Question

Turner's syndrome is a human female disorder characterized by short stature, obesity, and failure to mature sexually. It is caused by a missing X chromosome, so the affected person has only 45 chromosomes. They are always XO, as YO is a fatal condition prior to birth.

How can a genetic disorder, such as Turner's syndrome, occur?
Responses
A a translocation during meiosisa translocation during meiosis
B a translocation during mitosisa translocation during mitosis
C a frameshift mutation during meiosisa frameshift mutation during meiosis
D a frameshift mutation during mitosisa frameshift mutation during mitosis
E crossing over during meiosiscrossing over during meiosis
F crossing over during mitosiscrossing over during mitosis
G nondisjunction during meiosisnondisjunction during meiosis
H nondisjunction during mitosis

Answer 1

G — nondisjunction during meiosis.

Explanation: Turner syndrome (45,X) results from a gamete that lacked an X (or a sperm lacking a sex chromosome) because homologous chromosomes or sister chromatids failed to separate in meiosis, producing a gamete with 0 or 2 sex chromosomes. Fusion of a 0-X gamete with a normal gamete yields an XO zygote. (Note: rare mosaic cases arise from post‑fertilization mitotic nondisjunction, but the classic cause is meiotic nondisjunction.)