Every human life begins with a fertilized egg, which contains 46 chromosomes. If these 46 chromosomes happen to be of 23 different types, each with two copies, then there are 23 pairs of chromosomes in the fertilized egg, which can be written as 23Ⅱ or 22Ⅱ+XX, where "Ⅱ" represents a pair. A fertilized egg with 22Ⅱ+XX chromosomes will definitely develop into a female, while if the 46 chromosomes in the fertilized egg have 24 different types, with 22 of them paired and the other two unpaired, it can be written as 22Ⅱ+XY. Such a fertilized egg will inevitably develop into a male. In fact, the difference between men and women lies in whether one chromosome is X or Y. This sex-differentiating chromosome is called the sex chromosome, and the testis-determining gene is located on the Y chromosome. Regardless of whether it's a man or a woman, the 22 pairs of chromosomes are all paired, with the exception of the sex chromosomes, which are related to gender: male sex chromosomes are XY, while female sex chromosomes are XX.
How are sperm and eggs produced? It is now clear that sperm are produced by male somatic cells through meiosis, and eggs are produced by female somatic cells through meiosis. The meiosis process reduces the number of chromosomes by half. For example, the chromosomes in female somatic cells are of 23 different types, each in a paired form, namely 22Ⅱ+XX. Therefore, the chromosomes in the eggs produced by meiosis are all 22Ⅰ+X, with no other types. Men are different, as the chromosomes in male somatic cells are of 24 different types, namely 22Ⅱ+XY. The sperm produced by meiosis can either be 22Ⅰ+X or 22Ⅰ+Y, with two possibilities. When a 22Ⅰ+X egg and a 22Ⅰ+X sperm combine, the chromosome composition of the fertilized egg becomes 22Ⅱ+XX, which will develop into a female in the future; when a 22Ⅰ+X egg and a 22Ⅰ+Y sperm combine to form a fertilized egg, the chromosome composition of the fertilized egg becomes 22Ⅱ+XY, which will develop into a male under the action of the testis-determining gene on the Y chromosome. The sex chromosome in an egg can only be X, while the sex chromosome in a sperm can either be X or Y.