xyS

XYs refers to organisms with an XY sex chromosome complement, typically resulting in male development in species that use the XY sex-determination system. In these species, the presence of a Y chromosome—especially a functional SRY gene—activates a cascade that initiates testicular development and downstream male differentiation. By contrast, individuals with two X chromosomes usually develop ovaries, though there are exceptions.

In humans and many mammals, the Y chromosome is relatively small and gene-poor compared with the X. It contains genes important for male fertility and spermatogenesis, with SRY acting as a key sex-determining switch. Regions at the ends of the Y and X, called pseudoautosomal regions PAR1 and PAR2, recombine with their X homologs during meiosis; outside these regions, the Y does not recombine with the X, and most Y-linked genes are inherited from father to son.

Not all XY individuals develop as typical males. Variations in SRY or other genes can lead to disorders of sex development (DSD), resulting in phenotypes ranging from typical female to ambiguous gonads. Examples include Swyer syndrome (XY gonadal dysgenesis) and complete androgen insensitivity syndrome, where individuals are genetically XY but develop female-appearing features and are often infertile.

Evolutionarily, the Y chromosome has undergone substantial changes and gene loss in many lineages, and some species have evolved alternative primary sex-determining mechanisms. Dosage compensation mechanisms, such as X-chromosome inactivation in mammals, balance gene expression between XX and XY individuals.

In research and clinical contexts, ‘XYs’ is used to describe persons or organisms bearing an XY karyotype and to discuss associated developmental, reproductive, and evolutionary aspects.