Pseudoautosomal Region

One-sentence definition. The pseudoautosomal region (PAR) is the segment of the sex chromosomes where X and Y (or Z and W) still recombine with each other — behaving like autosomes despite sitting on sex chromosomes.

One-sentence analogy. The PAR is the small strip of shared border where two otherwise sovereign territories still freely exchange people; everything outside it is locked down and unique to one side.

Why it matters. PAR size is the central parameter in the fragile Y hypothesis: in species with chiasmatic meiosis, the rate of Y-chromosome aneuploidy and the size of the PAR are negatively correlated. A large PAR means obligate crossovers must occur there, which keeps meiosis stable — but it also means any sexually antagonistic allele near the PAR cannot be stably linked to one sex. A fusion that lands in the PAR is actively harmful rather than neutral: it reconstitutes maladaptive genotypes each generation.

Where you meet it in the wiki.

• Fragile Y hypothesis — PAR size is the key parameter driving Y aneuploidy risk.
• Sex chromosome evolution — PAR fusions vs. non-PAR fusions have opposing fitness consequences under sexual antagonism.

Primary citation.

“The fragile Y hypothesis proposes that in species with chiasmatic meiosis the rate of Y-chromosome aneuploidy and the size of the recombining region have a negative correlation.” — Blackmon & Demuth 2015, Finding 1

Prerequisites: heterogamety, recombination suppression Next, learn about: achiasmy, recombination suppression