A
chromosome anomaly,
abnormality,
aberration, or
mutation is a missing, extra, or irregular portion of
chromosomal DNA. It can be from an atypical number of chromosomes or a structural abnormality in one or more chromosomes.
Chromosome mutation was formerly used in a strict sense to mean a change in a chromosomal segment, involving more than one
gene. A
karyotype refers to a full set of chromosomes from an individual that can be compared to a "normal" karyotype for the
species via
genetic testing. A chromosome anomaly may be detected or confirmed in this manner. Chromosome anomalies usually occur when there is an error in
cell division following
meiosis or
mitosis. There are many types of chromosome anomalies. They can be organized into two basic groups, numerical and structural anomalies.