pseudoautosomal

Pseudoautosomal refers to regions of homology between the X and Y chromosomes that carry genes present on both chromosomes and participate in recombination during meiosis. In humans, two pseudoautosomal regions, PAR1 and PAR2, lie at the tips of the X and Y chromosomes—the short-arm (PAR1) and the long-arm (PAR2) ends. These regions enable pairing and genetic exchange between the sex chromosomes in male meiosis and help ensure proper segregation of X and Y.

Genes located in PARs are present on both sex chromosomes and typically escape X-inactivation, so they have two active copies in both males and females. Because these genes are shared between X and Y, their inheritance patterns resemble autosomal genes more than classic X-linked or Y-linked genes. In males, PAR genes are contributed by both parents (the X from the mother and the Y from the father), while in females they are present on both X chromosomes.

PARs are thought to be remnants of ancestral autosomes that began to differentiate into sex chromosomes but retained regions of homology to permit recombination. Their size and gene content can vary among species, but the basic principle remains: these regions provide the necessary genetic exchange between X and Y and contribute to dosage-sensitive traits.

Clinical relevance varies with the genes involved; deletions or duplications in PARs can affect growth and development due to dosage effects. The best-known PAR gene in humans is SHOX, whose dosage influences skeletal development.