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INVESTIGATION C2 C2 Allele Frequency and Evolution Key Question: What happens to the frequency of an allele in a population over time? This investigation teaches an advanced topic that builds on concepts explored in Investigations B5 through B8. Here, students learn to use a mathematical model used by scientists to track how alleles change over time in populations of the same species. They determine the genome of a Crazy Creature and identify the alleles it carries for two traits. Then they study those traits in two different populations of Crazy Creatures. Students will model how favorable alleles are passed on to offspring and how allele frequencies within one of the populations change over time. Finally, they compare the allele frequencies for the two traits in each population. GETTING STARTED Time 150 minutes Setup and Materials 1. Make copies of investigation sheets for students. 2. 3. Review all safety procedures with students. Materials for each group yy Crazy Chromosomes set Learning Goals ✔✔Explain how changes in allele frequency over time are an indication that evolution is occurring. ✔✔Calculate allele frequencies for populations given the frequency of homozygous recessive individuals. ✔✔Evaluate the importance of genetic variation to the survival of a species when changes in the environment occur. Watch the equipment video. Online Resources Available at curiosityplace.com yy Equipment Video: Crazy Chromosomes yy Skill and Practice Sheets yy Whiteboard Resources yy Animation: Galapagos Finches yy Science Content Video: Heredity yy Student Reading: Traits NGSS Connection This investigation builds conceptual understanding and skills for the following performance expectation. HS-LS4-3. Apply concepts of statistics and probability to support explanations that organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait. Science and Engineering Practices Analyzing and Interpreting Data Disciplinary Core Ideas LS4.B: Natural Selection Crosscutting Concepts Patterns LS4.C: Adaptation CC_TG.indb 145 Crazy Chromosomes 171 145 8/1/14 11:28 AM Allele Frequency and Evolution Vocabulary adaptation – an inherited characteristic that enhances an organism’s chance for survival in its current environment allele frequency – a number that relates the occurrence of a particular allele for a gene within a population crossover – a physical exchange of chromosome segments that most commonly occurs early in the first division of meiosis evolution – a change in the genetic makeup of a population over time fitness – the ability of an organism to survive and reproduce to pass its genes on to the next generation genetic variation – the diversity of alleles in a population of organisms Hardy-Weinberg formulas – mathematical relationships that are used to calculate allele frequencies in a population of organisms linked genes – genes that are found on the same chromosome pair genes and traits, they discovered patterns of inheritance that did not match Mendel’s results. This led to the discovery of linked genes. Linked genes are found on the same chromosome pair and do not follow the law of independent assortment. In linked genes, recombination can occur through a process called crossover. In crossover, segments of DNA are exchanged between non-sister chromatids of homologous pairs. The diagram below shows how crossover can occur. The chance of crossover happening is directly proportional to the distance of a gene from the centromere. The further from the centromere the gene is located, the greater the chance of crossover occurring. If a test cross is performed using genes that are known to be linked, and some of the offspring do not resemble either parent, we can deduce that crossover has occurred. Because crossover does not happen in every line of gametes, the ratios are different than the ratios in non‑linked genes. Students will discover that usually, most offspring resemble at least one parent and only a smaller number bear no resemblance to their parents. mutation – a change in the hereditary material of an organism Greater chance of crossover natural selection – the process by which organisms with favorable adaptations survive and reproduce at a higher rate than those with less-favorable adaptations Lesser chance of crossover non-linked genes – genes that are found on different chromosome pairs and are inherited independently from each other recombination – a mixing of alleles that occurs during meiosis BACKGROUND The law of independent assortment was derived from Gregor Mendel’s work with pea plant traits. Although he knew nothing about genes or chromosomes, Mendel deduced that there was a mixing force that caused alleles (he called them “factors”) to segregate into different gametes independently of each other. It turns out that Mendel’s data was collected from non-linked genes: genes found on different