Changes in the Gene Pool

Gene Pool Changes

All alleles within a species constitute the gene pool. New genes enter the gene pool via mutations or genetic leakage. Genetic variability is vital for a species' survival, enabling evolution and adaptation to environmental changes. Some traits offer a selective advantage, leading to increased viable, fertile offspring.

Factors Causing Changes in Allele Frequency

This section covers mutations, genetic leakage, and genetic drift, which alter alleles or their frequency in the gene pool.

Mutations

A mutation is a DNA sequence change resulting in a mutant allele. Mutant alleles contrast with wild-type alleles, which are normal, natural, and common in the population.

Causes of Mutations:
  • Ionizing radiation: UV rays and chemical exposures can damage DNA.
  • Mutagens: Substances causing mutations.
  • DNA polymerase errors: Mistakes during DNA replication, although proofreading mechanisms minimize these.
  • Transposons: Elements that insert and remove themselves from the genome. Insertion within a coding sequence disrupts the gene.
  • Flawed proteins: Incorrect nucleotide pairing during transcription/translation or tRNA charged with the wrong amino acid can lead to amino acid sequence errors.
Nucleotide Level Mutations

Occur at a single nucleotide level.

  • Point Mutations: One nucleotide is swapped for another (A, C, T, or G).
    • Silent Mutations: Nucleotide change has no effect on the final protein due to genetic code degeneracy (wobble).
    • Missense Mutations: Nucleotide change substitutes one amino acid for another in the final protein.
    • Nonsense Mutations: Nucleotide change substitutes a stop codon for an amino acid, prematurely terminating the protein.
  • Frameshift Mutations: Nucleotides are inserted or deleted, shifting the reading frame. This leads to altered amino acid sequences or premature truncation due to a nonsense mutation.
    • Insertion mutations
    • Deletion mutations
Chromosomal Mutations

Large-scale mutations affecting large DNA segments.

  • Deletion Mutations: Large DNA segment is lost from a chromosome. Small deletions are frameshift mutations.
  • Duplication Mutations: DNA segment is copied multiple times.
  • Inversion Mutations: DNA segment is reversed within the chromosome.
  • Insertion Mutations: DNA segment moves from one chromosome to another. Small insertions are frameshift mutations.
  • Translocation Mutations: DNA segment from one chromosome swaps with a segment from another.
Consequences of mutations

Mutations can be advantageous, detrimental, or inconsequential

  • Advantageous mutations: Confer a positive selective advantage, that may allow the organism to produce, fitter offspring. For example, sickle cell disease is a single nucleotide mutation that causes sickled hemoglobin. While the disease itself is detrimental to life, heterozygotes for sickle cell disease usually have minor symptoms, if any, and have natural resistance to malaria.
  • Detrimental or deleterious mutations: For example, x e r o d e r m a pigmentosium XP is an inherited defect in the nucleotide excision repair mechanism. In patients with XP DNA that has been damaged by ultraviolet radiation cannot be repaired appropriately. Ultraviolet radiation can introduce cancer causing mutations since they lack a repair mechanism patients with XP are frequently diagnosed with malignancies, especially of the skin.
  • Inborn errors of metabolism: Defects in genes required for metabolism. For example, in phenylketonuria (PKU), the enzyme phenylalanine hydrolase is defective. Toxic metabolites of phenylalanine accumulate causing seizures, impairment of cerebral function, and learning disabilities, as well as a musty odor to bodily secretions. If the disease is discovered shortly after birth, then dietary phenylalanine can be eliminated, and treatments can be administered to aid in metabolizing any remaining phenylalanine
Leakage

Genetic leakage is the flow of genes between species. For example, the male hybrid of a male horse and a female donkey are not able to reproduce because they have odd numbers of chromosomes. Horses have 64 chromosomes and donkeys have 62. So mules with 63 chromosomes cannot undergo normal homologous pairing in meiosis and cannot form gametes. A hybrid can reproduce with members of one species or the other, such as the beefalo, a cross between cattle and American bison. The hybrid carries genes from both parent species, so this can result in a net flow of genes from one species to the other.

Genetic Drift

Genetic drift refers to changes in the composition of the gene pool due to chance and is more pronounced in small populations.

  • Founder Effect: A small population becomes reproductively isolated due to natural barriers or catastrophic events, resulting in a drastic reduction in population size.
  • Inbreeding: Mating between genetically related individuals, common in small populations. It increases homozygosity, raising the prevalence of homozygous dominant and recessive genotypes.

Drift, the founder effect, and inbreeding reduce genetic diversity, increasing the prevalence of certain traits and diseases in small populations.

  • Inbreeding Depression: Reduced fitness of a population due to loss of genetic variation. For example, branched chain keto acid dehydrogenase deficiency (maple syrup urine disease) is common in Mennonite communities, implying a common origin in a small population.
  • Outbreeding (Outcrossing): Introduction of unrelated individuals into a breeding group. It increases variation within the gene pool and enhances population fitness.