The Y chromosome is one of two sex chromosomes in therian mammals and other organisms. The other sex chromosome is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or absence of Y that determines the male or female sex of offspring produced in sexual reproduction. In mammals, the Y chromosome contains the gene SRY, which triggers male development. The DNA in the human Y chromosome is composed of about 62 million base pairs, making it similar in size to chromosome 19. The Y chromosome is passed only from father to son. With a 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest-evolving parts of the human genome. The human Y chromosome carries 693 genes, with 107 of these being protein-coding, but some genes are repeated and that makes the number of exclusive protein-coding genes just 42, the numbers are given for telomere-to-telomere CHM13. The Consensus Coding Sequence (CCDS) Project only classified 63 out of 107. All single-copy Y-linked genes are hemizygous (present on only one chromosome) except in cases of aneuploidy such as XYY syndrome or XXYY syndrome. Because of fake gaps inserted in GRCh38 it may be not obvious that CHM13 added 30 million base pairs into the Y chromosome, which is almost half of it that was unknown before 2022 (and was present in many Genbank samples by mistake unknown that it was Y chromosome base pairs).