Genetic Mutations: Types, Causes, and Effects

Posted on Feb 26, 2025 in Biology

Genetic Mutations

Mutations are random alterations of genetic material. At the species level, gene variability allows for adaptation to environmental changes, potentially preventing extinction. Mutations have driven the evolution of species. There are two main types: somatic mutations and germline mutations; the latter are transmitted to offspring. Mutations can also be classified by their origin: natural (spontaneous) or induced.

Types of Mutations Based on Affected Genetic Material

Gene Mutations

Gene mutations, often called point mutations, are alterations in the nucleotide sequence of a gene. They occur in two primary ways:

  • Base substitutions: These account for 20% of spontaneous gene mutations.
  • Insertions or deletions (Indels): These mutations involve the loss or insertion of nucleotides, causing a frameshift in the normal reading order. The consequences can be serious. They constitute 80% of spontaneous gene mutations.

Chromosomal Mutations

Chromosomal mutations cause changes in the internal structure of a chromosome, altering the sequence of genes.

  • Deletion: Loss of a chromosome fragment. If the fragment contains many genes, the consequences can be severe, even lethal. In humans, a deletion on chromosome 5 causes “Cri du Chat” syndrome, characterized by intellectual disability; affected individuals often do not reach adulthood.
  • Duplication: Repetition of a chromosome segment. The copy can be on the same chromosome, a different chromosome, or exist independently with its own centromere. Duplications increase genetic material, and if this material undergoes further mutations, it can drive the evolution of species.
  • Inversion: Reversal of a chromosome fragment.
    • Pericentric inversion: Includes the centromere.
    • Paracentric inversion: Does not include the centromere.

Inversions are generally not harmful to the individual but can affect offspring, as meiosis may produce abnormal gametes.

Translocation: Change in position of a chromosome segment. A reciprocal translocation involves an exchange of fragments between non-homologous chromosomes. Transposition: A fragment moves to another part of the same chromosome or to a different chromosome.

Translocations do not typically affect the individual but can impact offspring, as meiosis can result in chromosomes with duplications or deletions.

Genomic Mutations

Genomic mutations are alterations in the number of chromosomes typical of a species.

  • Aneuploidy: Alteration in the normal number of copies of one or more chromosome types, without affecting the entire chromosome set.
    • Nulisomy (0 copies)
    • Monosomy (1 copy)
    • Trisomy (3 copies)
    • Tetrasomy (4 copies)
  • Euploidy: Alterations affecting entire chromosome sets.
    • Haploidy: Only one set of chromosomes.
    • Polyploidy: More than two sets of chromosomes. This is common in plants, often resulting in larger leaves and fruits.

Causes of genomic mutations include: centric fusion (joining of two non-homologous chromosomes), centric fission (splitting of a chromosome into two), and incorrect segregation during meiosis (failure of homologous chromosomes or sister chromatids to separate completely).

Mutagens

Mutagens are factors that significantly increase the normal frequency of mutations. They can be:

  • Physical agents (radiation):
    • Non-ionizing: Ultraviolet rays.
    • Ionizing: X-rays, gamma rays (?), and radiation from nuclear explosions.
  • Chemical agents: Examples include nitrous acid and mustard gas.