chromosome pairs that segregate independently. But as other scientists studied Greater chance of crossover Crossover A mutation is a change in the hereditary material of an organism. A mutation may lead to different alleles of a gene which, in turn, lead to variations of a trait. Mutated alleles may cause favorable or unfavorable traits to surface. An adaptation is an inherited trait that helps an organism survive. Adaptations include body structures that help an organism feed, move around, and protect itself. Adaptations are inherited; therefore, they must be carried on genes. Some mutations are harmful because they cause genetic disorders. Mutations may also be helpful because they contribute to genetic variation. Genetic variation refers to the diversity of alleles in a population, and ensures that a population has a better chance of survival should the environment change. 146 CC_TG.indb 146 172 8/1/14 11:28 AM C2 Imagine a population of brown squirrels that has a single gene that determines fur color. A mutated allele causes white fur instead of the usual brown fur. The squirrels with brown fur can hide from predators more easily than squirrels with white fur. Most of the squirrels that survive to reproduce are brown. This example illustrates the process of natural selection. Since brown fur is a favorable adaptation, the allele for brown fur is selected over the allele for white fur. In natural selection, organisms with favorable adaptations survive and reproduce. They pass favorable adaptations on to offspring. Over many generations, the alleles for favorable adaptations increase in the population. The Hardy-Weinberg formulas can be used by scientists to determine whether evolution has occurred. Using the equations, any changes in the allele frequencies in a population over time can be detected. An allele frequency is a measure of the occurrence of a particular allele of interest in a population. The Hardy-Weinberg principle states that if no evolution is occurring, an equilibrium of allele frequencies will remain in effect in each succeeding generation of sexually reproducing individuals. In order for equilibrium to remain in effect (or, in order to conclude that no evolution is occurring), the following conditions must be met: bo 1. No mutations can occur, so that new alleles do not enter the population. 2. No migration of individuals into, or out of, the population can occur. 3. Random mating must occur (individuals must pair by chance). 4. The population must be large. Hardy-Weinberg Formulas p + q = 1, where: p = frequency of the dominant allele in the population q = f requency of the recessive allele in the population Fitness refers to the ability of an individual to survive, reproduce, and contribute its alleles to the next generation in the population. The term "Darwinian fitness" is often used to distinguish this term from physical fitness. Over generations, the alleles with higher fitness become more common in the population. and p2 + 2pq + q2 = 1, where: p2 = frequency of homozygous dominant individuals 2pq = frequency of heterozygous individuals Natural selection is the driving force behind evolution. In evolution, the alleles of a population change over time as favorable phenotypes are selected over unfavorable phenotypes. Through evolution, the genetic makeup of the population changes over time. Eventually, evolution leads to the formation of new species from a common ancestor. The new species are genetically different from each other and they can no longer interbreed. q2 = frequency of homozygous recessive individuals bo CC_TG.indb 147 Crazy Chromosomes 173 147 8/1/14 11:28 AM Allele Frequency and Evolution 5E LESSON PLAN Engage Take your students outside into the schoolyard. Have them bring their notebooks. Ask students to find a single organism, either an animal or plant, and have them make a sketch of the organism or take a digital photo to print out and paste into their notebook later. Ask them to make a list of the characteristics of the organism. Then ask them to identify three characteristics from their list and describe how those characteristics may be adaptations to the organism’s environment. For example, a plant’s fuzzy leaves may help the plant trap moisture in a dry environment. Explore Complete Investigation C2, Allele Frequency and Evolution. In this investigation, students apply what they have learned about genes, mutations, and the processes of meiosis and fertilization, to studying the process of natural selection and evolution. They will study two fictional populations of organisms (Crazy Creatures) and compare allele frequencies using the Hardy-Weinberg formulas. They will deduce whether evolution is occurring in one of the populations based on their data. Explain Revisit the Key Question to give students an opportunity to reflect on their learning experience and verbalize understandings about the science