GENERAL BIOLOGY LAB 1 BSC1010L Lab 11 Population Genetics OBJECTIVES To gain a general understanding about the field of population genetics and how it can be used to study evolution and population dynamics Understand the concepts of evolution fitness natural selection genetic drift and mutations To simulate Hardy Weinberg equilibrium conditions INTRODUCTION A population is a group of individuals plant or animal of one species that occupy a defined geographical area and share genes through interbreeding Within a large population new genetically distinct subpopulations can arise through isolation by distance IBD In this mechanism as the subpopulations become geographically isolated genetic differentiation between groups in the general population increases Jensen Bohonak and Kelley 2005 These groups more commonly referred to as local populations or demes See Fig 1 consist of members that are far likelier to breed with each other than with the remainder of the population As such their gene pool differs significantly from that of the general population and with continued isolation demes may eventually evolve into new species Figure 1 Local populations demes present within a large population 1 Demes also arise from other mechanisms including geographical ecological temporal and or behavioral isolation Fig 2 Figure 2 Mechanisms of isolation Population genetics which emerged as separate branch of genetics in the early 1900s is a direct extension of Mendel s laws of inheritance Darwin s ideas of natural selection and the concepts of molecular genetics The field of population genetics focuses on the population to which an individual belongs rather than on the individual Within any given population every individual has its own set of alleles for diploid organisms there are 2 alleles for each gene one from the mother and one from the father Collectively every individual s set of alleles comprises the population s gene pool The role of a population geneticist is to study the allelic and 2 genotypic Formulas 1 and 2 respectively variation present within a population s gene pool and to assess how this variation changes from one generation to the next 1 Allelic frequency of copies of an allele in a population Total of all alleles for that gene in a population 2 Genotypic Frequency of individuals with a particular genotype in a population Total of all individuals in a population Example In a population of 100 students 64 are PTC tasters with genotype TT 32 are PTC tasters with genotype Tt and the last 4 are non tasters with genotype tt a What is the allelic frequency t Allelic frequency of t 2 t allele in recessive genotype tt t allele in heterozygous genotype Tt Total of alleles for the PTC gene in the population 2 of alleles in homozygous dominant condition 2 of alleles in heterozygous condition 2 of alleles in homozygous recessive condition 2 4 32 2 64 2 32 2 4 40 200 0 2 or 20 b What is the genotypic frequency of tt Genotypic frequency of tt off tt individuals Total of individuals in the population 4 64 32 4 4 100 3 0 04 or 4 In general populations are dynamic units that change from one generation to the next To be able to predict how a gene pool changes in response to fluctuations in size geographic location and or genetic composition population geneticists have developed mathematical models that quantify these parameters The most recognized of these is the Hardy Weinberg HW equation formulated by G Hardy and W Weinberg p q 2 1 or p2 2pq q2 1 This equation relates allele and genotype frequencies in a population and indicates the proportion of each allele combination that should exist within a population In this formula p2 the frequency of a homozygous dominant genotype e g BB q2 the frequency of a recessive genotype e g bb 2pq the frequency of a heterozygote genotype e g Bb The HW equation predicts equilibrium i e the allelic and genotypic frequencies remain constant over the course of many generations if the following five assumptions are met 1 Large population size 2 Random mating 3 No mutation 4 No migration 5 No natural selection In reality no population ever satisfies HW equilibrium completely Example 1 If in a population of 100 cats 84 carry a dominant allele for black coat B and 16 carry the recessive allele for white coat b then the frequency of the black phenotype is 0 84 and of the white phenotype is 0 16 a Using the HW equation calculate the frequencies of alleles B and b frequency of white bb cats 16 100 0 16 q2 0 16 therefore q 0 16 0 4 since p q 1 then p 1 q therefore p 1 0 4 0 6 4 Using the HW equation calculate the frequencies of the BB and Bb genotypes b From part a we know that p 0 6 and q 0 4 therefore the frequency of BB cats is p2 0 6 2 0 36 and the frequency of Bb cats 2pq 2 0 6 0 4 0 48 To check since p2 2pq q2 1 then 0 36 0 48 0 16 1 Example 2 In a population of fruit flies the genotypes of individuals present are 50 RR 20 Rr and 30 rr where R red eyes and r white eyes Assuming the population is in Hardy Weinberg equilibrium the proportion of each genotype would be determined as follows a Using Formula 1 calculate the frequency of each allele in this case R and r Frequency of r allele 2 30 20 2 50 2 20 2 30 80 200 0 4 Therefore q the frequency of the recessive allele equals 0 4 b Since q is known the p q 1 is equation is used to determine p the frequency of the dominant allele since p q 1 then p 1 q therefore p 1 0 4 0 6 c Now using the HW equation we can calculate the proportion of RR Rr and rr individuals in the population 5 From part a and b we know that p 0 6 and q 0 4 therefore the frequency of RR individuals is p2 0 6 2 0 36 the frequency of those with the Rr genotype 2pq 2 0 6 0 4 0 48 and the frequency of rr flies q2 0 4 2 0 16 d Since there are 100 flies in our population when the population is in HW equilibirium 36 flies are homozygous dominant RR 48 are heterozygous Rr and the remaining16 are homozygous recessive rr The genetic composition of a population s gene pool can be affected by several evolutionary factors including mutations migration non random mating genetic drift and natural selection Mutations changes in the DNA sequence are the ultimate source of genetic variation in a population s gene pool but because mutation rates are generally low mutations alone do not usually result in changes in allele frequency Allelic distributions in a particular group can also fluctuate due to migration i e the movement of individuals either into immigration or