Genetic Variation

ninja icon
Key Knowledge:
  • Mutations as the source of new alleles
  • Causes of changing allele frequencies in a population’s gene pool, including gene flow


Evolution requires genetic variation among members of a species (to allow for the differential selection of heritable traits)

  • Mutations function as a source of new alleles and (along with gene flow) increase the genetic diversity of a population


Mutations

A gene mutation is a change in the nucleotide sequence of a section of DNA encoding for a specific trait 

  • Mutations can give rise to new versions of a gene (called alleles) and hence change the characteristics of an organism 
  • Only germ line mutations (in gametes) produce heritable variation,somatic mutations (in body cells) are not passed to offspring


Point Mutations

Point mutations involve the substitution of a single base of DNA, which may alter the phenotype by changing the protein sequence

  • Silent mutations occur when the DNA change does not alter the amino acid sequence (due to degeneracy of the genetic code)
  • Missense mutations occur when the DNA change alters a single amino acid in the polypeptide chain (can create new alleles)
  • Nonsense mutations occur when the DNA change creates a premature STOP codon which truncates the polypeptide


point mutations


Frameshift Mutations

Frameshift mutations involve either the addition (insertion) or removal (deletion) of a single base of DNA, changing the reading frame

  • This change will affect every codon beyond the point of mutation and thus may dramatically change the amino acid sequence


frameshift mutation


Block Mutations

Block mutations are changes to segments of a chromosome, resulting in large scale changes in the DNA of an organism

  • Block mutations are commonly caused by transposons (mobile genetic elements that can change positions within the genome)


There are many different types of block mutations that can exist, including:

  • Duplications – a part of the chromosome is copied, resulting in duplicate segments (potentially increasing gene expression)
  • Deletions – a portion of a chromosome is lost (along with any genes contained within this segment)
  • Inversions – a segment of a chromosome is removed and then replaced in reverse order
  • Translocations – segments of two chromosomes are exchanged (may interrupt gene sequences)


block mutations


Gene Flow

Mutations are the only source of new alleles within a population, however alleles can also be introduced from other populations

  • The movement of alleles between interbreeding populations (as a result of migration and sexual reproduction) is called gene flow
  • Gene flow maintains the genetic compatibility between two separate populations and hence functions to prevent speciation


gene flow