concepts explored in the investigation. Curiosityplace.com resources, including student readings, videos, animations, and whiteboard resources, as well as readings from your current science textbook, are other tools to facilitate student communication about new ideas. Science Content Video Heredity Animation Galapagos Finches Elaborate The concept taught in this investigation, allele frequency, allows scientists to study genes at a population level. In previous investigations, students used Punnett squares to study the genes of individuals. The development of the Hardy-Weinberg formulas was an important milestone in the science of genetics and evolution. Before Hardy and Weinberg, it was thought that dominant alleles must, over time, inevitably drive recessive alleles out of existence. This incorrect theory was called “genophagy” (literally “gene eating”). According to this wrong idea, dominant alleles always increase in frequency from generation to generation. Hardy and Weinberg were able to demonstrate with their equation that dominant alleles can just as easily decrease in frequency. Use this example to illustrate an important idea about the process of science: that scientific ideas change and evolve over time as new information is acquired. Evaluate yy D uring the investigation, use the checkpoint questions as opportunities for ongoing assessment. yy A fter completing the investigation, have students answer the assessment questions on the Evaluate student sheet to check understanding of the concepts presented. 148 CC_TG.indb 148 174 8/1/14 11:28 AM C2 Explore INVESTIGATION Explore C2 C2 INVESTIGATION Name ____________________________________________ Date ________________________ Table 1: Creature genome C2 Allele Frequency and Evolution Materials: ✔ Crazy Chromosomes set What happens to the frequency of an allele in a population over time? ✔ Allele coins Gene color Trait Allele on chromosome 1 of the pair Black Skin Color T T TT red Light Purple Leg t t tt long Light blue Foot T t Tt webbed White Arms t T Tt long Red Hands t t tt claws Yellow Eye color t T Tt red/green Dark Purple Eyebrows T T TT unibrow Orange Beak T T TT trumpet Gray Ears t t tt mouse Green Antenna length T t Tt long Dark Blue Antenna shape t T Tt knob Allele on chromosome 2 of the pair Genotype for trait Phenotype for trait Chromosome pair #1 Evolution is a change in the genetic makeup of a population over time, driven by a process called natural selection. All organisms compete for limited resources such as food and shelter. In natural selection, individuals with favorable phenotype variations will be more likely to survive and pass the alleles for those phenotype variations on to their offspring. Eventually, the favorable phenotype becomes prevalent in the population. Imagine a population of Crazy Creatures that lives on the mainland where there are plenty of food sources. This is the original population. A huge storm carries some of that population to a secluded island called Walnut Island. The only food source for this population is rock hard walnuts that fall out of the trees. Will certain phenotypes be more favorable than others in this new environment? One hundred years later, how might we determine if evolution is occurring in the new population? Chromosome pair #2 Determining the genome of a creature in the original population Let’s start the investigation by studying the original population of Crazy Creatures that existed before the storm. Working by yourself, you will randomly determine the genome of an individual in the population. Follow the steps below to determine your creature’s genome. 1. Take out the small plastic bag with the coins in it. You will need to share these coins with other members of your group since everyone is going to flip for their own creature. a. Find the blue coin that has a T on one side and a t on the other. Use this coin to flip for the first chromosome in each pair. Find the green coin with a T on one side and a t on the other. Use this coin to flip for alleles on the second chromosome in each pair. We won’t worry about the sex of our creatures for this investigation, and will focus only on genes that code for other traits. Yellow-green Tail t t tt none Pink Wings T T TT none 2. The first trait you will flip for is skin color. Flip the blue coin and record the result in Table 1 in the skin color row. Flip the green coin and record the result in the skin color row as well. Pass the coins around in your group so everyone has a chance to flip for the trait. 3. Record the genotype for your creature’s skin color in the genotype column. An organism’s genotype is the set of alleles for a particular gene found in the organism’s genome. To record the genotype, simply write the letters in columns 2 and 3. In genetics, the capital letter is always written first, regardless of which chromosome it is located on. For example, if you recorded a t in column 2 and a T in column 3, the genotype for that trait would be Tt. 4. Flip the coins for the rest of the traits. Record the alleles on each chromosome in each pair as well as the genotypes for all of the traits. Share the coins with other members of your group so everyone can determine the genotypes for their creatures. 