Nondisjunction as a cause of trisomy

Especially in trisomy 21, the frequency of nondisjunction
depends on the age of the mother at
the time of conception (1). The age of the father
has very little or no influence.
Nondisjunction may occur during the first or
the second maturation division (meiosis I or
meiosis II, p. 116) (2). The difference can be established
by appropriate chromosomal
markers. If nondisjunction occurs in meiosis I,
the three chromosomes will be different
(1 + 1 + 1), whereas if nondisjunction occurs
during meiosis II, two of the three chromosomes
will be identical (2 + 1). In humans, about
70% of nondisjunctions occur in meiosis I, and
30% in meiosis II.