and you can’t even tell the ceiling from the floor. It was only in 2003 that scientists figured out that the large number of “shadow” genes on the Y chromosome were actually the so-called palindromic structure of the genetic code. For example, ABCDEFG and GFEDCBA constitute a pair of palindromic structures, which can also be said to be mirror images of each other. The mirrors that once frustrated scientists have become the gateway to the mysteries of the Y chromosome. These indistinguishable “shadow” genes have contributed greatly to ensuring that the weak Y chromosome does not continue to degenerate for billions of years. While all other chromosomes have the ability to pair and recombine with their twin partners, the Y chromosome can only be repaired and preserved by recombining and swapping with its partner, the X chromosome, at both ends. So how does the middle segment of the Y chromosome maintain its long-term stability? It turns out that by relying on these palindromes, the tiny Y chromosome can bend up from the middle, allowing itself to pair up with its reciprocal palindromes and undergo recombination and interchange, so that if a gene in the middle is unfortunately mutated, it can be repaired from its opposite palindromic backup. What a maverick Y chromosome that, in the absence of a twin partner, has managed to save its own life by being self-reliant! Saved by cooperation However, the escalation of genetic rivalry is a dangerous thing. It is easy to understand how the interests of a collective will be compromised if its members do not cooperate in a tacit way. A prolonged genetic war is bound to decimate the survival of the human body, the victim of the war. If all the genes on the X chromosome tried to kill the sperm containing the Y chromosome, there would be no more men in the human race, and eventually the genes on the X chromosome would lose their chance to reproduce. So the war between X and Y, which began hundreds of millions of years ago, gradually subsided into a very sensible and subtle game – a game of “selective purging” discovered by William Amos and others. Over the years, after the Y chromosome had largely succumbed, the X chromosome would only occasionally sweep the Y chromosome for genes that were identical to its own, while the X chromosome never bothered with rare genes such as SRY, which controls masculinity in normal cells (but occasionally the X chromosome would show its true colors and attack the SRY gene when it was abnormal, as in the case of the aforementioned X chromosome with two copies of the DAX gene). Those genes that perform the “sweeping” function are the genes that drive the feminization of the species, and without the balancing effect of the SRY genes to pull back towards masculinity, the human sex would eventually be out of balance. Of course, one should ask: Why are the same interchangeable genes present on both ends of the Y chromosome as on the X chromosome not “cleaved”? The answer is simple: these genes – about 5 in number – are not sex-determining specific genes, and their function is only to maintain the basic life activities of the cell, i.e. some “housekeeping genes “, so there seems to be no need for scavenging. In a sense, the hundreds of millions of years-long war between the X and Y chromosomes ended in a compromise that turned the Y chromosome into the weakest chromosome. But it is incredible that just this seemingly pathetic little guy has become the dominant force in human society today.