5. Use Table 2 to decode your creature’s genome and fill in the phenotype column of Table 1 for each trait. Copyright © CPO Science Can be duplicated for classroom use 1 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes Copyright © CPO Science Can be duplicated for classroom use 2 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes Guiding the INVESTIGATION Determining the genome of a creature in the original population If students have completed Investigation B1, they will be familiar with the process of flipping coins to determine the genotype and phenotype of a Crazy Creature. You may have them use their data from that investigation. Unlike in earlier investigations, each individual student should flip for his or her own creature for this investigation. This is because we want a larger starting population for the rest of the investigation. Students flip the allele coins that come with the Crazy Chromosomes set and take turns using them. Alternatively, you can have them use regular coins, with heads representing the dominant allele and tails representing the recessive allele. CC_TG.indb 149 Crazy Chromosomes 175 149 8/1/14 11:28 AM Allele Frequency and Evolution Explore INVESTIGATION C2 Table 2: Genotypes and phenotypes for Crazy Creatures Trait 1. Sex Genotypes and phenotypes XX – female XY – male 2. Skin color TT – red Tt – purple tt – blue 3. Leg TT – short Tt – short tt – long 4. Foot 5. Arms 6. Hands 7. Eye color TT – webbed Tt – webbed TT – long Tt – long tt – talon tt – short TT – paws Tt – paws tt – claws TT – red Tt – one red and one green TT – unibrow tt - green Tt – unibrow tt – separate 9. Beak TT – trumpet Tt – trumpet tt – crusher 10. Ears TT – elephant 8. Eyebrows 11. Antenna length TT – long 12. Antenna shape TT – knob 13. Tail TT – long 14. Wings TT – no wings Tt – elephant Tt – long Tt – knob Tt – short tt – mouse tt – short tt – star tt – none Tt – no wings Explore INVESTIGATION C2 Calculating allele frequencies using the Hardy-Weinberg formulas To determine if evolution is occurring, we can start by determining the allele frequencies for alleles in the original population, then comparing those frequencies with the population on Walnut Island today (100 years after the storm that carried them there). If there are differences in the allele frequencies, we will have evidence that favorable phenotypes are being selected by the environment (i.e., natural selection is occurring). The Hardy-Weinberg formulas can be used by scientists to determine whether evolution has occurred. Using the equations, any changes in the allele frequencies in a population over time can be detected. The law states that if no evolution is occurring, then equilibrium of allele frequencies will remain in effect in each succeeding generation of sexually reproducing individuals. In order for equilibrium to remain in effect (or, in order to conclude that no evolution is occurring), the following conditions must be met: 1. No mutations can occur, so that new alleles do not enter the population. 2. No migration of individuals into, or out of, the population can occur. 3. Random mating must occur (individuals must pair by chance). 4. The population must be large. Obviously, all of these conditions cannot be met in real-life situations. But we can use the formulas to take a “snapshot” of the allele frequency for a gene in a population. By comparing the allele frequencies of the original population and the present-day Walnut Island population, we can determine whether evolution is occurring on the island. If there is a difference in the allele frequency between the two populations, then all of the Hardy-Weinberg conditions have not been met, and evolution may be occurring on the island. If the two frequencies are the same, then we can hypothesize that evolution is not occurring. There are two Hardy-Weinberg formulas. tt – wings Hardy-Weinberg Formulas p + q = 1, where: p = frequency of the dominant allele in the population q= frequency of the recessive allele in the population and p2 + 2pq + q2 = 1, where: p2 = frequency of homozygous dominant individuals 2pq = frequency of heterozygous individuals q2 = frequency of homozygous recessive individuals Copyright © CPO Science Can be duplicated for classroom use 3 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes SCIENCE AND MATH Using the Hardy-Weinberg formulas If your students are good with algebra, they will quickly understand the Hardy-Weinberg formulas. These formulas are called “equilibrium formulas” because all of the variables add up to 1. In this equation (p² + 2pq + q² = 