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 by the British population geneticist J.B.S. Haldane. See Haldane (1957). Population genetics is the study of the distribution of and change in allele frequencies under the influence of the five evolutionary forces: natural selection, genetic drift, mutation, migration and nonrandom mating. ... For computer science algorithms that find the kth smallest number in a list, see selection algorithm. ... Mutations are permanent, sometimes transmissible (if the change is to a germ cell) changes to the genetic material (usually DNA or RNA) of a cell. ... John Burdon Sanderson Haldane (November 5, 1892 - December 1, 1964), who normally used J.B.S. as a first name, was a geneticist born in Scotland and educated at Eton and Oxford University. ...

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".

Mathematics

Consider a single gene locus with the alleles , which have the fitnesses w₁...w_n and the allele frequencies p₁...p_n respectively. Ignoring frequency-dependent selection, then genetic load (L) may be calculated as: An allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. ... Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ... Allele frequency is a term of population genetics that is used in characterizing the genetic diversity of a species population, or equivalently the richness of its gene pool. ... Frequency dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on the relative frequency of other phenotypes in a given population. ...

Where w_max is the maximum value of the fitnesses w₁...w_n and is mean fitness which is calculated as the mean of all the fitnesses weighted by their corresponding allele frequency: The largest and the smallest element of a set are called extreme values, or extreme records. ... In statistics, mean has two related meanings: the average in ordinary English, which is more correctly called the arithmetic mean, to distinguish it from geometric mean or harmonic mean. ... Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ...

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

When the w_max = 1, (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. For computer science algorithms that find the kth smallest number in a list, see selection algorithm. ... For the socioeconomic meaning, see social inequality. ...

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 . In population genetics, directional selection occurs when natural selection favors a single allele and therefore allele frequency continuously shift in one direction. ... mutation-selection balance is a balance between mutation and selection. ...

In contrast to directional selection, heterozygote advantage always exerts a load at equilibrium. A heterozygote advantage (heterozygous advantage or overdominance) describes the case in which in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. ...

If the mean fitness is 0, the load is equal to 1, but the population goes extinct. In biology and ecology, extinction is the ceasing of existence of a species or group of species. ...

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. Creationism is generally the belief that the universe was created by a deity, or alternatively by one or more powerful and intelligent beings. ... Henry M. Morris Dr Henry Madison Morris (born 1918) is an American young earth creationist and hydraulic engineer, considered the father of the creation science movement. ...

References

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

The Journal of Genetics (not to be confused with another journal called Genetics) is a scientific journal concerning genetics. ...

External links

• Genetic load, from Evolution A-Z by Mark Ridley
• Creationist Claim CB120: Genetic load from An Index to Creationist Claims by Mark Isaak