Pseudoautosomal region

The pseudoautosomal regions, PAR1 and PAR2 are homologous sequences of nucleotides on the X and Y chromosomes.

The pseudoautosomal regions get their name because any genes located within them (so far at least 29 have been found) are inherited just like any autosomal genes. PAR1 comprises 2.6 Mbp of the short-arm tips of both X and Y chromosomes in humans and other great apes (X and Y are 155 Mbp and 59 Mbp in total). PAR2 is located at the tips of the long arms, spanning 320 kbp.

Inheritance and function
Normal male mammals have two copies of these genes: one in the pseudoautosomal region of their Y chromosome, the other in the corresponding portion of their X chromosome. Normal females also possess two copies of pseudoautosomal genes, as each of their two X chromosomes contains a pseudoautosomal region. Crossing over between the X and Y chromosomes is normally restricted to the pseudoautosomal regions; thus, pseudoautosomal genes exhibit an autosomal, rather than sex-linked, pattern of inheritance. So, females can inherit an allele originally present on the Y chromosome of their father and males can inherit an allele originally present on the X chromosome of their father.

The function of these pseudoautosomal regions is that they allow the X and Y chromosomes to pair and properly segregate during meiosis in males.

Genes
Pseudoautosomal genes are found in two different locations: PAR1 and PAR2. These are believed to have evolved independently.

PAR1

 * pseudoautosomal PAR1
 * ASMT
 * ASMTL
 * CD99
 * CRLF2
 * CSF2RA
 * SFRS17A
 * DHRSXY
 * GTPBP6
 * IL3RA
 * P2RY8
 * PLCXD1
 * PPP2R3B
 * SHOX
 * SLC25A6
 * XG
 * ZBED1

In mice, some PAR1 genes have transferred to autosomes.

PAR2

 * pseudoautosomal PAR2
 * SPRY3
 * SYBL1
 * IL9R
 * CXYorf1, which is now mapped to the pseudogene "WASH6P", but is of interest due to its proximity to the telomere.

Pathology
Pairing (synapsis) of the X and Y chromosomes and crossing over (recombination) between their pseudoautosomal regions appear to be necessary for the normal progression of male meiosis. Thus, those cells in which X-Y recombination does not occur will fail to complete meiosis. Structural and/or genetic dissimilarity (due to hybridization or mutation) between the pseudoautosomal regions of the X and Y chromosomes can disrupt pairing and recombination, and consequently cause male infertility.

The SHOX gene in the PAR1 region is the gene most commonly associated with and well understood with regards to disorders in humans, but all pseudoautosomal genes escape X-inactivation and are therefore candidates for having gene dosage effects in sex chromosome aneuploidy conditions (45,X, 47,XXX, 47,XXY, 47,XYY, etc.).

Deletions have also been associated with Léri-Weill dyschondrosteosis and Madelung's deformity.