1), p is defined as the frequency of the dominant allele and q as the frequency of the recessive allele for a trait controlled by a pair of alleles (T and t). In other words, p equals all of the alleles in individuals who are homozygous dominant (TT) and half of the alleles in individuals who are heterozygous (Tt) for this trait in a population. Likewise, q equals all of the alleles in individuals who are homozygous recessive (tt) and the other half of the alleles in individuals who are heterozygous (Tt). Copyright © CPO Science Can be duplicated for classroom use 4 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes Go through the example in Part 2 of the investigation as a class to make sure students have a grasp of how to use these formulas. If they are having trouble, they will need practice before continuing. Here is another example to use in addition to the one presented in the investigation. Within a population of moths, brown color (B) is dominant to white color (b). If 30% of the population is white, what is the frequency of homozygous dominant and heterozygous individuals in the population? bb = q 2 = 0.30 q = 0.30 = 0.55 p = 1 − 0.55 = 0.45 p 2 = (0.45) = 0.20 = BB 2 2 pq = 2(0.45)(0.55) = 0.50 = Bb q = 0.30 = bb 150 CC_TG.indb 150 176 8/1/14 11:28 AM C2 Explore INVESTIGATION Explore C2 So, if an individual has claws, we know that its genotype is hh. Similarly, if the individual has a crusher beak, we know its genotype is bb. Follow the steps below to calculate the allele frequencies for the hand and beak genes for the class population. Here is an example of how to use the Hardy-Weinberg formulas. You have sampled a population in which you know the frequency of individuals out of the total population is 0.24 (which is equal to 24%) homozygous recessive for a trait (tt). Use the Hardy-Weinberg formulas to calculate the frequency of the T and t alleles in this population. Then find the frequency of each possible genotype (TT, Tt, tt). 1. Your teacher will guide the class in filling out Table 3. Use this data to calculate the allele frequencies for the hand and beak genes. The allele frequencies are equal to the number of creatures with the allele divided by the total number of creatures in the population. 1. What do you know? The value of q2 = the frequency of the population that is tt (0.24). 2. Use the Hardy-Weinberg formulas to calculate the frequency of the alleles of each gene and record the frequencies in the last two columns of Table 3. 2. What do you want to find out? The allele frequencies of T and t in the population, and the frequencies of each possible genotype (TT, Tt, and tt). Table 3: Frequency of homozygous recessives in original population 3. Formulas to use: p + q = 1 and p2 + 2pq + q2 = 1 4. Solution: Trait To find the frequency of t, simply take the square root of 0.24, since q2 = tt: 0.24 = 0.49 Since p + q = 1 and we now know the value of q, we can calculate p using this relationship: Number of Frequency of homozygous recessive homozygous recessive individuals individuals Total Population 24 24 Hands 3 5 0.13 0.21 Beak p + 0.49 = 1 p = 1 – 0.49 = 0.51 Frequency of recessive allele (h or b) Frequency of dominant allele (H or B) 0.36 0.46 0.64 0.54 Modeling reproduction on Walnut Island Since p2 + 2pq + q2 = 1, and we know the values of p and q, we can easily determine the frequencies of each genotype in this population. The storm blew a random sample of the mainland population to Walnut Island. In this part of the investigation, we will model reproduction in the P1 generation to produce the F1 offspring. p = 0.51 and q = 0.49, so: Frequency of TT = (0.51)2 = 0.26 Frequency of Tt = 2(0.51)(0.49) = 0.50 1. Randomly choose the genome of the creature in your group that you choose to be a parent. You can do this by choosing the creature made by the person with the earliest birthday in a calendar year. Frequency of tt we already know is 0.24 2. Build a diploid set of chromosomes for the chosen creature. To check your work, add the values to make sure they add up to 1: (0.26) + (0.50) + (0.24) = 1 3. Take the chromosomes through the process of meiosis to produce gametes. First, build a sister chromatid for each chromosome in the homologous pair and attach the sister chromatids together with a centromere. Calculating allele frequencies in the original population Now, let’s think about recombination. Will recombination occur during meiosis? Locate the hand and beak genes on your chromosomes. Since they are found on separate chromosome pairs, they are non-linked genes and will segregate independently into gametes. Recombination can also occur in the form of crossover on linked genes. The hand gene is located farthest from the centromere, so crossover is a possibility with the first chromosome pair. But the beak gene, located close to the centromere, is an unlikely candidate for crossover. For the purpose of this investigation, we will assume that crossover will not occur in the beak gene. We will study the genes for hands and beak in Crazy Creatures because we know that both traits follow the law of dominance. For the purpose of our study, let’s use new letters as symbols for each trait. For hands, let H = paws and h = claws For beak, let B = trumpet and b = crusher Copyright © CPO Science Can be duplicated for classroom use C2 INVESTIGATION 4. With that information, model crossover with the hand gene, then separate homologous pairs for the first division. 