Question

What is the primary cause of Down's Syndrome in approximately 95% of cases?

Options

• Nondisjunction during spermatogenesis

• Nondisjunction during oogenesis

• Translocation of chromosome 21

• Spontaneous mutation of chromosome 21

Answer 1

Maternal nondisjunction during oogenesis accounts for approximately 95% of Down's Syndrome cases, with risk increasing significantly with advanced maternal age. Nondisjunction errors during meiosis result in gametes with an extra chromosome 21, which when fertilized produces a trisomy 21 individual.