ABSTRACT

Human X and Y chromosomes fist formed about 200-300 million years ago in eutherian mammals. Afterwards, sex chromosomes evolved separately in birds, reptiles, amphibians and fih. Classic genetic studies showed that some species have completely lost their Y chromosomes. For example nematodes have XX and X0 chromosomes. The sex chromosomes began their evolutionary journey as an ordinary pair of autosomes. A mutation in the SOX3 gene (SRY-related HMG-box), produced the SRY gene, a critical determinant of maleness. However, another gene RPS4 (ribosomal protein small subunit, protein 4) retained a similar function on both the X and Y chromosomes. Internal recombination events caused a rearrangement of genes on the Y chromosome which meant that large portions of the X and Y chromosome no longer recombined. Deletions on the non-recombinant Y made it decrease in size. However, the donation of a block of genes from an autosome 130 million years ago to both the X and Y, allowed recombination between them. As a result of these changes, Y chromosome is about one-third the size of its partner, the X-chromosome. The knowledge about the evolution of the Y chromosome might shed light on identifying the reasons of infertility.