5 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes Copyright © CPO Science Can be duplicated for classroom use 6 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes Guiding the INVESTIGATION Guiding the INVESTIGATION Calculating allele frequencies in the Modeling reproduction on To increase your study population, you may wish to complete parts 1 and 2 of the investigation on the first day and spend some time working through Hardy-Weinberg calculations. Then, collect data from all of your classes and give it to the students on the second day to complete the rest of the investigation. Larger populations in this investigation yield better results from the activities. Work with the entire class to complete Table 3. This part of the investigation assumes that students have a solid understanding of meiosis and the recombination forces that occur during meiosis. If your students have not been exposed to these concepts before, be sure they complete investigations B5 through B8 prior to this investigation. If they do have a good understanding, you may need to review the processes of independent assortment and crossover with them. Work with the students as a class to make their gametes for Part 4; then they will be able to make them on their own for Part 5, where they will have to model the process two more times. original population Walnut Island CC_TG.indb 151 Crazy Chromosomes 177 151 8/1/14 11:28 AM Allele Frequency and Evolution Explore INVESTIGATION C2 5. Next, separate sister chromatids at the centromere to form your gametes. 6. Place your gametes into separate plastic bags, then put those bags into a paper bag. Mix them up and pull a gamete out of the bag. 7. Place your gametes next to the gametes of the group to which you were assigned. 8. Fill in the genotype and phenotype of the offspring for Trial 1 in Table 4. Be sure to use H/h for hands and B/b for beak instead of T/t. 9. Return the gametes to their original paper bags, mix them up, and repeat steps 6 through 8 for a total of 10 trials. 10. Fill in the last column of Table 4 with a value for fitness. The term fitness refers to the ability of an organism to survive and reproduce. In our system, a fitness of 10 means the greatest chance of surviving to reproduce and 1 means the smallest chance. Use the following key as your guide to assigning values: claws and crusher beak = 10 claws and trumpet beak = 7 paws and crusher beak = 5 paws and trumpet beak = 1 C2 Modeling two more generations In genetic variation, the alleles of organisms within a population can change through natural selection. Natural selection is the process by which organisms with favorable adaptations survive and reproduce at a higher rate than do those with less-favorable adaptations. Genetic variation is important to the process of natural selection. Variations in alleles and phenotypes caused by mutations and recombination happen by chance, but the process of natural selection is influenced by the environment. To model the influence of the environment, follow the steps below to produce the F2 and F3 generations. 1. Choose the creature from Table 4 that has the highest fitness value. 2. Build that creature’s diploid set of chromosomes. 3. Follow steps 3 through 7 of Part 4 to produce the F1 creature’s gametes. 5. Follow steps 1 through 4 above to complete Table 6 for the F3 generation. Table 5: F2 generation on Walnut Island Trial Genotype for hands Phenotype for hands 1 Hh 2 Trial Genotype for hands Phenotype for hands Genotype for beak Phenotype for beak Fitnessvalue Fitnessvalue 1 trumpet 1 2 Hh paws Bb trumpet 1 hh claws Bb trumpet Bb trumpet 1 7 3 hh claws BB trumpet Bb trumpet 7 7 4 hh claws bb crusher 10 bb Bb crusher 5 5 Hh paws bb crusher 5 trumpet 1 6 Hh paws bb crusher BB 5 trumpet 1 7 HH paws BB trumpet Bb 1 trumpet 1 8 hh claws Bb trumpet 7 bb crusher 5 9 hh claws bb crusher 10 Bb trumpet 7 10 Hh paws Bb trumpet 1 crusher 5 Genotype for beak Phenotype for beak paws BB Hh paws 3 hh claws 4 Hh paws 5 Hh paws 6 HH paws 7 Hh paws 8 HH paws 9 hh claws 10 Hh paws bb INVESTIGATION 4. Follow steps 8 through 10 of Part 4 to complete Table 5 for the F2 generation. Table 4: F1 generation on Walnut Island Copyright © CPO Science Can be duplicated for classroom use Explore 7 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes STEM CONNECTION A real-life Hardy-Weinberg problem