In the early 21st century, Brian Sykes, a professor of human genetics at the University of Oxford in the UK, put forward a theory that made men feel uneasy: the Y chromosome has already developed defects and has not been repaired until now. According to his calculations, 125,000 years later, the Earth will become a "daughter kingdom", at which time women with a chromosome composition of 22Ⅱ+XX will combine with sperm and eggs produced by stem cells, so the fertilized eggs will only develop into females.
Everything can be traced back to the origin of the Y chromosome: mammals evolved from reptiles. When mammals first appeared on Earth, sex chromosomes had not yet been formed, only sex genes. The sex of the embryo was determined by external environmental factors such as temperature, so there were often hermaphrodites.
300 million years ago, the X chromosome, where the human sex genes are located, developed defects. Instead of being repaired, these defects gradually expanded, causing many genes on the X chromosome to be lost. Eventually, the X chromosome became shorter and had fewer genes, forming the current Y chromosome.
However, there is a huge controversy about the time of the Y chromosome's extinction. Professor Jennifer Graves, a geneticist at the Australian National University, believes that when the Y chromosome first appeared 300 million years ago, there were about 1,438 genes, but now there are only about 45 genes left, of which 1,393 genes have been lost. According to the rate of gene loss, the Y chromosome will disappear in more than 10 million years.
While scientists in the UK and Australia are debating the time of the Y chromosome's disappearance, American geneticist David Page and his colleagues point out that the DNA molecular structure of the Y chromosome is unique. Once a gene defect occurs, the Y chromosome can repair itself, so there's no need to worry about the Y chromosome disappearing from the population.