b. What do you understand by ‘Genetic Load’ in a population? How is it measured and what are the important factors that can influence it? (2013, paper II Section A)
Genetic load is the difference between the fitness of an average genotype in a population and the fitness of some reference genotype, which may be either the best present in a population, or may be the theoretically optimal genotype. The average individual taken from a population with a low genetic load will generally, when grown in the same conditions, have more surviving offspring.
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.
A mutational load is a genetic load created by mutations that introduce either inferior or superior alleles to the population. Mutations affect the mean fitness of a population.
A 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.
1.Deleterious mutation load is the main contributing factor to genetic load overall. Most mutations are deleterious, and occur at a high rate. The Haldane-Muller theorem of mutation-selection balance says that the load depends only on the deleterious mutation rate and not on the selection coefficient.
2.New beneficial mutations create fitter genotypes than those previously present in the population. When load is calculated as the difference between the fittest genotype present and the average, this creates a substitutional load.
3.Inbreeding increases homozygosity. In the short run, an increase in inbreeding increases the probability with which offspring get two copies of recessive deleterious alleles, lowering fitnesses via inbreeding depression. In a species that habitually inbreeds, e.g. through self-fertilization, recessive deleterious alleles are purged.
Combinations of alleles that have evolved to work well together may not work when recombined with a different suite of coevolved alleles, leading to outbreeding depression. Segregation load is the presence of under dominant heterozygotes (i.e. heterozygotes that are less fit than either homozygote). Recombination load arises through unfavourable combinations across multiple loci that appear when favourable linkage disequilibria are broken down.Recombination load can also arise by combining deleterious alleles subject to synergistic epistasis, i.e. whose damage in combination is greater than that predicted from considering them in isolation.