 The trisomy 21 karyotype figure shows the chromosomal arrangement, with the prominent extra chromosome 21.Trisomy 21 is the cause of approximately 95% of observed Down syndrome, with 88% coming from nondisjunction in the maternal gamete and 8% coming from nondisjunction in the paternal gamete.Mitotic nondisjunction after conception would lead to mosaicism, and is discussed later.