From a biological point of view, both males and females have 44 autosomals + 2 sex chromosomes, of which the male sex chromosome is XY and the female sex chromosome is XX, so the question is, is there any other combination of XY or XX? For example, is there a combination of "YY", "XXY" or "XYY"?
heredity
To understand this problem, we must first understand heredity. It is said that people are composed of a variety of cells, and the cells are composed of nuclei and cytoplasm, there are chromosomes in the nucleus, and the chromosomes are composed of double-stranded DNA, and the functions on each chain are called genes. Genes, on the other hand, can achieve basic life activities by directing the synthesis of proteins.
That is to say, the basis of human life lies in the normal functioning of genes. Everyone's genetic material is actually obtained from the father and mother. Among them, the father can provide 22 autosomals + 1 sex chromosome (can be X or Y), and the mother can provide 22 autosomal + X chromosomes, adding up to 44 autosomals + 2 sex chromosomes (XX or XY).
The vast majority of people's sex chromosomes are either XX or XY. But in the genetic process, there are occasional errors, which are also called: mutations. Mutations are broadly divided into three broad categories: genetic mutations, genetic recombination, and chromosomal variations. If the genetic material provided by the father and mother is mutated, there is a small probability that the sex chromosomes will not match, such as XXY or XYY.
The situation of XXY may be a combination of the mother's side to XX and the father's side to Y, or it may be that the mother's side gives X and the father's side gives XY. In general, people with XXY chromosomes appear to have male physiological characteristics, but also appear in individuals with androgen deficiency, and may also appear with different degrees of feminized secondary sex characteristics.
XYY is the case because the father's germ cells are abnormal when they replicate, resulting in the germ cells containing 2 Y chromosomes, and then the mother normally provides an X chromosome, which eventually combines to form XYY, which is also known as: hyperandrogenism syndrome. According to the available statistics, the crime rate of people with XYY is several times, or even dozens of times higher than that of ordinary people.
Why is there no "YY" combination?
Since there can be a "XYY" situation, I believe many people will wonder if there is a YY situation?
First of all, we can imagine how "YY" might come from? If the father's germ cells are abnormal, YY can be provided, but because humans rely on sexual reproduction, the mother must provide another X, which eventually constitutes XYY, which is the case of super-male syndrome. Also, parents provide a Y to combine into YY, but since the mother only has the X chromosome, this is not possible. Of course, it is impossible for two males to produce offspring. Therefore, from the perspective of sexual reproduction, the "YY" situation cannot happen.
Some people may also wonder, is this achieved through gene editing technology?
The answer is again no. First, we don't have such technology. Even if we do have this technology, the edited embryo will not survive.
This is because some genes on the X chromosome are very special, and the X chromosome has about 900 to 1200 genes, compared with only about 70 genes on the Y chromosome. Among them, some of the genes on the X chromosome are genes related to brain function and immune function. If there is no X chromosome, then people will not have these genes, and they will not survive at all. Therefore, these genes are also called: house-keeping genes. According to statistics, the average life expectancy of human women is longer than that of men, and the ability to resist sepsis, infection, and trauma is stronger, mainly because the X chromosome plays a role.
Therefore, it is impossible for a person not to have an X chromosome, and once there is not, it will not live long at all, which has nothing to do with any technology.
Are genes on the Y chromosome disappearing?
Scientists have found that the Y chromosome is gradually disappearing. Scientists have found that the early Y chromosome was as long as the X chromosome, but now the length of the Y chromosome is only 1/3 of the X chromosome, which means that the Y chromosome is gradually disappearing.
The reason for the disappearance is also very simple, the Y chromosome has no homologous chromosome, and in the process of gene replication, the gene is not copied very precisely, but a slight deviation occurs. The X chromosome can repair a missing part of itself by copying the contents of another X chromosome, but the Y chromosome has only one and cannot repair itself by copying the other chromosome.
Therefore, the length of the Y chromosome is slowly getting shorter, and the rate of shorter is to lose 5-10 genes every 1 million years, compared with only 45 genes on the current Y chromosome.
Does this mean that the Y chromosome will disappear completely in the future? In fact, this has happened in the animal kingdom, for example: two types of mole rats in West Asia have lost all of their Y chromosome genes, but they still have male individuals, and researchers believe that they must have evolved genes responsible for making males on other chromosomes.
However, whether humans will be able to evolve this ability is not known for the time being.