Fishy Frequencies Overview: Objectives: Before doing this lab you should understand: After doing this lab you should be able to: Introduction:

Transcription

1 Fishy Frequencies Overview: In this lab you will:. learn about the Hardy-Weinberg law of genetic equilibrium, and. study the relationship between evolution and changes in allele frequencies by using your class to represent a sample population. Objectives: Before doing this lab you should understand: how natural selection can alter allelic frequencies in a population; the Hardy-Weinberg equation and its use in determining the frequency of alleles in a population; and the effects on allelic frequencies of selection against the homozygous recessive or other genotypes. After doing this lab you should be able to: calculate the frequencies of alleles and genotypes in the gene pool of a population using the Hardy-Weinberg formula, and discuss natural selection and other causes of microevolution as deviations from the conditions required to maintain Hardy-Weinberg equilibrium. Introduction: Understanding natural selection can be confusing and difficult. People often think that animals consciously adapt to their environments - that the peppered moth can change its color, the giraffe can permanently stretch its neck, the polar bear can turn itself white - all so that they can better survive in their environments. In this lab you will use taste tests and fish crackers to help further your understanding of natural selection and the role of genetics and gene frequencies in evolution. Hardy-Weinberg: In 908 G. H. Hardy, an English mathematician, and W.R. Weinberg, a German physician, independently worked out the effects of random mating in successive generations on the frequencies of alleles in a population. This is important for biologists because it is the basis of hypothetical stability from which real change can be measured. This also allows you to figure out the frequency of genotypes from phenotypes. You assume that in the total population of fish crackers, you have the following genotypes, FF, Ff, and ff. You also assume that mating is random so that ff could mate with ff, Ff, or FF; or Ff could mate with ff, Ff, or FF, etc. In addition, you assume that for the gold and brown traits there are only two alleles in the population - F and f. If you counted all the alleles for these traits, the fraction of f alleles plus the fraction of F alleles would add up to.

2 The Hardy-Weinberg equation states that: p + pq + q = This means that the fraction of p (or FF) individuals plus the fraction of pq (or Ff) individuals plus the fraction of q (ff) individuals equals. The pq is multiplied by because there are two ways to get that combination. You can get F from the male and f from the female OR f from the male and F from female. If you know that you have 6% recessive fish (ff), then your qq or q value is.6 and q = the square root of.6 or.; thus the frequency of your f allele is. and since the sum of the f and F alleles must be, the frequency of your F allele must be.6 Using Hardy-Weinberg, you can assume that in your population you have.6 FF (.6 x.6) and.8 Ff ( x. x.6) as well as the original.6 ff that you counted. Hardy and Weinberg also argued that if five conditions are met, the population s allele and genotype frequencies will remain constant from generation to generation. These conditions are as follows:. The breeding population is large. (The effect of chance on changes in allele frequencies is thereby greatly reduced.). Mating is random. (Individuals show no mating preference for a particular phenotype.). There is no mutation of alleles. (No alteration in the DNA sequence of alleles.). No differential migration occurs. (No immigration or emigration.). There is no selection. (All genotypes have an equal chance of surviving and reproducing.) The Hardy-Weinberg equation describes an existing situation. If the five conditions are met, then no change will occur in either allele or genotype frequencies in the population. Of what value is such a rule? It provides a yardstick by which changes in allele frequency, and therefore evolution, can be measured. One can look at a population and ask: Is evolution occurring with respect to a particular gene locus? Since evolution is difficult to observe in most natural populations (except for quickly dividing populations such as yeast, bacteria, or certain protists), we will model the evolutionary process using the class as a simulated population. The purpose of this simulation is to provide an opportunity to test some of the basic tenets of population genetics and evolutionary biology. Warm-up: Estimating Allele Frequencies for a Specific Trait within a Sample Population Using the class as a sample population, the allele frequency of a gene controlling the ability to taste a specific chemical (Mr. LaFleur will specify before lab) could be estimated. A taster or the ability to taste the chemical is evidence of the presence of a dominant allele in either the homozygous condition (AA) or the heterozygous condition (Aa). The inability to taste the chemical at all is indicative of homozygous recessive alleles (aa). To estimate the frequency of the chemical tasting allele in the population, one must find p. To find p, one must first determine q (the frequency of the nontasting allele), because only the genotype of the homozygous recessive individuals is known for sure (i.e., those that show the dominant trait could be AA or Aa). Warm-up Procedure:. Using the test papers provided, tear off a short strip and press it to your tongue tip. Tasters will immediately notice a different and distinct taste in their mouths indicating their genotypic condition (either AA or Aa).. A decimal number representing the frequency of tasters (p + pq) should be calculated by dividing the number of tasters in the class by the total number of students in the class. A decimal number representing the frequency of nontasters (q ) can be obtained by dividing the number of nontasters by the total number of students. You should then record these numbers in the warmup table below.. Use the Hardy-Weinberg equation to determine the frequencies (p and q) of the two alleles. The frequency of q can be calculated by taking the square root of q. Once q has been determined, p can be determined because q = p. Record these values in the warm-up table below for the class and also calculate and record values of p and q for the North American population.

