PAR gene family

Genes in the pseudoautosomal (PAR) gene family are located near the ends of the X and Y chromosomes in areas known as the pseudoautosomal regions. The X and Y chromosomes are the human sex chromosomes. Females have two X chromosomes in each cell, while males have one X chromosome and one Y chromosome in each cell. Although many genes are unique to either the X or Y chromosome, genes in the pseudoautosomal regions are present on both sex chromosomes. Males and females have two functional copies of most pseudoautosomal genes in each cell.

The pseudoautosomal regions on the X and Y chromosomes are known as PAR1 and PAR2. PAR1 is located at the end of the short (p) arm of the chromosomes and is about 2.7 million DNA building blocks (2.7 megabases) long. It contains at least 24 known genes. PAR2 is located at the end of the long (q) arm of the X and Y chromosomes and is about 330,000 DNA building blocks (0.33 megabases) long. It contains at least five known genes.

In males, the pseudoautosomal regions are needed for the X and Y chromosomes to line up correctly during cell division. X-Y pairing in PAR1 appears to be critical for the normal formation of sperm (spermatogenesis), and studies suggest that a loss of PAR1 is associated with an inability to father a child (infertility). The shorter PAR2 does not appear to be necessary for normal fertility.

Although many of the genes in PAR1 and PAR2 play important roles in normal development, only one pseudoautosomal gene, the SHOX gene in PAR1, has been definitively associated with human disease. Changes involving this gene are responsible for two related skeletal disorders, Léri-Weill dyschondrosteosis and Langer mesomelic dysplasia. One copy of the SHOX gene is also missing in females with Turner syndrome, causing them to have short stature and skeletal abnormalities. Additionally, variations in the SHOX gene have been associated with short stature in people without other features of a genetic syndrome (idiopathic short stature).