N a m e : A v a n i Rathod G r p. N o. : 1 9 5 - B T o p i c : G e n e t i c L

population

of an individual is below the optimum for the population

as a whole due to the deleterious alleles that the individual carries in its genome.
Genetic load : The average number of lethal mutations per individual in a population. Such mutations result in the premature death of the organisms carrying them.

population and the fitness of the best genotype. It measures

the probability of selective death of an individual in a population.
W = average fitness
Genetic load (L) = 1 - W

be recognized:
A.Input Load: in which inferior alleles are introduced into

the gene pool of a population either by mutation or immigration;
B. Balanced Load: which is created by selection favouring allelic or genetic combinations that, by segregation and recombination, form inferior genotypes every generation; and

replacement of an existing allele by a new allele.
Originally called

the ‘cost of natural selection’ by the geneticist J. B. S. Haldane, substitutional load is the genetic load associated with transient polymorphism.
The term ‘genetic load’ was originally coined by H. J. Muller in 1950

that an individual will die without reproducing as a consequence

of natural selection. [e.g.,15% of offspring in above]

genetic load overall.
Most mutations are neutral or slightly deleterious,

and occur at a constant 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.
High load can lead to a small population size, which in turn increases the accumulation of mutation load, culminating in extinction via mutational meltdown.

present in the population.
When load is calculated as the difference

between the fittest genotype present and the average, this creates a substitutional load.
The difference between the theoretical maximum (which may not actually be present) and the average is known as the "lag load.

inbreeding increases the probability with which offspring get two copies

of a recessive deleterious alleles, lowering fitnesses via inbreeding depression.
In a species that habitually inbreeds, e.g. through self-fertilization, recessive deleterious alleles are purged.

together may not work when recombined with a different suite

of coevolved alleles, leading to outbreeding depression.
Segregation load is the presence of underdominant heterozygotes (i.e. heterozygotes that are less fit than either homozygote).

appear when favorable 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.

balance school:

Well known examples exist; Haemoglobin, MHC locus, etc.

this time period

aren’t adapted to a particular environment coming into that environment.

If they breed with individuals who are adapted to that environment, their offspring will not be as fit as they would have been if both of their parents had been adapted to that particular environment.

selective values under all the conditions under which they may

coexist in a population. … cases of neutral polymorphism do not exist … it appears probable that random fixation is of negligible evolutionary importance”
-------Ernst Mayr

an extreme of the range of phenotypes


Directional selection: can be

subdivided into two broad categories.
1.Positive Darwinian selection
2.Negative Darwinian selection

Type 1:

Positive Darwinian selection: directional selection for fixation of a new and beneficial mutation in a population.

Type 2:

Negative Darwinian selection: directional selection for removal of a new and deleterious mutation from a population.

Negative selection: same as “negative Darwinian selection”.

Purifying election: same as negative selection.