3 Warm-up Phenotypic Proportions of Tasters and Nontasters and Frequencies of the Determining Alleles Class Population N. America Population Tasters (p + pq) Phenotypes Nontasters (q ) # % # % Allele Frequency Based on the H-W Equation p q Warm-up Questions:. What is the percentage of heterozygous tasters (pq) in your class?. What percentage of the North American population is heterozygous for the taster trait? Case Studies: Background: Facts about the Fish ) These little fish are the natural prey of the terrible fish-eating sharks - YOU! ) Fish come with two phenotypes - gold and brown: a) gold: this is a recessive trait (ff) b) brown: this is a dominant trait (FF or Ff) ) In the first simulation, you, the terrible fish-eating sharks, will randomly eat whatever color fish you first come in contact with. (There will be no selection.) ) In the second simulation, you will prefer to eat the gold fish (these fish taste yummy and are easy to catch) you will eat ONLY gold fish unless none are available in which case you resort to eating brown fish in order to stay alive (the brown fish taste salty, are sneaky and hard to catch). ) In the third simulation, you will flip a coin to determine the fate of the brown fish simulating something called heterozygous advantage. In this case, we will say that having one gold allele is advantageous while having two (ff) or not having one (FF) is deadly in our environment (like sickle-cell trait in humans where malaria is prevalent). 6) New fish are born every year ; the birth rate equals the death rate. You simulate births by reaching into the pool of spare fish and selecting randomly. 7) Since the gold trait is recessive, the gold fish are homozygous recessive (ff). Because the brown trait is dominant, the brown fish are either homozygous or heterozygous dominant (FF or Ff). Procedure (Selection): ) Get a random population of 0 fish from the ocean. ) Count gold and brown fish and record in your chart; you can calculate frequencies later. ) Eat fish, chosen randomly, without looking at the plate of fish ) Add fish from the ocean. (One fish for each one that died.) Be random. Do NOT use artificial selection. ) Record the number of gold and brown fish. 6) Again eat fish, randomly chosen. 7) Add randomly selected fish, one for each death. 8) Count and record. 9) Repeat steps 6, 7, and 8 two more times. 0) Provide your results for the class. Fill in the class results on your chart.

4 Procedure - Without Selection CHART (without selection): Partners generation gold brown q q p p pq CHART (without selection): Class generation gold brown q q p P pq Procedure (No Selection): ) Get a random population of 0 fish from the ocean. ) Count gold and brown fish and record in your chart; you can calculate frequencies later. ) Eat gold fish; if you do not have gold fish, fill in the missing number by eating brown fish. ) Add fish from the ocean. (One fish for each one that died.) Be random. Do NOT use artificial selection. ) Record the number of gold and brown fish. 6) Again eat fish, all gold if possible. 7) Add randomly selected fish, one for each death. 8) Count and record. 9) Repeat steps 6, 7, and 8 two more times. 0) Provide your results for the class. Fill in the class results on your chart. Procedure - With Selection CHART (with selection): Partners generation gold brown q q p P pq

