Population Genetics (Learning Objectives) • Recognize the quantitative nature of the study of population genetics and its connection to the study of genetics and its applications. • Define the terms population, species, allelic and genotypic frequencies, gene pool, and fixed allele, genetic drift, bottle-neck effect, founder effect. • Explain the difference between microevolution and macroevolution. • Learn how to calculate the genotypic and allelic frequencies in a population. Given the appropriate information about a population you should be able to calculate the genotypic and allelic frequencies of homozygous dominant, recessive, or heterozygous individuals (following the example discussed in class), and the chance or probability that two unrelated individuals in a particular population will have an affected child for an autosomal recessive condition • Visit this website to learn the factors that lead to changes in genotypic and allelic frequencies between generations, i.e . the factors that lead to microevolution: http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf • Recognize Hardy-Weinberg equilibrium of a non-evolving population and the factors necessary to satisfy this equilibrium. • Learn the only origin of new alleles in populations. Population genetics Study of the extensive genetic variation within populations that already exist Recognizes the importance of quantitative characters Population Genetics A population is a localized group of individuals that belong to the same species. A species is a group of populations whose individuals have the potential to interbreed and produce fertile offspring in a nature. Alu genotypic and phenotypic frequencies among Bio 210A students 42 36 23 29 48 33 32 34 35 30 37 28 31 46 47 26 45 49 38 39 40 Spring 2013 (Tuesday) 77 82 83 90 74 78 95 97 98 84 73 88 94 93 81 80 85 96 76 87 89 91 92 Spring 2013 (Thursday) Calculating the allelic frequencies from the genotypic frequencies What is the allelic frequency (of R and r) in this population? Allele Frequencies # of particular allele Allele frequency = Total # of alleles in the population Count both chromosomes of each individual Allele frequencies affect the frequencies of the three genotypes Genotypic frequency RR= 320/500 = 0.64 Rr = 160/500= 0.32 rr = 20/500 = 0.04 What is the allelic frequency in a population of 500 flowers? How many total alleles are there? 500 X 2 = 1000 Frequency of R allele in population RR + Rr = 320 X 2 + 160= 640+160= 800 800/1000 = 0.8 =80% Frequency of r allele = 1- 0.8 = 0.2 =20% or rr +Rr = 20 X 2+ 160= 200 200/1000 = 0.2 Population Genetics Calculations Determine the genotypic and phenotypic frequencies in an existing population, using Hardy-Weinberg equilirium. In the earlier calculations of allelic frequencies in flower population, as each gamete has only one allele for flower color, we expect that a gamete drawn from the gene pool at random has a 0.8 chance of bearing an R allele and a 0.2 chance of bearing an r allele. Population geneticists use p to represent the frequency of one allele and q to represent the frequency of the other allele. The combined frequencies must add to 100%; therefore p + q = 1. If p + q = 1, then p = 1 - q and q = 1 - p. Calculating the genotypic frequencies of RR, Rr, rr in next generation based on allelic frequency of p = 0.8 and q =0.2 The genotype frequencies should add to 1: p2 + 2pq + q2 = 1 In the wildflower example p is the frequency of red alleles (R) and q of white alleles (r). – The probability of generating an RR offspring is p2 (an application of the rule of multiplication). In this example, p = 0.8 and p2 = 0.64. – The probability of generating an rr offspring is q2. In this example, q = 0.2 and q2 = 0.04. – The probability of generating Rr offspring is 2pq. In this example, 2 x 0.8 x 0.2 = 0.32. This general formula is the Hardy-Weinberg equation is used to calculate - frequencies of alleles in a gene pool if we know the frequency of genotypes or - the frequency of genotypes if we know the frequencies of alleles Applications of Population Genetics 1. Calculation of the % carriers in the population for a certain disorder 2. Calculating the chance or probability that two unrelated individuals in a particular population will have an affected child for an autosomal recessive condition Example Phenylketonuria (PKU) in an autosomal recessive genetic disease that can lead to mental retardation, if unmanaged – All babies born in the United States are screened for PKU. – Information can be used to calculate the % carriers in the population http://www.ygyh.org/pku/whatisit.htm Phenotypic Frequencies vary between populations Example: PKU an autosomal recessive trait Table 14.1 Calculation of % PKU carriers from screening About 1 in 10,000 babies in US are born with PKU - The frequency of homozygous recessive individuals = q2 = 1 in 10,000 or 0.0001. - The frequency of the recessive allele (q) is the square root of 0.0001 = 0.01. - The frequency of the dominant allele (p) is p = 1 - q or 1 - 0.01 = 0.99. The frequency of carriers (heterozygous individuals) is 2pq = 2 x 0.99 x 0.01 = 0.0198 or about 2%. • About 2% of the U.S. population carries the PKU allele. The Carrier Frequency of an Autosomal Recessive (Cystic Fibrosis) Table 14.3 Calculating the chance or probability that two unrelated individuals within a population will have an affected child Probability that both are carriers = 1/23 x 1/23 = 1/529 Probability that their child has CF = 1/4 Therefore, probability = 1/529 x 1/4 = 1/2,116 Figure 14.3 Definitions • Gene pool = The collection of all alleles in the members of the population • Population genetics = The study of the genetics of a population and how the alleles vary with time • Gene Flow = Movement of alleles between populations when people migrate and mate Calculation of genotypic & allelic frequencies in populations Evolution Microevolution small changes due to changing allelic frequencies within a population from generation to generation Macroevolution large changes in allelic frequencies over 100’s and 1000’s of generations leading to the formation of new species Microevolution: • A change in the allele frequencies in the gene pool of a population from generation to generation • Populations not individuals are the units of evolution - If all members of a population are homozygous for the same allele, that allele is said to be fixed - Meiosis and random fertilization do not change the allele and genotype frequencies between generations - The shuffling of alleles that accompanies sexual reproduction does not alter the genetic makeup of the population The frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations unless acted upon by factors other than Mendelian segregation and recombination of alleles The Hardy-Weinberg theorem describes the gene pool of a non-evolving population Hardy Weinberg animation http://zoology.okstate.edu/zoo_lrc/biol1114/t utorials/Flash/life4e_15-6-OSU.swf practice questions http://nhscience.lonestar.edu/biol/hwe.html Macroevolution Caused by factors: 1. Non-Random mating 2. Genetic drift – due to sampling/ bottleneck & founder effects, geographic & cultural separation 3. Migration- of fertile individuals 4. Mutation- in germline cells transmitted in gamete 5. Natural selection- accumulates and maintains favorable genotypes in a population Populations at Hardy-Weinberg equilibrium must satisfy six conditions. (1) Very large population size. (2) Random mating. (3) No migrations. (4) No natural selection. (5) No genetic drift (6) No net mutations. Evolution results when any of these five conditions are not met - when a population experiences deviations from the stability predicted by the Hardy-Weinberg theory. Genetic Drift changes allelic frequencies in populations The bottleneck effect The founder effect New alleles originate only by mutation – rare and random. – mutations in somatic cells are lost when the individual dies. – Only mutations in cell lines that produce gametes can be passed along to offspring. .