Figure 1. The human Y chromosome is divided into two ‘arms’ by the centromere. There are small pseudoautosomal regions (pale green) at the tips of the short and long arms. About half of the long arm is composed of highly repeated ‘junk’ DNA (pink). The rest of the differential region (mid green) contains about 20 known genes; some present in many copies.

The position of the testis-determining gene was pinpointed within a critical sex-determining region (dark green) on the short arm by deletion analysis in patients with different parts of the Y chromosome missing (vertical lines). If this region is deleted, the patient is a sex reversed (XY) female. SRY was cloned from this region.

The position of the spermatogenesis gene(s) was also pinpointed within a critical azoospermia region (dark green) on the long arm, by deletion analysis in patients who are oligospermic (make few sperm) or azoospermic (no sperm). This region contains RBM, one of several candidates for a spermatogenesis gene.

Figure 2. Ancient and added regions of the mammalian X and Y chromosomes. The ancient region (blue) is conserved on the X and Y chromosomes of all therian mammals (XCR and YCR), and contains at least four X-Y shared genes including SRY, SMCY and RBM. The added regions (pink, XAR and YAR) are autosomal in marsupial and monotreme mammals, and were therefore added since marsupials diverged from eutherians 150 million years ago.