Chromosomal Structural Aberrations and It’s Genetical Effects

Chromosomal Structural Aberrations are changes in the physical structure of chromosomes. These changes can affect gene expression and function, often leading to genetic disorders or abnormalities. Structural aberrations are categorized based on the type of chromosomal change involved.

1. Deletion

• Definition: A segment of the chromosome is missing or lost.
• Types:
  • Terminal deletion: Loss from the end of the chromosome.
  • Interstitial deletion: Loss from the middle of the chromosome.
• Example:
  • Cri-du-chat Syndrome – Caused by deletion of a portion of the short arm of chromosome 5 (5p deletion). Affected infants have a high-pitched cry, intellectual disability, and facial abnormalities.

2. Duplication

• Definition: A segment of the chromosome is duplicated, resulting in extra genetic material.
• Types:
  • Tandem duplication: Duplicated segment is adjacent to the original.
  • Displaced duplication: Duplicated segment is located elsewhere on the same or a different chromosome.
• Example:
  • Charcot-Marie-Tooth Disease Type 1A – Caused by duplication of the PMP22 gene on chromosome 17, leading to muscle weakness and nerve damage.

3. Inversion

• Definition: A chromosome segment breaks off, rotates 180 degrees, and reattaches in the reversed direction.
• Types:
  • Paracentric inversion: Inversion does not include the centromere.
  • Pericentric inversion: Inversion includes the centromere.
• Example:
  • Inversion of Chromosome 9 – A common variant; usually benign but may be associated with infertility or miscarriages in some cases.

4. Translocation

• Definition: A segment of one chromosome breaks off and attaches to another chromosome.
• Types:
  • Reciprocal translocation: Exchange of segments between two chromosomes.
  • Robertsonian translocation: Fusion of two acrocentric chromosomes (chromosomes with very short p arms).
• Example:
  • Chronic Myelogenous Leukemia (CML) – Caused by a reciprocal translocation between chromosomes 9 and 22, forming the Philadelphia chromosome.

5. Ring Chromosome

• Definition: A chromosome breaks in two places and the ends join to form a ring.
• Example:
  • Ring Chromosome 14 Syndrome – Associated with developmental delay, seizures, and intellectual disability.

6. Isochromosome

• Definition: A chromosome with identical arms due to misdivision at the centromere.
• Example:
  • Turner Syndrome (isochromosome Xq) – A structural abnormality where both arms of the X chromosome are long arms (q arms), leading to symptoms like short stature and infertility.