Biology 212 General Genetics Spring 2007Lecture 26: Population Genetics IReading: Chapter 14 pp. 500-506Lecture outline:1. Definitions2. Genotype frequencies and allele frequencies3. Hardy-Weinberg EquilibriumLecture1. DefinitionsPopulation genetics: applying genetic principles to groups of individuals from the same species.Population: group of individuals of some species living within a prescribed geographical area.Gene pool: complete set of genetic information contained within the individuals in a population.Genotype frequencies: how often a particular genotype occurs in a population; expressed as a fraction.Allele frequencies: the frequency of a particular allele among all versions of a particulargene. 2. Genotype frequencies and allele frequenciesExample: Analysis of a particular population in France for alleles of the CCR5 receptor gene.CCR5 receptor: Chemokine receptor; protein on the surface of lymphocytes that binds chemokines, a signaling molecule. Also serves as the coreceptor for the HIV virus.Human populations are polymorphic for the CCR5 receptorA=normal allele; susceptible to HIV infectiona=Δ32 deletion- removes 32 bp from gene - creates a frameshift mutation in region encoding receptor protein- individuals with mutant receptor are less susceptible to HIV infection1000 French people genotyped for CCR51Population: 795 AA 190 Aa 15 aaGenotype frequencies:AA Aa aa795/1000 190/1000 15/10000.795 AA 0.190 Aa 0.015 aaAllele frequency = frequency homozygotes + 1/2 frequency heterozygotesAllele frequency of A = frequency AA + 1/2 frequency Aa 0.795 + 1/2 (0.190) = 0.89Allele frequency of a = frequency aa + 1/2 frequency Aa 0.015 aa + 1/2 (0.190) = 0.11sum of allele frequencies = 1frequency (A) + frequency (a) = 10.89 + 0.11 =13. Hardy Weinberg equilibrium- derived by G. Hardy and W. Weinberg independently in 1908- a mathematical prediction of genotype frequencies and allele frequencies in populations based ono Mendel's lawso Random mating=organisms in population form mating pairs independent of genotypeo No natural selectiono No mutationo No migrationo No genetic drift: random fluctuations in allele frequencies due to chanceLet p = allele frequency for ALet q = allele frequency for aLet the genotypes equate to the following termsAA Aa aap22pq q22These terms are based on mating that occurs when gametes combine at randompA qapA p2AA pqAaqa pqAa q2aaPredictions of the H-W equilibrium- If assumptions are met, H-W equilibrium will be established in one generation- Once a population is in H-W equilibrium allele frequencies remain constant from one generation to the next.Use of a chi-square test to determine whether population has reached H-W equilibrium- French population with CCR5 receptor polymorphismPhenotypes Genotypes Observed Expected d d2d2/expNormal AA 795 792.1 2.9 8.41 0.0106Some resistance to HIVAa 190 195.8 -5.8 33.64 0.172Resistant to HIVaa 15 12.1 2.9 8.41 0.6951000 1000 χ2=0.878- Note: degrees of freedom is defined differently here. Not number of phenotypes -1. Since p and q are only variables once p is known, q = 1 - p, therefore only onedegree of freedom.- Chi-square test will identify large deviations in expected genotype frequencies compared to that expected by H-W