Chromosome Abnormality - Structural Abnormalities

Structural Abnormalities

When the chromosome's structure is altered, this can take several forms:

• Deletions: A portion of the chromosome is missing or deleted. Known disorders in humans include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome, also called the terminal 11q deletion disorder.
• Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Known human disorders include Charcot-Marie-Tooth disease type 1A which may be caused by duplication of the gene encoding peripheral myelin protein 22 (PMP22) on chromosome 17.
• Translocations: A portion of one chromosome is transferred to another chromosome. There are two main types of translocations:
  • Reciprocal translocation: Segments from two different chromosomes have been exchanged.
  • Robertsonian translocation: An entire chromosome has attached to another at the centromere - in humans these only occur with chromosomes 13, 14, 15, 21 and 22.
• Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.
• Insertions: A portion of one chromosome has been deleted from its normal place and inserted into another chromosome.
• 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.
• Isochromosome: Formed by the mirror image copy of a chromosome segment including the centromere.

Chromosome instability syndromes are a group of disorders characterized by chromosomal instability and breakage. They often lead to an increased tendency to develop certain types of malignancies.