In population genetics, genetic load or genetic burden is a measure of the cost of lost alleles due to selection (selectional load) or mutation (mutational load). It is a value in the range , where 0 represents no load. The concept was first formulated by the British population geneticist J.B.S. Haldane. See Haldane (1957).

Definition

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Genetic load may be defined as "the extent to which the average individual in a population is inferior to the best possible kind of individual," which is equivalent to "the relative chance that an average individual will die before reproducing because of the deleterious genes that it possesses."

Mathematics

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Consider a single gene locus with the alleles $\backslash mathbf\{A\}\; \_1\; ...\; \backslash mathbf\{A\}\; \_n$, which have the fitnesses $w\_1\; ...\; w\_n$ and the allele frequencies $p\_1\; ...\; p\_n$ respectively. Ignoring frequency-dependent selection, then genetic load ($L$) may be calculated as:

$L\; =\; -\; \backslash bar\; w\}\backslash over\; w\_\{\backslash mathrm\{max\}\}\}~~~~~~~~~~(1)$

Where $w\_\{\backslash mathrm\{max\}\}$ is the maximum value of the fitnesses $w\_1\; ...\; w\_n$ and $\backslash bar\; w$ is mean fitness which is calculated as the mean of all the fitnesses weighted by their corresponding allele frequency:

$\backslash bar\; w\; =\; \{\backslash sum\_\{i=1\}^n\; \{p\_i\; w\_i\}\}\; ~~~~~~~~~~(2)$

Where the $i^\backslash mathrm\{th\}$ allele is $\backslash mathbf\{A\}\_i$ and has the fitness and frequency $w\_i$ and $p\_i$ respectively.

When the $w\_\{\backslash mathrm\{max\}\}\; =\; 1$, $(1)$ simplifies to:

$L\; =\; 1\; -\; \backslash bar\; w\; ~~~~~~~~~~(3)$

Causes of genetic load

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Load may be caused by selection and mutation.

Mutational load

Load caused by mutations is known as mutational load.

Selectional load

Selection occurs when the fitnesses of particular alleles are inequal, hence selection always exerts a load.

With directional selection, the allele frequencies will tend towards an equilibrium position with the fittest allele reaching a frequency in mutation-selection balance. As mutations are rare, this is effectively fixation. Consider two alleles $\backslash mathbf\{A\}\_1$ and $\backslash mathbf\{A\}\_2$. If $w\_1\; >\; w\_2$, then at equilibrium, $p\_1\; \backslash approx\; 1$ and $p\_2\; \backslash approx\; 0$, hence $\backslash bar\{w\}\; \backslash approx\; w\_\{\backslash mathrm\{max\}\}$, and $L\; \backslash approx\; 0$.

In contrast to directional selection, heterozygote advantage always exerts a load at equilibrium.

If the mean fitness is 0, the load is equal to 1, but the population goes extinct.

Creationist criticism

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Some creationists (such as Henry M. Morris) have suggested that mutational load would increase over time and thus make populations inviable. However, they ignore the effect of selection acting to weed out deleterious mutations.

References

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• Haldane, J.B.S. (1957) The cost of natural selection Journal of Genetics 55:511-524 link (pdf file)

External links

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• The Cost of Natural Selection Revisited, Leonard Nunney, Ann. Zool. Fennici 40:185-194 (pdf file)
• Genetic load, from Evolution A-Z by Mark Ridley
• Creationist Claim CB120: Genetic load from An Index to Creationist Claims by Mark Isaak

population genetics