Structural Aberrations of Chromosomes in Human Genetics

Structural Aberrations of Chromosomes

Types of Structural Chromosomal Aberrations

  • Structural chromosomal aberrations can be classified into several types, including deletions, duplications, inversions, and translocations.

Breaks on Chromosomes and Causes of Breaks

  • Chromosomal breaks can occur due to various factors such as environmental stresses, radiation, and errors during DNA replication.

Chromosome Deletions

  • Definition: A deletion is the loss of a segment of a chromosome.
  • Mechanism of Occurrence: Can occur during DNA replication or due to environmental factors.
  • Types of Deletions:
    • Terminal Deletions: occur at the end of a chromosome.
    • Intercalary Deletions: occur within the chromosome.

Pure Deletions

  • Types:
    • Homozygous deletions: both chromosome copies are deleted.
    • Heterozygous deletions: one chromosome copy is missing.
  • Mechanism of Occurrence: Often involves misalignment during meiosis.

Chromosome Duplications

  • Definition: A duplication involves the presence of an extra segment of a chromosome.
  • Mechanism of Occurrence: Often results from unequal crossing over during meiosis.
  • Consequences for Health: Can lead to genetic disorders due to gene dosage effects.

Ring Chromosome, Isochromosome, and Dicentric Chromosome

  • Ring Chromosome: Circular chromosome due to fusions.
  • Isochromosome: A chromosome with identical arms resulting from misdivision.
  • Dicentric Chromosome: A chromosome that has two centromeres, usually resulting from translocation events.

Chromosome Inversions

  • Definition: An inversion occurs when a segment of the chromosome is reversed end to end.
  • Mechanism of Occurrence: Caused by breaks in the chromosome followed by reinsertion of the segment in reverse orientation.
  • Types of Inversions:
    • Pericentric Inversions: involve the centromere, affecting both arms of the chromosome.
    • Paracentric Inversions: do not involve the centromere, affecting only one arm.

Genetic Risk for Offspring from Inversions

  • Pericentric Inversions:
    • Gametogenesis can lead to unbalanced gametes.
  • Paracentric Inversions:
    • Result similarly in potential genetic imbalances.

Difference Between Inversion and Transposition

  • Inversion: involves reversal of chromosome segments; results in different gene orientations.
  • Transposition: involves movement of segments from one chromosome to another.
  • Examples:
    • Inversion: 1q21 inversion.
    • Transposition: gene jumping (transposons).

Translocations

  • Definition: A translocation involves the movement of chromosomal segments between non-homologous chromosomes.
  • Types:
    • Reciprocal Translocations: exchange of segments between chromosomes (e.g., Philadelphia chromosome).
    • Non-reciprocal Translocations: transfer of a segment without reciprocal exchange.

Gametogenesis in Heterozygous Reciprocal Translocation Carriers

  • Results in various types of gametes, some of which can be genetically unbalanced, increasing the risk for genetic disorders in offspring.

Genetic Risk for Offspring of Heterozygous Reciprocal Translocation Carriers

  • Increased probability of offspring having chromosomal abnormalities due to unbalanced genetic material.

Robertsonian Translocations

  • Definition: A type of translocation where two acrocentric chromosomes fuse.
  • Mechanism of Occurrence: Fusion at the centromere.
  • Types:
    • Homologous (21,21): involves two similar chromosomes.
    • Heterologous (14,21): involves different chromosomes.
  • Genetic Risk for Offspring:
    • Homologous (21,21): High risk, usually resulting in Down syndrome.
    • Heterologous (14,21): Moderate risk of genetic disorders due to unbalanced gametes.