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.