This lecture covers the organization and evolutionary instability of the major human pseudoautosomal regions (PAR1 and PAR2), the role of sex-specific genes in the differentiation of the X and Y chromosomes, the shrinking of the Y chromosome over time, and the implications of male meiosis in relation to PAR1 and PAR2. Additionally, it discusses the genetic basis and clinical manifestations of Langer mesomelic dysplasia, a rare subtype of mesomelic dysplasia characterized by limb shortening and skeletal abnormalities.