GENETIC LOAD

BY: RAHUL ANDHARIA (MSIWM001)

Introduction:

  • Genetic load can be defined as the reduction in average or mean fitness of the population due to deleterious alleles (lethal alleles) which may cause death or sterility of the affected genotype.
  • The term genetic load was first used by American geneticist H.J Muller.
  • Genetic load is given as:
  • Equations used to establish genetic load consider single locus systems with alleles.                      (L= wmax w / wmax))(1). Wmax is hypothetical fitness.
  • L= 1-w (2) where w,   is the average or mean fitness of population.

{P2w1 + 2pqw2 + q2w3}   (where w1, w2 and w3 are relative fitness of the genotype AA, Aa and aa.)   

Factors affecting Genetic load: Selection, Mutation, Balance/segregation and heterozygote advantage.

Types of genetic load:

  1. Mutational load:
  2. Genetic load arising due to deleterious alleles caused by mutations is known as mutational load.
  3. If a recessive gene is deleterious in homozygous condition, the loss in frequency of individuals incurred by genetic load= sq2
  4. Suppose, if N individuals were in a population before selection than sq2 * N are eliminated because of genetic load (genetic deaths).
  5. Therefore, recessive load caused by deleterious recessive allele is given as

L= sq2= u (mutation rate)

  • For a dominant deleterious  allele, loss in frequency of individuals due to genetic load is:

Frequency of affected individuals * selection co-efficient, that is

                          =2p * s

                          =2 * u/s*s= 2u

Therefore, L= 2u.

  • Segregational/balanced load:
  • Due to heterozygote advantage, as the frequency of heterozygote’s increases, homozygotes are produced that die either before birth or are sterile.
  • Therefore there is wastage of alleles or genetic cost which we call as genetic load.
  • The balanced load gets created during selection, favouring allelic or genetic combinations which form inferior genotypes every generation by segregation.
  • In case of heterozygote advantage, mean fitness of population is, w= 1- sp2– tq2.
 AAAaaa
Initial zygotic frequencyP22pqq2
Fitness (w)(1-s)1(1-t)
Each genotype contribution to next generationP2(1-s)2pqq2(1-t), q2– sp2– tq2

Allele frequencies are as follows:

  • P’= t/s+t   and  q’= s/s+t, (equation m), where ‘s’ and ‘t’ are selection co-efficient against AA and aa genotype respectively. Substituting the (equation m) allele frequencies in equation frequency of w, we get:   w= 1-sp2-tq2

                                     = 1-s(t/s+t)2  – t(s/s+t)2

                                        = 1-st2-ts2/(s+t)2   = 1-{st(t+s}/(s+t)2 (so, here t+s and the square gets cancelled)

          Therefore, w= 1-st/s+t  and segregational load is L= st/s+t.

  • The above equation is applicable to one locus. For several loci the segregational load will increase dramatically.
  • Since population fitness will decline rapidly with increase in number of loci, thus:

W= (1-L)i here i, is no of loci showing over-dominance.

  • The required increase in the reproduction rate of surviving individuals will be quite large.
  • In most species, however the possible increase in the reproduction rate per surviving individual will be usually much smaller than required to compensate for this genetic load.
  • As a result, the population would most likely become extinct over time.

Application and Significance:

  • Theory known as Fitness indication theory was build on the assumption that in humans, deleterious germ line mutations can influence fitness outcomes related to their pleiotropic effects on traits.(can influence different or multiple traits at once). There are assumptions and predictions that there exists a fitness factor(F) among traits that signals sensitivity of development to perturbation stemming from deleterious mutations that are present. Therefore, among sexually reproducing taxa, there should exist genetic correlation between distinct traits. The theory was proposed in the year 2000 by Houle and Miller.
  • In future micro-evolutionary trends, in particular for fitness stemming from purifying selection and from mutation accumulation in industrialised population is significant in terms of concept of genetic load.

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