Final Study Guide Chapter 8 Linkage Disequilibrium Sex 1 Define linkage disequilibrium When two loci are in a nonrandom association between a chromosomes s genotype at one locus and its genotype at the other locus Chromosome freq are NOT product of appropriate allele freq On a bar chart mid lines will not align What creates LD o Selection on multilocus genotypes Grass hoppers are green and brown After each generation some grass hoppers have both colors mismatched colors Selection kills these ones because they stand out more in the brown and green leaves o Selection in small populations genetic drift Population has one allele A at one locus but two alleles B and b at another Mutation creates new favorable allele a but it is on a chromosome with b allele Selection increases freq of a and freq of b increases Results in LD o Admixture of populations 2 populations have different chromosome frequencies Recombination destroys LD If 2 loci are in LD selection on 1 locus can cause hitchhiking by the other locus admixture of the two populations creates LD Conversely two loci are in linkage equilibrium when the genotype of a chromosome at one locus is independent of its genotype at the other locus Chromosome freq are product of appropriate allele freq On bar chart mid lines will align D g AB g ab g Ab g aB If equals 0 then linkage equilibrium exhibited Ranges from 0 25 to 0 25 Measures degree of linkage disequilibrium in a population 2 What is genetic recombination Genetic recombination is the creation of new combinations of alleles from sexual reproduction Crossing over during meiosis Reduces LD 3 Why does genetic recombination reduce linkage disequilibrium Genetic recombination reduces LD because it tends to randomize genotypes at one locus with respect to genotypes at another Varies from 0 to 0 5 o 0 meaning linkage is very tight Reduces LD less effectively o 0 5 when loci are on different chromosomes Reduces LD most effectively Rate of decline in LD between pair of loci is proportional to rate of recombination between them 4 What is the consequence of sex at the level of population genetics It has 2 population genetic effects Restores H W equilibrium Restores linkage equilibrium Without sex asexual reproduction females produced twice as many offspring and increased the population more each generation Compared to with sex which did worse but because it only has 2 population genetic effects it is beneficial Sex is beneficial in host parasite arms race Selection repeatedly changes direction Sex is favored because it facilitates evolutionary response to those reversals in selection 5 What is Muller s ratchet A process by which asexual populations accumulate deleterious mutations in a irreversible manner Sex can rescue a population from Muller s Ratchet o In a population individuals with fewest deleterious mutations can be lost due to drift o Sex is able to bring back the lost genotypes 6 Why might genetic recombination be favored in a variable environment Because genetic recombination sex recreates favorable multilocus genotypes that might have been eliminated by selection Then the genes progress to high frequencies in the high fitness genotypes they help to create 7 Why did Sewall Wright consider linkage disequilibrium to be a doubly unfortunate name He proved that the probability that any given allele in a population will be the one that drifts to fixation is equal to that allele s initial frequency Considered LD to be doubly unfortunate name because 8 Linkage disequilibrium can be maintained a stable level in a population when two opposing forces are balance What are those forces Natural selection creates and genetic recombination destroys 9 Do two loci have to be on the same chromosome to be in linkage disequilibrium Not it does not matter if they are on the same chromosome 10 What is the technical equivalent of sex for a population geneticist Reproduction involving Meiosis with crossing over Mating between unrelated individuals o Ex random mating Chapter 9 Quantitative Genetics Quantitative Genetics branch of evolutionary biology that provides tools for analyzing evolution of multilocus traits Quantitative traits characters with continuously distributed phenotypes o Continuous distribution o Affected by many genes Human height affected by at least 180 genes o Affected by environment o More loci more phenotypic classes to be expected o Many genes contribute to quantitative variation Identifying Loci that contribute to quantitative traits QTL quantitative trait loci portions of genome that influence quantitative traits fixed differences o Detect presence of QTL by crossing parents from populations with Marked loci for one parent species for one allele that was all homozygous and marked loci for other parent species for a different allele that was all homozygous o In F2 grand offspring look for association between phenotype and genotype at marker loci Case of 2 monkey flowers cardinalis and lewisii o Crossed 2 species F1 generation Parents homozygous o Bred pairs of individuals in F1 to produce F2 F1 all heterozygotes o F2 you see quantitative variation in many traits F2 heterozygotes and homozygotes QTL Mapping employ marker loci to scan chromosomes and identify regions containing genes that contribute to a quantitative trait o Trial 1 QTL and marker physically linked close and on same chromosome Lewisii parent ML QL ML QL Cardinalis parent MC QC MC QC F1 all MC QC ML QL F2 has 3 genotypes ML QL ML QL MC QC MC QC ML QL MC QC Homozygous for Lewisii marker lewisii phenotype Homozygous for cardinalis marker cardinalis phenotype Heterozygote intermediate phenotype Marker locus and QTL are in LD o Trial 2 QTL and marker unlinked Same parents and F1 genotypes F2 has 9 genotype because unlinked No association between genotype at the marker and phenotype influenced by QTL o Explanation If phenotypes differ among individuals with different genotypes at particular marker top right then we can infer that the marker locus sits near a locus that contributes to the quantitative trait Heritability of a trait fraction of the total variation in a trait that is due to variation in genes Number between 0 and 1 o If 0 it means that variation among individuals is due to environment not genes and vice versa if 1 Resemblance between parents and offspring Line of regression can be made o Slope of this line estimates heritability h 2 Measuring Selection Predicting Response to Selection Selection differential S