Genetic load


The genetic load (also genetic burden) is a number between 0 and 1 which describes the fitness of an individual relative to the ‘ideal’ individual. It is the relative chance that an individual will die before reproducing due to the deleterious alleles that it carries. Ignoring frequency-dependent selection, it is calculated as follows:

L = (Wopt – v) / Wopt

Where L is the genetic load, Wopt is the fittest genotype, and v is mean fitness (the fitness of each genotype multiplied by its frequency). In a population where Wopt = v, the genetic load, L = 0. Types of genetic load include mutational load and selectional load.


Mutational load is the genetic load created by mutations that introduce either inferior or superior alleles to the population. Mutations affect the mean fitness of a population.

Segregational load is a type of genetic load caused when a population is segregating less-fit homozygotes due to the reproductive fitness of heterozygotes. The large segregational load of heterozygote advantage is the reason why the balanced school's view on maintaining genetic polymorphisms was considered flawed by other geneticists.