http://www.phschool.com/science/biology_ place/labbench/lab8/intro.html AP Biology Lab Population Genetics {Hardy Weinberg} AP Biology A Large Breeding Population Helps to ensure that chance alone does not disrupt genetic equilibrium. AP Biology Random Mating Population at equlibrium: mating must be random. Assortative mating: individuals tend to choose mates similar to themselves AP Biology does not alter allelic frequencies, but results in fewer heterozygous individuals than you would expect in a population where mating is random. No Change in Allelic Frequency Due to Mutation Any mutation in a particular gene would change the balance of alleles in the gene pool Mutations may remain hidden in large populations for a number of generations, but may show more quickly in a small population. AP Biology No Immigration or Emigration Both immigration and emigration can alter allelic frequency. AP Biology No Natural Selection If selection occurs, those alleles that are selected for will become more common. AP Biology Lab 8: Population Genetics Description simulations were used to study effects of different parameters on frequency of alleles in a population selection heterozygous advantage genetic drift AP Biology Lab 8: Population Genetics Concepts Hardy-Weinberg equilibrium p+q=1 p2 + 2pq + q2 = 1 required conditions large population random mating no mutations no natural selection no migration AP Biology gene pool heterozygous advantage genetic drift founder effect bottleneck Lab 8: Population Genetics Conclusions recessive alleles remain hidden in the pool of heterozygotes even lethal recessive alleles are not completely removed from population know how to solve H-W problems! to calculate allele frequencies, use p + q = 1 to calculate genotype frequencies or how many individuals, use p2 + 2pq + q2 = 1 AP Biology Estimating Allelic Frequency If a trait is controlled by two alternate alleles, how can we calculate the frequency of each allele? For example, let us look at a sample population of pigs. The allele for black coat is recessive to the allele for white coat. Can you count the number of recessive alleles in this population? Can you count the number of dominant alleles? AP Biology The Hardy-Weinberg Equation Used to estimate the frequency of alleles in a population, According to this equation: p = the frequency of the dominant allele (represented here by A) q = the frequency of the recessive allele (represented here by a) For a population in genetic equilibrium: p + q = 1.0 (The sum of the frequencies of both alleles is 100%.) (p + q)2 = 1 So p2 + 2pq + q2 = 1 The 3 terms of this binomial expansion indicate the frequencies of the 3 genotypes: p2 = frequency of AA (homozygous dominant) 2pq = frequency of Aa (heterozygous) q2 = frequency of aa (homozygous recessive) This page contains all the information you need to calculate allelic frequencies when there are two different alleles. We start with some sample problems that will give you practice in using the Hardy-Weinberg equation. AP Biology Sample Problem 1 The allele for black coat is recessive. 1. 2. 3. 4. We can use the Hardy-Weinberg equation to determine the percent of the pig population that is heterozygous for white coat. Calculate q2 Find q. Find p. Find 2pq. AP Biology Sample Problem 1 The allele for black coat is recessive. 1. 2. 3. 4. We can use the Hardy-Weinberg equation to determine the percent of the pig population that is heterozygous for white coat. Calculate q2 Find q. Find p. Find 2pq. Answer: Four of the sixteen individuals show the recessive phenotype, so the correct answer is 25% or 0.25. AP Biology What % of the pig population is heterozygous for white/pink coat color? 1. Calculate q2 Count the homozygous recessive individuals in the illustration. Calculate the percent of the total population they represent. This is q2. 2. Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele. 3. Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p, the frequency of the dominant allele? 4. Find 2pq. The frequency of the heterozygotes is represented by 2pq. This gives you the percent of the population that is heterozygous for white coat: AP Biology What % of the pig population is heterozygous for white/pink coat color? 1. Calculate q2 Count the homozygous recessive individuals in the illustration. Calculate the percent of the total population they represent. This is q2. 2. Find q. Take the square root of q2 to obtain q, the frequency of the recessive allele. 3. Find p. The sum of the frequencies of both alleles = 100%, p + q = l. You know q, so what is p, the frequency of the Answers: dominant allele? Four of the 16 individuals show the 4. Find 2pq. recessive phenotype, The frequency of the heterozygotes is so the correct answer is 25% or 0.25. represented by 2pq. This gives you 2. q = 0.5 the percent of the population that is 3. p = 1 - q, so p = 0.5 4. 2pq = 2(0.5) (0.5) = 0.5 , heterozygous for white coat: so 50% of the population is AP Biology heterozygous. How many individuals would you expect to be homozygous for red eye color? In a certain population of 1000 fruit flies, 640 have red eyes while the remainder have sepia eyes. The sepia eye trait is recessive to red eyes. Hint: The first step is always to calculate q2! Start by determining the number of fruit flies that are homozygous recessive. If you need help doing the calculation, look back at the Hardy-Weinberg equation. AP Biology How many individuals would you expect to be homozygous for red eye color? In a certain population of 1000 fruit flies, 640 have red eyes while the remainder have sepia eyes. The sepia eye trait is recessive to red eyes. Answer: You should expect 160 to be homozygous dominant. Calculations: q2 for this population is 360/1000 = 0.36 q = = 0.6 p = 1 - q = 1 - 0.6 = 0.4 The homozygous dominant frequency = p2 = (0.4)(0.4) = 0.16. Therefore, you can expect 16% of 1000, or 160 individuals, to be homozygous dominant. AP Biology FRQ Evolution is one of the unifying themes of biology. Evolution involves change in the frequencies of alleles in a population. For a particular genetic locus in a population, the frequency of the recessive allele (a) is 0.4 and the frequency of the dominant allele (A) is 0.6. (a) What is the frequency of each genotype (AA, Aa, aa) in this population? What is the frequency of the dominant phenotype? (b) How can the Hardy-Weinberg principle of genetic equilibrium be used to determine whether this population is evolving? (c) Identify a particular environmental change and describe how it might alter allelic frequencies in this population. AP Biology Lab 8: Population Genetics FRQ ESSAY 1989 Do the following with reference to the Hardy-Weinberg model. a. Indicate the conditions under which allele frequencies (p and q) remain constant from one generation to the next. b. Calculate, showing all work, the frequencies of the alleles and frequencies of the genotypes in a population of 100,000 rabbits of which 25,000 are white and 75,000 are agouti. (In rabbits the white color is due to a recessive allele, w, and agouti is due to a dominant allele, W.) c. If the homozygous dominant condition were to become lethal, what would happen to the allelic and genotypic frequencies in the rabbit population after two generations? AP Biology Application The Hardy-Weinberg equation is useful for predicting the percent of a human population that may be heterozygous carriers of recessive alleles for certain genetic diseases. Phenylketonuria (PKU) is a human metabolic disorder that results in mental retardation if it is untreated in infancy. In the United States, one out of approximately 10,000 babies is born with the disorder. Approximately what percent of the population are heterozygous carriers of the recessive PKU allele? AP Biology Design of the Experiment The experiment in this laboratory is a test of Hardy-Weinberg equilibrium in a "mating population," represented by you and your classmates. Your instructor will assign each class member a genotype (AA, Aa, or aa). The initial allelic frequency of the population will be 0.5 A and 0.5 a. Your population will simulate several different conditions that might alter allelic frequency, and you will determine the allelic frequencies over several generations. AP Biology Analysis of Results Your analyses should have included Hardy Weinberg and Chi square. http://www.phschool.com/science/biology_place/labbench/lab8/analysis.html AP Biology Any Questions?? AP Biology
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