Первый слайд презентации: 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 o a d I n H u m a n p o p u l ation
Слайд 2: Genetic load
Genetic load : the extent to which the fitness 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. G enetic load : T he average number of lethal mutations per individual in a population. Such mutations result in the premature death of the organisms carrying them.
Genetic load : the difference between the average fitness of the 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
Слайд 4: Types o f G e n e t i c L o a d
Three main kinds of genetic load may be recognized: A. I n p u t L o a d : in which inferior alleles are introduced into the gene pool of a population either by mutation or immigration; B. B a l a n c e d L o a d : which is created by selection favouring allelic or genetic combinations that, by segregation and recombination, form inferior genotypes every generation; and
C. S u b s t i tutio n a l L o a d : W hich is generated by selection favouring the replacement of an existing allele by a new allele. Originally called the ‘cost of natural selection’ b y the geneticist J. B. S. Haldane, substitutional load is the genetic load associated with transient polymorphism. T he term ‘genetic load’ was originally coined by H. J. Muller in 1950
Слайд 7: Genetic load an Example… Selective death (or genetic death ): the chance that an individual will die without reproducing as a consequence of natural selection. [ e.g.,15% of offspring in above ]
Two alleles ( A and a) with frequencies p = q = 0.5: Survival to reproduce: AA = 40% Aa = 50% aa = 30% The relative fitness values are: AA = 0.8 Aa = 1 aa = 0.6 The mean fitness of the population = 0.25(0.8) + 0.5(1) + 0.25(0.6) = 0.85 The load of this population (L) = 1 – 0.85 = 0.15 [ Note that if every member of the population had the same genotype the average fitnes would equal 1 and the load on the population would be zero.]
Слайд 8: C a u s e s o f G e n e t i c L o a d
1.Deleterious mutation 2.Beneficial mutation 3.Inbreeding 4.Recombination/segregation load
Слайд 9: D E L E T E R I O U S M U T A T IONS
Deleterious mutation load is the main contributing factor to 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.
Слайд 10: Beneficial mutation
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. The difference between the theoretical maximum (which may not actually be present) and the average is known as the "lag load.
Слайд 12: Inbreeding
Inbreeding increases homozygosity. In the short run, an increase in inbreeding increases the probability with which offspring get two copies of a recessive deleterious alleles, lowering fitnesses via inbreeding depression. I n a species that habitually inbreeds, e.g. through self-fertilization, recessive deleterious alleles are purged.
Слайд 13: Recombination/segregation load
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 underdominant heterozygotes (i.e. heterozygotes that are less fit than either homozygote).
Recombination load arises through unfavorable combinations across multiple loci that 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.
Слайд 15: Genetic load : Mutation
Let’s assume: (i) new mutations are deleterious alleles, and (ii) recessive. Remember the approximation of the equilibrium frequency of deleterious alleles [See population genetics, Topic 5 for a review]: q = ( µ /s) 1/2 Remember that population load is: L = 1 - W And remember that the average fitness under these assumptions was: W = 1 – sq 2 We can make substitutions: L = 1 - W L = 1 – (1 – sq 2 ) L = 1 – (1 – s( µ /s)) L = 1 – (1 – µ ) L = µ It is interesting that we estimate that the load is equal to the mutation rate. Because it suggests that the load is approximately independent of the reduction in fitness caused by the mutant ( s ).
Слайд 16: Genetic load: segregational
Segregational load is a big problem for the balance school: The model Genotype AA Aa aa Frequency p 0 2 2 p 0 q 0 q 0 2 w 1 – s 1 1 1 – s 2 Well known examples exist; Haemoglobin, MHC locus, etc.
There is a cost to selection, in genetic death, during this time period
Слайд 18: Migration load
Migration load is the result of nonnative organisms that 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.
“ It is altogether unlikely that two genes would have identical 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
Слайд 20: Defining Directional Section
Directional selection : selection that favours the phenotype at an extreme of the range of phenotypes Directional selection : can be subdivided into two broad categories. 1. Positive Darwinian selection 2. Negative Darwinian selection
Слайд 21: Defining two types directional 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.