This definition appears very rarely
7 Mb pseudoautosomal region PAR1. Homology between the X and Y chromosomes in PAR1 is maintained by an obligatory recombination in male meiosis; gene loci in this region are present in two copies in both males and females and are not subject to dosage compensation by XCI. At the tip of the long arm of X and Y is a second pseudoautosomal region, the 330 kb PAR2, which was created by duplication of material from X to Y since the divergence of human and chimpanzee lineages40. Some genes in PAR2 are subject to XCI, presumably reflecting their status on the X chromosome before the duplication event. Outside the PARs, homologies between the X and Y chromosomes are in non-recombining regions, predominantly in other parts of the XAR, together with a large 'X-transposed region.