Human chromosome
Normal human cells have 23 pairs of chromosomes (46 chromosomes), including 22 pairs of autosomes and sex chromosomes 1 pair. Sex chromosomes determine a person's sex. If the sex chromosome is XX, it is female, and XY is male.
Among them, the Y chromosome is unique to men, while the X chromosome exists in both men and women. From the appearance, the Y chromosome is only one third as long as the X chromosome. Gene sequencing shows that there are 78 effective genes in human Y chromosome, while there are 54 alleles in human X chromosome with 1098 effective genes. Non-allelic genes on Y chromosome endow men with special body structures different from those of women, such as SRY gene (testis).
Y chromosome inheritance
After meiosis of human germ cells, women's eggs have only one X chromosome, and men's X and Y chromosomes account for half of sperm cells. After eggs and sperm cells are randomly combined, the probability of forming XX and XY is 50%, so in the case of natural conception, the probability of giving birth to boys and girls is the same.
Because Y chromosome is a unique feature of men, Y chromosome can only be inherited in men. By studying the Y chromosome, we can trace back to the ancestors of a family. X chromosome may come from parents or mothers, so X chromosome cannot be traced back to the next generation. For example, two strangers have a very high similarity in the Y chromosome gene (there will be a mutation in chromosome replication), so it can be basically concluded that these two people have the same father in a certain generation.
Evolution of y chromosome
Biological research shows that human Y chromosome was as long as X chromosome before1.800 million years ago, and there were more than 1.000 genes. However, for some reason, the genes on Y chromosome either disappeared or transferred to other autosomes.
Some biologists also pointed out that the human Y chromosome is getting shorter and shorter, and it is expected to disappear after 4 million years, just as the mole has only X chromosome but no Y chromosome. Of course, the disappearance of human Y chromosome does not mean that men will disappear, but some genes of Y chromosome will be transferred to other autosomes, and some invalid genes may disappear.
A similar chromosome disappearance occurred in humans millions of years ago. For example, gorillas have 24 pairs (48 chromosomes). Biologists speculate that about 5 million years ago, two pairs of chromosomes of human ancestors fused, making a pair of human chromosomes missing.
mitochondrial dna,mtdna
Y chromosome is inherited only between men. In fact, human beings also have special DNA inherited from women, such as mitochondrial circular DNA, which is an important occasion to generate energy in human cells and has genetic material independent of the nucleus.
Sperm cells produced by meiosis of male germ cells will discard excess substances in the cells. There are only a few mitochondria in the head and tail of sperm cells, while all mitochondria are retained in egg cells, so most of the mitochondrial DNA in human cells comes from the mother and can also be used as traces of maternal origin.