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Encyclopedia :
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Genetic load |
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Genetic loadIn 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 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". MathematicsConsider a single gene locus with the alleles , which have the fitnesseses and the allele frequencies respectively. Ignoring frequency-dependent selection, then genetic load () may be calculated as:
When the , simplifies to:
Selectional loadSelection 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 , then at equilibrium, and , hence , and . 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 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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