What is the cause of Turner syndrome?

• A. Absence of One X Chromosome.
• B. Extra Chromosome X.
• C. Deletion of Chromosome Y.
• D. Triplication of Chromosome 21.

Answer: (A)

Turner syndrome is caused by the loss of one X chromosome in a person who is genetically female, resulting in a 45,X karyotype in many cases. This single X (monosomy X) disrupts ovarian development and leads to features such as short stature and gonadal dysgenesis, though intelligence is typically normal. Most cases arise from nondisjunction during the formation of reproductive cells, and some individuals are mosaic with some cells 45,X and others 46,XX. The other conditions described correspond to different chromosomal changes: an extra X chromosome is seen in other syndromes (like Klinefelter or triple-X), deletion of a Y chromosome is a different sex-chromosome issue, and triplication of chromosome 21 is Down syndrome.