According to foreign media reports, our sex at birth is mainly determined by the probability of X and Y chromosomes. If you have two X chromosomes, you develop ovaries. If you have an X chromosome and a Y chromosome, you develop testes. The difference between these combinations of genetic material is not just what parts of the body make us grow. There are about 1000 genes on the X chromosome, but only 45 on the Y chromosome, which is very small compared with the X chromosome, and research shows that the Y chromosome has been shrinking for years. < / P > < p > to answer these questions, let’s look at a long time ago. Our sex chromosomes are not always X and y, and the determinants of male and female sex are not 100% related to them. < / P > < p > about 100-200 million years ago, when the first mammals on earth had just evolved, they had no sex chromosomes at all. The X and Y chromosomes were the same size and structure as any other pair of chromosomes. < / P > < p > here’s to point out that animals don’t need sex chromosomes, which used to be, and still are. All our chromosomes are a combination of genes related to sex and genes that are not related to sex. The only special feature of the Y chromosome is a gene called SRY, which is the “switch” that determines whether an organism will develop testicles. Even this switch is not necessary for crocodiles and turtles, whose sex is determined by the temperature at which the embryo develops. Our mammalian ancestors may have the same trait. But one day, a gene with this “switch” function suddenly appeared in one of our ancestors’ unexplained, sex independent chromosomes, and things turned out to be what it is today: you have to have the Y chromosome to develop male reproductive organs. However, the size of Y chromosome has been shrinking ever since its appearance. Over time, genes mutate, and many of them are harmful. Chromosomes can be combined with another chromosome to avoid passing on these mutations. However, in meiosis (i.e. the process of sperm and egg formation), the paternal chromosome and maternal staining experience random cross exchange. This process breaks down the genes, making it more likely that the really useful genes will be passed on. All chromosomes go through this process, and chromosome 1 from the mother is crossed with chromosome 1 from the father. However, the Y chromosome does not have a pair of chromosomes that can be exchanged. The two X chromosomes can be recombined with each other, but the similarity between Y chromosome and X chromosome is not high enough to recombine. In addition, it is rare to have two Y chromosomes at the same time, so the Y chromosome can not recombine with another Y chromosome. < / P > < p > in the event of a harmful genetic variation, this chromosome is generally interchangeable with the paired chromosome. But Y chromosome can’t do this, so more and more harmful variation will be accumulated in Y chromosome, and these harmful variation will be eliminated gradually by natural selection. As a result, the Y chromosome gets smaller and smaller. < / P > < p > research shows that there were 1669 genes on the Y chromosome before 166 million years, which is the same as “the X chromosome at that time”. It is not difficult to calculate that assuming that the rate of gene loss of Y chromosome remains constant and stable, if 10 genes are reduced every 1 million years, the whole Y chromosome will disappear completely after 4.5 million years. < / P > < p > “always the same” is critical. Recent studies have shown that the degradation rate of Y chromosome is actually slowing down. In a study published in the journal Nature in 2005, the researchers compared the Y chromosome of humans with that of chimpanzees. In 2012, the same team sequenced the Y chromosome of rhesus monkeys and published the results in nature. Researchers have found that since humans and rhesus monkeys split on the evolutionary path 25 million years ago, only one gene has been lost on the human Y chromosome; and since it was separated from chimpanzees six million years ago, there has been no shortage of one gene on the Y chromosome. These results suggest that the degeneration of the Y chromosome is not linearly decreasing at the rate of 10 genes per 1 million years, as initially indicated. However, this does not mean that the Y chromosome will not disappear, which has happened in other species. For example, two species of underground rodents have lost their Y chromosomes, and three species of endangered Rattus living on several small islands in Japan. However, these species showed that the absence of Y chromosome would not pose a threat to the survival of organisms. The species mentioned above are still male and female. People always think of gender as a very certain thing, that as long as you have Y chromosome, you are male; if you don’t have Y chromosome, you are female. This is not the case. < / P > < p > in fact, 95% of the genes expressed differently in males and females are not dependent on X and Y chromosomes. For example, the gene ESR1, which encodes the estrogen receptor, is located on chromosome 6. The receptor plays an important role in female growth and sexual development. The loss of Y chromosome does not mean the loss of male. In fact, the deletion of the Y chromosome may mean that another gene will take over the task of determining gender, the “switch” mentioned above, and many genes can do it perfectly. But how likely is that to happen? “It can happen, but we won’t see it in our life.” Chinese version of K-car: reading a10e design drawing exposure