Final answer:
Trisomy 21, or Down syndrome, is primarily caused by nondisjunction of chromosome 21 during meiosis, resulting in an extra chromosome. The likelihood of this chromosomal abnormality increases with maternal age, particularly after the age of 35.
Step-by-step explanation:
The majority of all cases of Trisomy 21, also known as Down syndrome, can be attributed to a chromosomal abnormality.
Specifically, Down syndrome occurs due to nondisjunction of chromosome 21 during meiosis, resulting in an individual having three copies of this chromosome instead of the usual two.
The incidence of having a fetus with Trisomy 21 is known to increase dramatically with maternal age, particularly in mothers over the age of 35.
Trisomy 21 is noteworthy as the only autosomal trisomy where affected individuals are more likely to survive into adulthood, in contrast to other trisomies which typically result in early lethality.