Genetic Load


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.

Keywords: load; mutations; segregation; drift

Figure 1.

Fixation probability of a deleterious allele as a function of the selection coefficient against the homozygous mutant. The probability of fixation of deleterious alleles decreases rapidly with effective population size, Ne. The solid line represents Ne=100, short dashes Ne=50, and long dashes Ne=20.

Figure 2.

Amount of load expected per new deleterious mutation as a function of the selection coefficient against the homozygous mutant. Intermediate values of the selection coefficient (near 0.4/Ne) give the most load per new mutation, because strongly selected alleles rarely fix but weakly selected alleles have little effect on fitness even when fixed. Drift load increases strongly as Ne gets small. The solid line represents Ne=100, short dashes Ne=50, and long dashes, Ne=20.



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Further Reading

Halligan DL and Keightley PD (2009) Spontaneous mutation accumulation studies in evolutionary genetics. Annual Review of Ecology, Evolution, and Systematics 40: 151–172.

Lynch M (2010) Rate, molecular spectrum, and consequences of human mutation. Proceedings of the National Academy of Sciences of the USA 107: 961–968.

Wallace B (1970) Genetic Load: Its Biological and Conceptual Aspects. New Jersey: Prentice‐Hall.

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How to Cite close
Whitlock, MC, and Davis, B(Jul 2011) Genetic Load. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001787.pub2]