Deviation from the normal chromosome number
in a single pair of chromosomes is referred
to as aneuploidy. In humans, numerical chromosome
aberrations occur in about 1 in 400
newborns. An abnormality of the number of
chromosomes occurs as a result of their abnormal
distribution (nondisjunction) during meiosis
I or II (meiotic nondisjunction). With meiotic
nondisjunction, the aberration occurs in all
cells of a resulting organism. Nondisjunction in
meiosis I and meiosis II can be differentiated
(see pp. 116). Abnormal chromosomal distribution
during mitosis leads to an aberration in
only a proportion of the cells (chromosomal
mosaicism).
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