Final answer:
Trisomy 13, 18, and 21 are caused by chromosomal nondisjunction, resulting in an extra chromosome. Trisomy 21, or Down syndrome, allows for survival into adulthood, unlike trisomies 13 and 18.
Step-by-step explanation:
Trisomy 13, 18, and 21 are all the result of a chromosomal nondisjunction during meiosis. This genetic anomaly leads to an individual having three copies of a particular chromosome instead of the usual two. For example, trisomy 21, also known as Down syndrome, occurs when there is an extra copy of chromosome 21. While individuals with trisomy 21 may survive into adulthood with certain impairments and are the most likely to do so compared to trisomy 13 or 18, those with trisomies 13 and 18 often do not survive infancy.
The presence of an extra chromosome in these conditions disrupts normal development, leading to severe health challenges. The incidence of trisomy conditions, particularly trisomy 21, increases with maternal age, especially in women over 36 years old. A karyotype analysis can identify the presence of an extra chromosome, confirming the diagnosis of a trisomy condition.