Structural disorders

Structural abnormalities are a consequence of chromosomal breakage. Once broken, there is the possibility that repair may join two unrelated sections of chromosome. The spontaneous mutation rate is 1 per 1000 gametes. Breakage is facilitated by ionizing radiation, mutatogenic chemicals and some rare inherited conditions.

A number of types of structural aberration are recognized:

• translocation - the transfer of chromosomal material between chromosomes. Three subtypes are recognized according to which region of the chromosomes swap. Often, the carrier with a 'balanced translocation' is not affected, but offspring are affected.
  
  
• deletion - where this occurs at both ends of a chromosome, a ring chromosome can result.
  
  
• duplication - the presence of two copies of a segment of chromosome, often with little harmful consequences.
  
  
• inversion - breakage at two ends of a chromosome with a rotation and rejoining of the part inbetween so that it lies the wrong way around. There is an increased risk of chromosomally imbalanced offspring. Carriers may appear normal.
  
  
• isochrome - a chromosome which has deletion of one arm and duplication of the other.
  
  
• centric fragments - small, remaining material left after translocations.