Describe the evolution of Y chromosome
Some 100s million years ago, sex was determined by temperature. Then the SRY gene developed. 2 chromosomes became more and more distinct. Because the SRY gene is the only necessary part in the Y-chromosome in determining the sex (male), the size of the Y-chromosome decreases until there left the SRY gene and the remaining top of the Y-chromosome, to which the X-chromosome recombines.