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  nondisjunctionb  insertionc  crossing overd  translocation