There are many types of chromosome abnormalities. However, they can be organized into two basic groups: numerical abnormalities and structural abnormalities. Numerical Abnormalities: When an individual is missing one of the chromosomes from a pair, the condition is called monosomy or when an individual has more than two chromosomes instead of a pair, the condition is called trisomy. Trisomies account for the most common genetic abnormalities – conditions such as Down’s syndrome (having a extra copy of chromosome 21), Patau’s syndrome (having a extra copy of chromosome 13) and Edward’s syndrome (having a extra copy of chromosome 18). Structural Abnormalities: A chromosome's structure can be altered in several ways: Deletions: A portion of the chromosome is missing or deleted. Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Translocations: A portion of one chromosome is transferred to another chromosome. Inversions: A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic material is inverted. Rings: A portion of a chromosome has broken off and formed a circle or ring.