U01.01.066 Robertsonian translocation

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Learning Objectives

  • Define the mechanism of Robertsonian Translocation in acrocentric chromosomes.
  • Distinguish between Balanced and Unbalanced translocations.
  • Identify the clinical consequences, specifically its role in Down Syndrome (Trisomy 21).

1. Mechanism of Fusion

A Robertsonian translocation occurs when the long arms (q arms) of two acrocentric chromosomes fuse at the centromere. The two short arms (p arms) are lost, but since they contain redundant ribosomal RNA genes, their loss is usually clinically insignificant in balanced carriers.

  • Acrocentric Chromosomes Involved: 13, 14, 15, 21, and 22.
  • Key Feature: These chromosomes have centromeres very near the end, making them prone to this specific type of fusion.

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2. Balanced vs. Unbalanced Translocations

Type Genetic Material Clinical Outcome
Balanced No significant gain or loss. A carrier has 45 chromosomes. Normal phenotype; however, high risk for offspring with chromosomal imbalances.
Unbalanced Significant gain (trisomy) or loss (monosomy) of genetic material. Miscarriage, stillbirth, or syndromes (e.g., Down syndrome, Patau syndrome).

3. Clinical Relevance: Down Syndrome

While 95% of Down syndrome cases are due to meiotic nondisjunction, approximately 4% are due to an unbalanced Robertsonian translocation (most commonly between chromosomes 14 and 21).

  • Inheritance: Unlike nondisjunction (which is usually sporadic), a translocation can be inherited from a balanced carrier parent.
  • Karyotype: An affected child would have 46 chromosomes, but one chromosome 14 would have a chromosome 21 attached to it, resulting in three effective copies of chromosome 21.

 

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