The Y-chromosome is the sex chromosome that determines male sexual identity in humans. The Y-chromosome in humans is small, yet contains a pseudoautosomal region in which there are significant homologies to the X-chromosome. This region facilitates chromosome pairing at meiosis. Active genes in this region include those for steroid sulfatase and the Xg blood group. The Y-chromosome also contains the sex-determining region Y-gene (SRY gene). The SRY gene is expressed only in the male genital ridge during differentiation of the gonad. The SRY gene product encodes a DNA-binding protein that is a transcriptional regulator.
It has been proposed that X and Y-chromosomes evolved by translocating transcribed regions from the Y-chromosome to the X-chromosome. Then the Y-chromosome would be reduced in size, whereas the X-chromosome would progressively acquire the capacity to inactivate the domains of the chromosome that could not pair in meiosis. The fact that males have only one of each X- and Y-chromosome, whereas females have two X-chromosomes, means that males are more susceptible to genetic mutations on these chromosomes that lead to disease. These include, among very many, Duchenne muscular dystrophy, hemophilia, and fragile X-linked mental retardation.
