| Genetic Load |
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The concept was first formulated in 1937 by JBS Haldane , independently formulated, named and applied to humans in 1950 by H. J. Muller 1, 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 Allele s , which have the Fitness es and the Allele Frequencies respectively. Ignoring Frequency-dependent Selection , then genetic load () may be calculated as:
where the allele is and has the fitness and frequency and respectively. When the , then (1) simplifies to : CAUSES OF GENETIC LOAD 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 and . If , 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 In contrast to directional selection, Heterozygote Advantage always exerts a load 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. REFERENCES EXTERNAL LINKS
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