3. What structural chromosomal abnormalities may occur and give three 
     examples of how these may affect the eye.
Abnormalities in chromosome structure follow a chromosome break and, during the repair process, the reunion of the wrong segments of the chromosome.When a chromosome breaks two unstable sticky ends are produced. Generally,
repair mechanisms rejoin these two ends without delay. However, if more than one break has occurred, repair mechanisms may not be able to distinguish one sticky end from another and may rejoin the wrong ends.

If, following repair, there is a loss or gain of chromosomal material (an unbalanced rearrangement) there can be significant clinical consequences. If there is no loss or gain of chromosomal material (a balanced rearrangement), then the individual is phenotypically normal.

However, there is an increased risk of having chromosomally abnormal offspring because individuals who carry balanced chromosome rearrangements may produce chromosomally unbalanced gametes.

The spontaneous rate of chromosomal breaking can be increased by: 
 

  •                Exposure to ionizing radiation
  •                Inheritance of rare conditions 
Structural aberrations are subdivided into:
    • Translocations 
    • Deletions 
    • Duplications 
    • Inversions 
Examples of chromosomal structural abnormalities that affect the eyes include:
 
    • long arm chromosome 13 deletion which is associated with retinoblastoma
    • short arm chromosome 11 deletion seen in aniridia
    • short arm chromosome 5 deletion or cri du chat syndrome which is associated with iris coloboma, optic atrophy, epicanthal folds, myopia, hypertelorism and exotropia.
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