chromosome abnormality

A chromosomal abnormality or chromosomal anomaly is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where there is an atypical number of chromosomes, or as structural abnormalities, where one or more individual chromosomes are altered. Chromosome mutation was formerly used in a strict sense to mean a change in a chromosomal segment, involving more than one gene. Chromosome anomalies usually occur when there is an error in cell division following meiosis or mitosis. Chromosome abnormalities may be detected or confirmed by comparing an individual's karyotype, or full set of chromosomes, to a typical karyotype for the species via genetic testing.

Sometimes chromosomal abnormalities arise in the early stages of an embryo, sperm, or infant. They can be caused by various environmental factors. The implications of chromosomal abnormalities depend on the specific problem, they may have quite different ramifications. Diseases and conditions caused by chromosomal abnormalities are called chromosomal disorders or chromosomal aberrations. Some examples are Down syndrome and Turner syndrome. However, chromosomal abnormalities do not always lead to diseases. Among abnormalities, structural rearrangements of genes between chromosomes can be harmless if they are balanced, which means that a set of the chromosomes remains complete and there are no gene breaks across the chromosomes.