5 CHART (with selection): Class generation gold brown q q p P pq Procedure (Heterozygote Advantage): ) Get a random population of 0 fish from the ocean. ) Count gold and brown fish and record in your chart; you can calculate frequencies later. ) Randomly grab fish; of the, eat all the gold fishes. If brown, flip a coin to determine its fate! Heads the fish dies (you eat him), tails the fish gets away and you do NOT get to eat him. ) Add the correct number of fish from the ocean. (One fish for each one that died.) Be random. Do NOT use artificial selection. ) Record the number of gold and brown fish. 6) Again, randomly grab three fish. As before, of the fish you grabbed eat all the gold fishes. If brown, flip a coin to determine its fate! Heads the fish dies (you eat him), tails the fish gets away and you do NOT get to eat him. 7) Add the correct number of randomly selected fish, one for each death. 8) Count and record. 9) Repeat steps 6, 7, and 8 two more times. 0) Provide your results for the class. Fill in the class results on your chart. Procedure Heterozygote Advantage CHART (without selection): Partners generation gold brown q q p p pq CHART (without selection): Class generation gold brown q q p p pq

6 FINALLY: Fill in your data chart and calculations, prepare a graph showing the frequency of the alleles in each generation (see analysis and questions #) and complete the analysis and questions below. Analysis and Questions: ) Prepare one graph using all sets of class data (without selection, with selection, AND heterozygote advantage). On the x axis put generations - and on the y axis put frequency (0-). Plot both the q and p for both sets of class data. Label lines clearly (without selection, with selection, AND heterozygote advantage). ) Procedure Without Selection Questions: a. What does the Hardy-Weinberg equation predict for the final p and q values? b. Do the results you obtained in this simulation agree? If not, why? c. What major assumption(s) (H-W conditions) were not strictly followed in this simulation? ) Procedure With Selection Questions: a. How do the final frequencies of p and q compare to the initial frequencies? b. What major assumption(s) (H-W conditions) were not strictly followed in this simulation? c. Predict what would happen to the frequencies of p and q if you simulated another five generations. d. In a large population would it be possible to completely eliminate a deleterious recessive allele? Explain. ) Procedure Heterozygote Advantage Questions: a. Explain how the changes in p and q frequencies in Procedure (with selection) compare Procedure (without selection) and Procedure (heterozygote advantage). b. Do you think the recessive allele will be completely eliminated in either Procedure or? c. What is the importance of heterozygotes (the heterozygote advantage) in maintaining genetic variation in populations? Hardy-Weinberg Problems (show all work!):. In Drosophila the allele for normal-length wings is dominant over the allele for vestigial wings (if you remember from earlier in the school year, vestigial wings are stubby little curls that cannot be used for flight). In a population of,000 individuals, 80 show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait? What would be the total number of dominant and recessive alleles be in the population?. The allele for unattached earlobes is dominant over the allele for attached earlobes. In a population of 00 individuals, % show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait? What would be the total number of dominant and recessive alleles be in the population?. The allele for the hair pattern called widow s peak is dominant over the allele for no widow s peak. In a population of,000 individuals, 70 show the dominant phenotype. How many individuals would you expect of each of the possible three genotypes for this trait?. In the United States about 8% of the population is Rh negative. The allele for Rh negative is recessive to the allele for Rh positive. If the student population of a high school is,00, how many students would you expect for each of the three possible genotypes?. In Nigeria, % of all children are born with sickle-cell anemia killing over 00,000 infants annually. Sickle-cell anemia is a recessive trait that has many symptoms and complications. Out of a random population of,000 Nigerian babies, how many would you expect for each of the three possible genotypes? 6. In a certain population, the dominant phenotype of a certain trait occurs 79% of the time. What is the frequency of the dominant allele?