BIOL 4003 Week 14 Lecture Notes
BIOL 4003 Week 14 Lecture Notes 4003
U of M
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This 5 page Class Notes was uploaded by Rachel Heuer on Tuesday April 26, 2016. The Class Notes belongs to 4003 at University of Minnesota taught by Robert Brooker in Spring 2016. Since its upload, it has received 10 views. For similar materials see Principles of Genetics in Biology at University of Minnesota.
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Date Created: 04/26/16
Chapter 26: - Population genetics is an extension of mendel’s laws of inheritance, molecular genetics, and the ideas of Darwin o Focus is shifted away from the individual and toward the population of which the individual is a member o All of the alleles of every gene in a population make up the gene pool § Only individuals that reproduce contribute to the gene pool of the next generation o Population geneticists study genetic variation within the gene pool and how it changes from one generation to the next - Population is a group of individuals of the same species the occupy the same region and can interbreed with eachother o Large populations are usually composed of smaller groups called local populations § or also called demes • Members of local population are more likely to breed with each other than with members of the general population o Some genes are monomorphic (genes are all the same), while most are polymorphic (a lot of variation in genes) o Polymorphism: Observation that many traits display variation within a population § Can also correspond to differences in genes (single nucleotide polymorphism) - Population genetics is concerned with allele and genotype frequencies o Two calculations are central to population genetics § Allele frequency: Number of copies of allele in a population / total number of all alleles for that gene in a population • Must multiply by two if a homozygote! § Genotype frequency: # of individuals with a particular genotype in a population / total # of individuals in a population o Hardy-Weinberg Equilibrium § Simple mathematical expression that relates alleles and genotype frequencies in a population § Called an equilibrium à True under certain set of conditions: • Allele and genotype frequencies do not change over course of many generations o One allele frequency is denoted p while the other is q § Equations: • p + q = 1 • p + 2pq + q = 1 o p = genotype frequency of homozygous dominant o 2pq = genotype frequency of heterozygote o q = genotype frequency of homozygous recessive • Only true if population is in Hardy-Weinberg equilibrium à must follow assumptions • Can make a punnet square using these § Conditions: (makes a lot of assumptions) • No new mutations • No genetic drift o The population is so large that allele frequencies do not change due to random sampling effects • No migration • No natural selection • Random mating § Useful to show that the population is not in equilibrium • Leads researcher to think one of the above conditions is not true in the population • could illustrate inbreeding or natural selection - Overview of microevolution o Genetic variation in natural populations changes over many generations o Microevolution describes changes in a population’s gene pool from generation to generation § Driven by (table 26.1): • Mutation o Does not have much impact on changing allele/genotype frequency because it occurs too slowly o Used as a mean to change things • Random genetic drift • Migration • Natural Selection • Nonrandom mating § Natural selection • There is a struggle for existence o Individuals that are most adapted to their particular environment will survive and reproduce • Can also be related to mating efficiency and fertility, not just to survival • Must consider the fitness of a species o Fitness: Likelihood that a genotype will survive and contribute to the gene pool of the next generation o Fitness is used as a measure of reproductive superiority § Don’t confuse with physical fitness o Give genotype with highest fitness a value of 1 § Assign fitness values to the genotypes in relation to this § Fitness value is denoted by variable W • Natural selection acts on phenotypes (which are derived from an individual’s genotype) • Natural selection can operate in four ways o Directional selection: favors the survival of one extreme phenotype that is better adapted to an environmental condition o Balancing selection: Favors the maintenance of 2 or more alleles § Heterozygote advantage § Negative frequency-dependent selection o Disruptive (diversifying) selection: Favors the survival of two or more different phenotypes; typically due to heterogeneous environment o Stabilizing selection: favors the survival of individuals with intermediate phenotypes (eliminates the extremes) § Genetic Drift • Refers to random changes in allele frequency due to random fluctuations • allele frequency may drift from generation to generation as a matter of chance o Favors either the loss or fixation (monomorphic/can’t fluctuate) of an allele o Rate of drift varies based on population size § Larger populations are most resistive § Rate of drift depends on the population size § Drift occurs very rapidly in small populations • Operates in a directional manner towards either allele loss or fixation • Genetic drift has a larger impact in small populations o Some populations are geographically broken into small local populations which are more susceptible to drift • Examples of Genetic Drift o Bottleneck effect: Large population is reduced to smaller numbers by an environmental event § Species can expand again but with less genetic variation in the population o Founder Effect: Small group from a large population leave and go somewhere else § Less genetic variation in breeding § Migration: can alter allele frequencies • Gene flow: transfer of alleles from donor population to recipient population, changing the gene pool • New population (recipient population) is called a conglomerate • Typically, it is common for individuals to migrate between populations in both directions o Bidirectional migration has two consequences: § Tends to reduce allele frequency differences between populations § Can enhance genetic diversity within population • Mutations in one population can be introduced into the neighboring group § Nonrandom mating: only affects genotype frequencies • In random mating, mates are chosen regardless of genotype or phenotype o violated frequently, especially in humans • Mating and phenotypes: o Assortative mating: occurs when individuals do not mate randomly § Positive assortative mating: Occurs when individuals are more likely to mate due to similar phenotypic characteristics § Negative assortative: occurs when individuals with dissimilar phenotypes mate preferentially • Mating and genotypes: o Inbreeding: Mating between genetically related individuals § Inbreeding raises the proportion of homozygotes and decreases the proportion of heterozygotes o Outbreeding: Mating between genetically unrelated individuals § Inbreeding increases the proportion of heterozygotes and decreases the proportion of homozygotes o In absence of other evolutionary forces, in/out breeding does not affect allele frequency o 5 factors of microevolution lead to evolution over long periods of time § Can lead to new species over time
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