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?
A nondisjunction
B insertion
C crossing over
D translocation