University:
California State University, Northridge
Course:
BIOL 360 | Genetics
Academic year:

2006
Views:

91

Pages:

70
Author:

ngepakgcs

Mendelian Population Genetics • Evolution: change in allele frequencies within a population over time A mouse is a vehicle for mouse gene replication Mice with Aa genotypes IF no mutation…Gametes IF random mating Zygotes Sum = 1 Then zygotes grow up to be adults IF no survival selection, and IF no biased migration, and IF no biased random death THEN you get these adult genotypes, which produce gametes…… Adults produce gametes IF no mutation…. Those are the same allele frequencies we started with!! A = 0.6 a = 0.4 There was no evolutionary change….IF • • • • • No mutation Random mating No biased migration No biased selected mortality (selection) No biased random mortality (drift) The general case Developed by Hardy 1908 Mind your p’s and q’s • If p = frequency of A1 allele • And q = frequency of A2 allele • And if there are only two alleles in the population at this locus • Then p + q = 1 – Alternatively q = 1- p Randomly combining gametes in the general case • Sperm are A1 with probability p • Eggs are A1 with probability p • So A1A1 zygotes are produced with probability of p X p = p2 And so on…. • • • • A1A1 genotype has frequency of p2 A1A2 genotype has frequency of pq A2A1 genotype has frequency of pq A2A2 genotype has frequency of q2 • But A1A2 is the same as A2A1 = 2 pq • Frequencies must add to 1 p2 + 2pq + q2 = 1 Hardy-Weinberg Equilibrium • If a simple set of assumptions holds, then the allele frequencies in a population will not change • If we symbolize allele frequencies as p and q, then genotype frequencies are p2, 2pq, and q2 What are those simple assumptions? • No selection: genotypes survive at equal rates (i.e. no survival selection) and contribute gametes to the next generation equally (i.e. no sexual selection) • No mutation: the alleles we are accounting for stayed the same, none disappeared or were created anew by mutation What are those simple assumptions? • No biased migration: genotypes do not enter or exit the population non-randomly • No biased random events: genotypes do not get zapped by lightning (or whatever) non-randomly; if so, called genetic drift • Mating is random by genotype. Violating this assumption affects genotype frequency, not allele frequency. This is NOT the same thing as sexual selection (unequal gametic contribution of genotypes). What a bunch of malarcky! • Obviously those five conditions are almost never met! • So when those conditions are not met, allele frequencies WILL change, i.e. evolution will occur Still, what good is that? • Hardy-Weinberg equilibrium model is a null model • It is a random expectation given a specific set of assumptions • Specification of those assumptions allows us to see when and how evolutionary change does occur And what about the only two alleles nonsense? Non-equilibrium: violations of Hardy-Weinberg • • • • • Mutation Selection Migration Drift Non-random mating Pop Quiz • A has frequency of 0.9, what is the frequency of a (if only two alleles)? • p2 + 2pq + q2……= • What are the HW genotypic frequencies? Hardy-Weinberg recap • If a set of five assumptions is met: – Then, allele frequencies won’t change – And, – Genotypic frequencies follow from • p2 + 2pq + q2 = 1 Mutation as an evolutionary Force • Mutation critical for genetic variation • But can mutation, by itself, have much evolutionary impact on allele frequencies? A = functional allele; a = mutant knockout • Mutation from A to a – Mutation rate of 1 per 10,000 – plausible but high • Mutation from a to A negligible • Suppose, initially A allele has a frequency of 0.9 What is the effect of mutation? Effect over time? Mutation - selection balance • Mutations are random • In finely tuned organisms, most mutations are bound to be bad • Bad mutations selected against • Also created anew • Rate of creation by mutation = rate of removal by selection • Called mutation-selection balance Deleterious mutations selected against • How common should any particular deleterious mutation be? • Depends on how bad (strength of selection) • Depends on how often re-created (mutation rate) Is that useful information? • Two examples • Spinal muscular atrophy, study by Wirth et al. – Caused by mutation in telSMN – Telomeric survival motor neuron • Cystic fibrosis, study by Pier et al. – Most common lethal autosomal recessive disease in Caucasians – Proximate cause bacteria Pseudomonas aeruginosa lung damage Spinal muscular atrophy • telSMN knock out mutants frequency of 0.01 • Selection coefficient ~ 0.9 • Can use equation to estimate what mutation rate would be needed to counter s = 0.9, and give q^ of 0.01 • And the calculated answer is 0.9 X 10-4 mutations per allele per generation • Wirth et al. directly measured rate of 1.1 X 10-4 • Mutation-selection balance Cystic fibrosis • Chromosome 7 mutations for protein called Cystic fibrosis transmembrane conductance regulator (CFTR) • Expressed in lining of lungs and intestines • In lungs, enables cells to destroy Pseudomonas aeruginosa • Until recently most cistic fibrosis sufferers died before reproducing (i.e. selection very very strong, approaching s = 1) Cystic fibrosis mutation-selection balance? • Could mutation selection balance maintain mutants at frequency of 0.02 with s = 1? • Yes, but mutation rate would have to be about 4 X 10-4 • Observed mutation rate much lower, ca. 6.7 X 10-7 Does something else maintain cystic fibrosis causing mutations at that level? • This is an important question to ask in this case; note that it was not necessary to ask this in the case of Spinal Muscular Atrophy • The answer could have important medical implications A possible answer • What if mutation(s) favored in another context? • Recall CFTR expressed in intestine as well as lung • Pier et al hypothesized a function for most common disease causing allele – Prevents intestinal infection by Salmonella typhi • Agent causes Typhoid fever DeltaF508 allele, cystic fibrosis, and Typhoid fever • deltaF508 most common cystic fibrosis causing allele • In humans, homozygotes would get cystic fibrosis • Could heterozygotes be protected from Typhoid fever? Mouse study • Heterozygotes more resistant to Salmonella typhi • Homozygotes almost completely protected Pleiotropy and genetic correlation • deltaF508 single allele with multiple (pleiotropic) effects • Genetic correlation – Lung lining phenotype will show some similarity to intestine lining phenotype – Not completely separate traits for selection to act upon independently • And that’s what is next: selection Selection as an evolutionary force • Hardy-Weinberg null model assumptions • Individuals of all genotypes survive at equal rates, and contribute gametes equally to the gene pool • Bottom line is differential reproductive success: some genotypes out reproduce others example Result • 7.5% increase in frequency of B1 allele 7.5% decrease in frequency of B2 allele • Population has evolved • Also true for much weaker selection Selection across generations Empirical Example: Adh • Alcohol dehydrogenase – Enzyme breaks down ethanol • Cavener & Clegg Drosophila melanogaster experiment – Two replicate populations on ethanol spiked food – Two replicate populations on ethanol free food • Tracked allele frequency across time Adh semi-natural selection experiment Selection and genotypic frequency • If a set of five assumptions is met: – Then, allele frequencies won’t change – And, – Genotypic frequencies follow from • p2 + 2pq + q2 = 1 • Does selection alter genotype frequency? – Sometimes, for one generation • Heterozygote advantage Selection and genotype frequency Allele freq. 0.5, 0.5 Heterozygotes fitter After selection Allele freq. 0.5, 0.5 Predicts genotype Frequency of 0.25, 0.5, 0.25 Observe 0.167, 0.667, 0.167 Generalizing Selection • Recall development of Hardy-Weinberg argument • We took special case of A = 0.6 and a = 0.4 • Then generalized using p as frequency of the A allele and q as frequency of the alternative allele a • Can we develop a general model of selection? Change in allele frequency depends on genotypic fitness • Why?, because selection acts on phenotypes not alleles • Fitness symbolized as w • Fitness of a genotype symbolized with subscripts, so that • Fitness of A1A1 genotype has fitness w11 • Fitness of A1A2 genotype has fitness w12 • Fitness of A2A2 genotype has fitness w22 How will allele frequency change with unequal genotypic fitness? • Unequal fitness needs a standard • Fitness of a genotype is expressed relative to a standard of the average fitness of all genotypes Genotypic frequency change • Genotypic frequency change depends on the fitness of the genotype relative to the average fitness How have allele frequencies changed? • How many A1 gametes produced? • A1 gametes in proportion to A1A1 genotype frequency multiplied by relative genotype fitness • Plus 1/2 of A1A2 genotype frequency multiplied by relative genotype fitness 1/2 of 2pq = pq Same for A2 alleles • A2 gametes proportional to frequency of A2A2 genotype (q2) multiplied by the relative fitness of the q2 genotype • Plus 1/2 of the alleles contributed by heterozygotes that is, 1/2 of the frequency of heterozygotes (2pq) multiplied by heterozygote relative fitness 1/2 of 2pq = pq Allele frequency change • Change = new allele frequency - old allele frequency Ta Da!!! • We have made a general expression that quantitatively describes how allele frequency will change depending on the average fitness of that allele • Similarly, Patterns of Selection • Selection on dominant versus recessive alleles • Selection on heterozygotes and homozygotes • Frequency dependent selection Dominant and recessive alleles • • • • • • • A dominant to a if AA genotype = A phenotype AND Aa genotype = A phenotype but aa genotype = a phenotype If so, A called dominant, a called recessive Suppose homozygous recessive is lethal • waa = 0 • How will frequency of a change over time? – If a is common? – If a is rare? • If a is common then genotype aa is pretty common – If frequency of a = q then genotype frequency = q2 What do we expect • If at first a is common, it will decrease rapidly • As it becomes more rare, it will decrease more slowly – Because when rare, there are few aa genotypes with fitness = 0 Data from flour beetles • Dawson started experiment with heterozygotes, so p = q = 0.5 • waa = 0 Data agree with qualitative prediction • Rapid change when aa genotype common • Slow change when aa genotype rare Selection against recessive Selection for recessive Heterozygotes • Heterozygotes don’t always express dominant phenotype • Heterozygotes can have fitness – Intermediate • Changes rate of evolution, not outcome – Higher • Heterozygote superiority or overdominance – Lower • Heterozygote inferiority or underdominance Overdominance • Suppose genotypic fitness is as follows: wAA = 0.735 wAa = 1 waa = 0 • Equilibrium frequency of a allele > 0 • Example seen in Drosophila melanogaster study of Muaki and Burdick (1958) • Specific gene effects unknown Four populations, 2 replicates Overdominance generalized • Mean fitness across genotypes underdominance • • • • What if heterozygotes have lower fitness? Considerably more complicated Outcome depends on starting conditions One allele goes to fixation (100%) – Other allele lost Underdominance generalized Adaptive landscape Heterozygote selection outcomes s and t are selection terms for homozygotes Both negative = heterozygote superiority (overdominance) Both positive = heterozygote inferiority (underdominance) Frequency dependent selection • Strength of selection depends on frequency • For example, if the allele that is rare is favored • Two examples – Book scale eating fish – Sex Scale eating fish Perissodus microlepis Lake Tanganyika • Mouth twisted to side, bites scales off other fish • Left mouth bites right side of victim; Right mouth bites left; Victim fish wary Sex • Sex ratio (males:females) usually about 1:1 • Imagine the fitness of a male if males were really rare – That rare male would make lots of babies • Female fitness if females rare? – Most males would not reproduce • Imagine male female controlled by single locus with two alleles

Mendelian Population Genetics

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