genetic load

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).

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

\mathbf{A} _1 ... \mathbf{A} _n

, which have the fitnesses

w_1 ... w_n

and the allele frequencies

p_1 ... p_n

respectively. Ignoring frequency-dependent selection, then genetic load (

L

) may be calculated as:

L =  - \bar w}\over w_{\mathrm{max}}}~~~~~~~~~~(1)

Where

w_{\mathrm{max}}

is the maximum value of the fitnesses

w_1 ... w_n

and

\bar w

is mean fitness which is calculated as the mean of all the fitnesses weighted by their corresponding allele frequency:

\bar w = {\sum_{i=1}^n {p_i w_i}} ~~~~~~~~~~(2)

Where the

i^\mathrm{th}

allele is

\mathbf{A}_i

and has the fitness and frequency

w_i

and

p_i

respectively. When the

w_{\mathrm{max}} = 1

(1)

simplifies to:

L = 1 - \bar w ~~~~~~~~~~(3)

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

\mathbf{A}_1

and

\mathbf{A}_2

. If

w_1 > w_2

, then at equilibrium,

p_1 \approx 1

and

p_2 \approx 0

, hence

\bar{w} \approx w_{\mathrm{max}}

, and

L \approx 0

. 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

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

External links

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