Why is Turners syndrome XO and not YO?

Turner syndrome is characterized by the presence of a single X chromosome (XO) instead of the typical two sex chromosomes (XX in females or XY in males). The absence of a second sex chromosome, specifically a Y chromosome, leads to the development of female physical characteristics. A Y chromosome is essential for male development, and without it, an individual cannot develop male traits. Therefore, individuals with Turner syndrome are genetically female due to the lack of a Y chromosome.