F2024 GEM 5 Population Genetics Student Version PDF

Populati on genetics Dr. Guri Tzivion Office: GC18 [email protected]  Learning Objectives GEM.5. Apply the principles of population genetics to predict allele, genotype and phenotype frequencies in populations. Given a clinical or research scenario, graph or table,...

Populati on genetics Dr. Guri Tzivion Office: GC18 [email protected]  Learning Objectives GEM.5. Apply the principles of population genetics to predict allele, genotype and phenotype frequencies in populations. Given a clinical or research scenario, graph or table, students should be able to: GEM.5.1. Explain the significance of allele and phenotype frequency in populations that are in Hardy-Weinberg equilibrium. GEM.5.2. Predict carrier frequency and risk based on the relationship between allele and phenotype frequencies in a population. GEM.5.3. Describe the factors that alter Hardy-Weinberg equilibrium and the predicted consequences for a population. Textbook Reading: Human Genetics: From Molecules to Medicine. Chapter 14. Risk Estimation and Calculation Section: “Hardy-Weinberg Law” Textbook Reading: Thompson & Thompson – Genetics and Genomics in Medicine, 9th ed. Chapter 10. Population Genetics for Genomic Medicine Section: Allele and Genotype Frequencies Hardy-Weinberg Equilibrium  Populations and Variation Population... Is a group of the same species, living within a particular geographical area, at a given time. Variation exists between members of a population and may be: Structural Developmental Biochemical Behavioural Physiological 4  Genetics of Populations Population  a localized group of individuals belonging to same species Gene pool = The total genes in a population Evolution on the smallest scale occurs when the relative frequency of alleles in a population changes over a succession of generations = microevolution  Genetics of a Non-evolving Population The gene pool is in stasis This is described by Hardy-Weinberg principal The frequencies of alleles in a population’s gene pool remain constant over the generations unless acted on by agents other than sexual recombination e.g., shuffling the deck has no effect on the overall genetic make-up of the population  The Hardy-Weinberg Principle The frequency of an allele in a population will remain constant over time, provided that the following conditions are met: The population is large and randomly breeding There are no conditions acting on the population to change the allele frequency Population Genetics = 1  The Hardy-Weinberg Equation This example is the simplest case: 2 alleles in the population and one is dominant. For this case: if p = frequency of one allele q = the frequency of the other allele Then: p + q = 1 probability of AA genotype = p2 probability of aa genotype = q2 probability of Aa genotype = 2pq  The Hardy-Weinberg principal Example: In pink flowers, pink (A) is dominant over white flowers (a), 2 alleles at this locus Sample 500 plants: 20 white flowers (aa) 480 pink (AA + Aa)  What will be the frequencies of the white and pink alleles?  Example (Continued)  How many of the pink flowers will be homozygotes and how many heterozygotes? In Sickle-cell anemia, normal homozygous individuals (SS) have normal blood cells that are easily infected with the malarial parasite. Thus, many of these individuals become very ill from the parasite and many die. Individuals homozygous for the sickle-cell trait (ss) have red blood cells that readily collapse when deoxygenated. Although malaria cannot grow in these red blood cells, individuals often die because of the genetic defect. However, individuals with the heterozygous condition (Ss) have some sickling of red blood cells, but generally not enough to cause mortality. In addition, malaria cannot survive well within these "partially defective" red blood cells. Thus, heterozygotes tend to survive better than either of the homozygous conditions. If 9% of the population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene? There are 120 students in a class. Ninety did well in the course whereas 30 blew it totally and received a grade of F. In the highly unlikely event that these traits are simple genetic traits rather than multifactorial, and involve dominant and recessive alleles, and the 30 students represent the frequency of the homozygous recessive condition, what is the frequency of the dominant allele? A. 0.7 B. 0.75 C. 0.25 D. 0.5 E. 0.225  The Hardy-Weinberg Equation For A, dominant allele; a, recessive allele: p+q=1 probability of AA (AA genotype) = p2 probability of aa (aa genotype = a phenotype) = q2 probability of Aa (Aa genotype) = 2pq Dominant phenotype = A phenotype = p 2 + 2pq p2 + 2pq + q2 = 1  Uses of Hardy-Weinberg  You can calculate the frequency of a gene in a population if you know the frequency of the phenotypes  Important in genetic disease counseling  The Hardy-Weinberg Equation Calculate the frequency of color blindness in females sing the HW formula to estimate allele frequencies in populations  Relevance to Evolution  A population at genetic equilibrium does not evolve  Hardy-Weinberg tells us what to expect in non-evolving populations  Therefore, it is a baseline for comparing actual populations where gene pools may be changing.  