Segregation load
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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 0 < L < 1, where 0 represents no load. The concept was first formulated in 1937 by JBS Haldane, independently formulated, named and applied to humans in 1950 by H. J. Muller¹, and elaborated further by Haldane in 1957.²

Definition

Genetic load is the reduction in selective value for a population compared to what the population would have if all individuals had the most favored genotype.³ It is normally stated in terms of fitness as the reduction in the mean fitness for a population compared to the maximum fitness.

Mathematics

Consider a single gene locus with the alleles , which have the fitnesses and the allele frequencies respectively. Ignoring frequency-dependent selection, then genetic load (L) may be calculated as:

where w_max is the maximum value of the fitnesses and is mean fitness which is calculated as the mean of all the fitnesses weighted by their corresponding allele frequency:

where the i^th allele is and has the fitness and frequency w_i and p_i respectively.

When the w_max = 1, then (1) simplifies to

Causes of genetic load

Load may be caused by selection and mutation.

Mutational load

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Mutation load is caused when a mutation at a locus produces a new allele of either lesser or greater fitness. This lowers the average fitness of the population; a deleterious mutation has a lower relative fitness, lowering average load, while an advantageous mutation effectively lowers the relative fitness of the existing allele, and thus also lowers average fitness.

Selectional load

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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 and . If w₁ > w₂, then at equilibrium, and , hence , and .

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

Segregational load

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In contrast to directional selection, in which one homozygote has a higher fitness than both the heterozygote and other homozygote, heterozygote advantage (also called overdominance) always exerts a load against the less fit homozygotes at equilibrium.

Creationist criticism

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 selectional load acting to weed out (decrease frequency of) deleterious mutations.^{citation needed}

References

1. ^ Muller, H. J. (1950). "Our load of mutations". Am J Hum Genet 2 (2): 111–76. 
2. ^ JBS Haldane (1957). "The cost of natural selection". Journal of Genetics 55: 511–524, http://www.blackwellpublishing.com/ridley/classictexts/haldane2.pdf. 
3. ^ JF Crow (1958). "Some possibilities for measuring selection intensities in man". Hum. Biol 30: 1–13. 

External links

• 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