How do Turner syndrome and Klinefelter syndrome involve sex chromosomes?

Turner syndrome and Klinefelter syndrome are both conditions involving abnormalities in the number of sex chromosomes, which typically determine biological sex.1

Turner Syndrome

Turner syndrome occurs in females and is characterized by the absence of a second sex chromosome, resulting in a 45,X karyotype. Because the second sex-determining chromosome is missing, individuals with this condition often experience incomplete development of secondary sex characteristics and infertility. While many cases are diagnosed as mosaics—where only some cells lack the second X chromosome—the classic form involves the total absence of that chromosome.23

Klinefelter Syndrome

Klinefelter syndrome occurs in males and results from the presence of at least one extra X chromosome, most commonly leading to a 47,XXY karyotype. This condition arises from an unequal distribution of sex chromosomes during cell division. Despite the extra X chromosome, individuals with Klinefelter syndrome generally have a male phenotype, though they often experience infertility and decreased testosterone levels. The clinical impact is often mitigated by X inactivation, a process where extra X chromosomes are silenced in somatic cells.245