Genetic burden

The genetic term of burden , used for the first time per H.J. Muller in 1950, indicates the whole of the unfavourable genetic mutations in a population. The presence, within a population, individuals with different Génotype S indicates that the selective Valeur ( fitness ) of the population is not optimal. The genetic burden points a deficit of adaptation in a population and a fall of the total selective value.

In the years 1960, when Richard Lewontin initiates the study of the molecular evolution, the description of the genetic polymorphism of the populations within a species, completely unsuspected, led to popularize the concept of genetic burden.

Calculation of the genetic burden

The genetic burden, noted L, can be measured by:

L = 1 - W / W _max

Where:

W is the average selective value of the population
W _max is the selective value of the best genotypes of the population

The genetic burden is thus defined as the relative difference between the optimal selective value of a population of which all the individuals would have the best possible genotype and the real selective value of the population.

Components of the genetic burden

The genetic burden has three components:

the burden of Change : appearance of noxious changes at the individuals.
the burden of substitution : the transitory genetic polymorphism which creates for itself time that an allele replaces another in a population of them.
the burden of segregation : the maintenance of a stable genetic polymorphism during time, owing to the fact that the segregation mendélienne, with each generation, manufactures individuals of less selective value while making reappear the least optimal genotype.

Limits of the concept of burden genetic

The genetic burden, as a deficit of adaptation, is conceived only for one isolated population, which exploits a stable and constant medium:

stable Medium in space : if a species exploits mediums different and so exchanges from individuals between the populations of these mediums take place then - or of the exchanges of genes through the reproduction between members of the various mediums - it is impossible that an optimal genotype for each one of these mediums is selected. The migration of genes maintains the genetic burden each population.

stable Medium in time : the modification of a medium during time makes that the optimal genotypes at a certain time will not be optimal any more at another time. In fact the variations of the environment maintain the genetic burden.

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