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Genetic Load

  1. MC Whitlock,
  2. B Davis

Published Online: 15 JUL 2011

DOI: 10.1002/9780470015902.a0001787.pub2



How to Cite

Whitlock, M. and Davis, B. 2011. Genetic Load. eLS. .

Author Information

  1. University of British Columbia, Vancouver, Canada

Publication History

  1. Published Online: 15 JUL 2011


Genetic load is the reduction in the mean fitness of a population relative to a population composed entirely of individuals having optimal genotypes. Load can be caused by recurrent deleterious mutations, genetic drift, recombination affecting epistatically favourable gene combinations, or other genetic processes. Genetic load potentially can cause the mean fitness of a population to be greatly reduced relative to populations without sources of less fit genotypes. Mutation load can be difficult or impossible to measure. Many species have mutation rates low enough that substantial genetic load is not expected, but for others, such as humans, the mutation rate may be great enough that load can be substantial. In extremely small populations, drift load, caused by the fixation by drift of weakly deleterious mutations, can threaten the probability of persistence of the population. Migration from other populations adapted to different local conditions can bring in locally maladapted alleles, resulting in migration load.

Key Concepts:

  • Genetic load is the reduction in mean fitness of a population caused by some population genetic process.

  • Mutation load is the reduction in fitness caused by recurrent deleterious mutations.

  • Mutation load may be as great as 95% for the human population.

  • Drift load is the reduction in mean fitness caused by genetic drift. In extreme cases, deleterious alleles can reach a frequency of one in a population because of genetic drift.

  • Genetic load can also be caused by recombination breaking up beneficial combinations of alleles, segregation reducing the frequency of fit heterozygotes, or migration bringing less fit alleles into a local population.


  • load;
  • mutations;
  • segregation;
  • drift