Can determine if the population is evolving  Genetic Equilibrium Hardy-Weinberg equilibrium is maintained only if the population meets all 5 of the following criteria: Very large population size Isolation from other populations migration can affect the gene pool No net mutations Random mating No natural selection no difference in reproductive success Describes an ideal conditions that rarely exists in nature  Altering Genetic Equilibrium For evolution to take place something must upset the genetic equilibrium of the population: Factors that change genetic equilibrium are: Genetic drift Migration (Gene flow) Non-random mating (Isolation) Mutation Natural selection  Genetic Change:  Gene pool is the total number of alleles present in a population.  Genetic change is the change in frequency of alleles in the gene pool of a population.  The processes of mutation, natural selection, migration and genetic drift all affect the gene pool and change the frequency of the alleles in that gene pool.  Frequency of an allele =  occurrence of allele/total number of alleles  Evolution  Is the process by which new species of organisms develop from earlier forms.  Process normally occurs slowly.  Most often in response to a change in a species’ environment.  Is drived by changes in the frequency of the alleles in a population (some alleles ‘do better’ than others).  Evolution acts on populations (i.e. it is populations that evolve, not individuals).  Natural Selection  The theory of natural selection was proposed by Charles Darwin over 150 years ago.  Populations typically produce more offspring than environmental resources can maintain – there is a competition for survival.  Individuals with the best adaptations survive and reproduce (this is what is meant by fitness) and pass their successful alleles onto their offspring.  The frequency of these successful alleles will then increase in the gene pool.  Environmental factors (both biotic and abiotic) act as selecting agents of phenotypes.  When environmental factors change, different phenotypes will be selected for.  As phenotype is largely determined by genotype, successful genotype alleles will increase in frequency in the gene pool.  Favourable alleles increase in frequency in a gene pool, while unfavourable alleles decrease.  If the frequency of alleles changes, evolution is occurring.  After a certain number of generations, the frequency of alleles and phenotypes might change so markedly that the population becomes reproductively isolated from others of that species.  It is now a new species.  DD = warm tolerant Original Dd = warm ancestral tolerant population dd = cold tolerant Cold environmen t Genotypicall y isolated gene pools Environment changes Warm environment Cold region Further environment al changes Mild region Selection for different genotypes Warm as climate changes. region  Genetic Drift Changes in gene frequency of a very small population due to chance Controlled by the laws of probability & chance  Bottleneck effect Chance sampling error due to small population  Founder’s effect a few individuals colonize a remote spot causes drift Illustrating Genetic Drift The Bottleneck Effect  Gene Flow (Migration)  Movement of organisms into or out of a population  Takes their genes out of the gene pool  Most populations are not completely closed gain & lose alleles  Non-random Mating  More apt to mate with close neighbors  Promotes inbreeding  Assortive mating seek mate like self (e.g., size, appearance) Disassortative mating: individuals with diverse traits mate more frequently than would be expected in random mating Isolation  Mutation  A change in a gene  An alteration of DNA  The original source of variation  Raw material on which natural selection works  Natural Selection  If one type produces more offspring than another, upsets the balance of equilibrium  There are three types of natural selection: Stabilizing Selection Directional Selection Disruptive or Diversifying Selection  Sample Question Within a population of butterflies, the color brown (B) is dominant over the color white (b), and 40% of all butterflies are white. Given this information, what is the frequency of homozygous dominant individuals: A. 60% B. 40% C. 20% D. 13.5% E. 80% Practice Questions Instructions: Go to https://socrative.com/ Log in using your Student Login Room Name: TZIVIONMD1 Questions?