Understanding Genetic Mutations: Types, Causes, and Effects

Posted on Oct 26, 2024 in Biology

1. Genetic Mutations: An Overview

Genetic mutations are alterations in the DNA sequence that can be passed down to offspring. The term “mutation” was introduced by Hugo de Vries, who rediscovered Mendel’s laws while studying the Oenothera Lamarckian. Mutations describe changes in the sequence or number of nucleotides within a cell’s DNA.

Mutations can be:

  • Harmful: Potentially causing death or reducing the likelihood of survival.
  • Beneficial: Increasing the likelihood of survival.
  • Neutral: Producing neither benefit nor harm.

Many genes have different versions called alleles, which arise from mutations in an original gene. It’s virtually impossible for two individuals to have identical genetic makeup, even within the same species. For example, there are alleles for blue eyes and dark eyes.

Types of Mutations (Based on Cell Type)

  • Germline Mutations: Affect gametes (sex cells) and are heritable, meaning they are passed on to offspring.
  • Somatic Mutations: Affect somatic cells (non-sex cells) and are not heritable, thus playing no role in evolution.

Types of Mutations (Based on Genetic Material Affected)

  • Gene Mutations: Changes in the nucleotide sequence of a specific gene.
  • Chromosomal Mutations: Affect the arrangement of genes on a chromosome, but not the gene’s nucleotide sequence itself.
  • Genomic Mutations: Alter the number of chromosomes within a species.

Causes of Mutations

Mutations can occur spontaneously due to natural causes. The mutation rate is lower in bacteria and higher in more complex organisms, varying considerably from gene to gene. Environmental factors, such as physical or chemical agents (mutagens), can also induce mutations.

2. Gene Mutations

Gene mutations are typically small-scale changes. There are two main types:

Base Substitutions

These account for 20% of gene mutations and involve swapping one DNA base for another. They can be further classified into:

  • Transitions: A purine base is replaced by another purine (A to G or G to A), or a pyrimidine is replaced by another pyrimidine (C to T or T to C).
  • Transversions: A purine is replaced by a pyrimidine or vice versa (e.g., A to C, A to T, G to C, G to T).

These mutations affect a single nucleotide and, therefore, only one codon. If the mutation doesn’t change the resulting amino acid, it’s called a silent mutation. However, some mutations can result in shorter or longer proteins. In some cases, a mutation may produce a protein that improves upon the original, providing a benefit to the carrier that can be passed on to their descendants.

Frameshift Mutations

These mutations involve insertions or deletions—the addition or loss of a nucleotide. They cause a shift in the reading frame during translation, resulting in a completely different protein.

3. Chromosomal Mutations

These mutations involve changes in the structure of chromosomes. Most organisms are diploid, meaning they have two copies of each chromosome. If one chromosome has a mutation, the other copy is usually normal.

Types of Chromosomal Mutations

  • Deletions: A segment of a chromosome is missing (e.g., Cri du Chat syndrome, caused by a deletion on chromosome 5).
  • Duplications: A chromosomal segment appears more than once on the same chromosome, playing a role in evolution.
  • Translocations: A segment of a chromosome changes location. These can be reciprocal (exchange between two non-homologous chromosomes) or non-reciprocal (transposition).
  • Inversions: A segment of a chromosome is reversed. These can be paracentric or pericentric.

Consequences of Chromosomal Mutations

Translocations and inversions have minimal impact unless the number of genes is altered. Deletions and duplications, even if affecting only one chromosome, can have serious consequences. It’s not enough to possess all the genes; they must also be present in the correct numbers. These mutations can disrupt meiosis, hindering the pairing of homologous chromosomes and potentially leading to non-viable offspring or offspring with mutations.

4. Genomic Mutations

These mutations involve variations in the total number of chromosomes within a species.

Euploidy

  • Monoploidy (Haploidy): Only one set of chromosomes is present. This is rare in nature, occurring in some plant species.
  • Polyploidy: More than two sets of chromosomes are present (e.g., triploid (3n), tetraploid (4n)). This is more common in plants than in animals.

Aneuploidy

  • Trisomies: An extra chromosome is present (2n + 1). These can affect both autosomes and sex chromosomes (e.g., Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), Klinefelter syndrome, Triple X syndrome).
  • Monosomies: A chromosome is missing (2n – 1) (e.g., Turner syndrome).