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Chromosomal Abnormality
Abnormalities exist when chromosomes exhibit atypical numerical properties (i.e., there is more than or less than one pair of chromosomes) or structural properties (i.e., there is extra chromosomal material, or a fraction of the chromosome is missing). Such abnormalities can lead to conditions such as Down syndrome, mental retardation, autism, or Turner's syndrome. Although some chromosomal abnormalities are believed to be inherited, most are purely accidental and occur during the conception or gestation process.