Down syndrome is most often the result of an extra copy of chromosome 21, a condition known as trisomy 21. The extra chromosome originates from the failure of homologous pairs to separate during gamete formation in one parent. Subsequent fertilization results in a zygote with three copies of chromosome 21. Which term best classifies the event in the passage above that leads to trisomy 21?
AÂ nondisjunction
BÂ insertion
CÂ crossing over
DÂ translocation
