Genetic load
Encyclopedia
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 in 1937 by JBS Haldane, independently formulated, named and applied to humans in 1950 by H. J. Muller, and elaborated further by Haldane in 1957.
as the reduction in the mean fitness for a population compared to the maximum fitness.
s
, which have the fitness
es
and the allele frequencies
respectively. Ignoring frequency-dependent selection, then genetic load (
) may be calculated as:
where
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
allele is 
and has the fitness and frequency 
and 
respectively.
When the
, then (1) simplifies to
occurs when the fitnesses of particular alleles are inequal, hence selection always exerts a load. This occurs due to natural selection in cases were a particular combination of alleles is favoured on another. In some cases like for example in the case of sickle cell anaemia, heterozygous for this trait or carriers are favoured in cases of malaria and other cases, that is, homozygous normal and sickle cell anaemia are not favoured and die off.
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
.
(also called overdominance) always exerts a load against the less fit homozygotes at equilibrium.
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
, genetic load or genetic burden is a measure of the cost of lost alleles due to selection
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
(selectional load) or mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
(mutational load). It is a value in the range
, 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. Muller, and elaborated further by Haldane in 1957.
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. It is normally stated in terms of fitnessFitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
as the reduction in the mean fitness for a population compared to the maximum fitness.
Mathematics
Consider a single gene locus with the alleleAllele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s
, which have the fitnessFitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
es
and the allele frequenciesAllele frequency
Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for...
respectively. Ignoring frequency-dependent selection, then genetic load () may be calculated as:where
is the maximum value of the fitnesses and is meanMean
In statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....
fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
which is calculated as the mean of all the fitnesses weighted by their corresponding allele frequency:
where the
allele is and has the fitness and frequency and respectively.When the
, then (1) simplifies toMutational load
Mutational load is caused when a mutation at a locus produces a new allele of either lesser or greater fitness than the mean fitness of the population. This lowers the average fitness of the population; a deleterious mutation has a lower relative fitness, lowering average load, while an advantageous mutation effectively decreases the relative fitness of the existing alleles, and thus also decreases the mean fitness.Selectional load
SelectionSelection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
occurs when the fitnesses of particular alleles are inequal, hence selection always exerts a load. This occurs due to natural selection in cases were a particular combination of alleles is favoured on another. In some cases like for example in the case of sickle cell anaemia, heterozygous for this trait or carriers are favoured in cases of malaria and other cases, that is, homozygous normal and sickle cell anaemia are not favoured and die off.
With directional selection
Directional selection
In population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction...
, the allele frequencies will tend towards an equilibrium position with the fittest allele reaching a frequency in mutation-selection balance
Mutation-selection balance
The mutation-selection balance is a classic result in population geneticsfirst derived in the 1920s by John Burdon Sanderson Haldane and R.A. Fisher.A genetic variant that is deleterious will not necessarily disappear immediately from apopulation...
. 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
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
.
Segregational load
In contrast to directional selection, in which one heterozygote has a higher fitness than other homozygote, heterozygote advantageHeterozygote advantage
A heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...
(also called overdominance) always exerts a load against the less fit homozygotes at equilibrium.