What are chromosome abnormalities?

A chromosome abnormality reflects an abnormality of chromosome number or structure. There are many types of chromosome abnormalities. However, they can be organized into two basic groups:

Numerical Abnormalities:

When an individual is missing either a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (trisomy). An example of a condition caused by numerical abnormalities is Down Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two). Turner Syndrome is an example of monosomy, where the individual - in this case a female - is born with only one sex chromosome, an X.

Structural Abnormalities:

When the chromosome's structure is altered. This can take several forms:

• Deletions: A portion of the chromosome is missing or deleted.
  
  
• Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.
  
  
• Translocations: When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere.
  
  
• Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.
  
  
• Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.

Most chromosome abnormalities occur as an accident in the egg or sperm. Therefore, the abnormality is present in every cell of the body. Some abnormalities, however, can happen after conception, resulting in mosaicism, where some cells have the abnormality and some do not.

Chromosome abnormalities can be inherited from a parent (such as a translocation) or be "de novo" (new to the individual). This is why chromosome studies are often performed on parents when a child is found to have an abnormality.

Source: http://www.genome.gov/11508982