Sickle-cell anemia is a well-studied genetic disease to discuss with your students. It also acts as a good example of why it is advantageous to carry alleles for a genetic disorder in a population. In sickle-cell anemia, being normal is dominant (S) to having the disease (s). Homozygous dominant individuals (SS) are easily infected with the parasite carried by mosquitoes that causes malaria. Eventually, individuals infected with malaria will become sick and perhaps die. Homozygous recessive individuals (ss) have red blood cells that are “sickle” shaped and cannot properly carry oxygen. These individuals often die at a younger age. However, individuals who are heterozygous (Ss) have some sickle-shaped blood cells, but not enough to Copyright © CPO Science Can be duplicated for classroom use 8 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes cause death. In addition, the malarial parasite cannot survive in the blood of heterozygous individuals. Have your students solve this problem using the Hardy-Weinberg formulas: Suppose 11% of a population is born with sickle-cell anemia. What percentage of the population would be resistant to malaria? q 2 = 0.11 = ss q = 0.11 = 0.33 p = 1 − 0.33 = 0.67 p 2 = (0.67 ) = 0.45 = SS 2 2 pq = 2 (0.67 )(0.33) = 0.44 = Ss In this example, 44% of the population would be heterozygous and resistant to malaria. 152 CC_TG.indb 152 178 8/1/14 11:28 AM C2 Explore INVESTIGATION Explore C2 INVESTIGATION C2 a. How do the allele frequencies compare between the original population and the population on Walnut Island? Table 6: F3 generation on Walnut Island Trial Genotype for hands Phenotype for hands Genotype for beak Phenotype for beak Fitnessvalue 1 hh claws Bb trumpet 7 2 hh claws bb crusher 10 3 hh claws bb crusher 10 4 Hh paws bb crusher 5 5 Hh paws Bb trumpet 1 6 Hh paws Bb trumpet 1 7 hh claws bb crusher 10 8 hh claws Bb trumpet 7 9 Hh paws bb crusher 5 10 hh claws bb crusher 10 The frequency of the recessive allele for both genes increased, while the frequency of the dominant allele for both genes decreased. b. Does this data provide evidence that evolution is occurring on Walnut Island? Use evidence from your data to argue your explanation. Yes. Since the only food source on Walnut Island is walnuts, having claws is favorable to having paws. Also, having a crusher beak is favorable to having a trumpet beak. Both phenotypes are recessive and the data shows that the frequency of the recessive allele for both genes is much higher than it was in the original population. c. If the dominant allele completely disappeared for both traits in the Walnut Island population, would that be a good or a bad thing? Explain your answer. It would be a bad thing, because genetic variation is important to the survival of a population. If the environment changed somehow, genetic variation would allow for other phenotypes that could be better adaptations to the new environment. Analyzing the data Now, 100 years after the storm, we are studying the F3 generation on Walnut Island. Our population consists of all of the individuals in Table 6 for the entire class. Your teacher will help you fill out columns 3 and 4 of Table 7. Using that information, calculate the allele frequencies for hands and beak in the Walnut Island population. Then, transfer the allele frequencies for the original population from Table 3 into Table 7. Table 7: Allele frequencies for Walnut Island Trait Number of Frequency of Frequency of recessive homozygous recessive homozygous recessive allele individuals individuals (h or b) 40 Total population Hands Beak 18 16 0.45 0.40 Allele frequencies of the original population Trait Hands Beak Copyright © CPO Science Can be duplicated for classroom use d. Look at the phenotypes of your original creature in Table 1. Does it have any other phenotypes that you think would be favorable for the Walnut Island habitat? Choose one phenotype and describe why it would be favorable. What would you expect to happen to the allele frequency for that allele if your creature ended up on Walnut Island? Frequency of dominant allele (H or B) Recessive allele (h or b) Dominant allele (H or B) 0.36 0.46 0.64 0.54 9 of 10 0.67 0.63 My creature has long arms, which would be a good adaptation for picking walnuts off of the trees if there are none on the ground. Long arms are dominant to short arms so I would expect the allele frequency for T to increase on Walnut Island and the allele for t (short legs) to decrease over time. 0.33 0.37 Allele frequencies of the Walnut Island population Recessive allele Dominant allele (h or b) (H or B) 0.67 0.63 0.33 0.37 C2 Allele Frequency and Evolution Crazy Chromosomes Copyright © CPO Science Can be duplicated for classroom use 10 of 10 C2 Allele Frequency and Evolution Crazy Chromosomes Guiding the INVESTIGATION Analyzing the data You may want to collect data from all of your classes prior to completing Part 6, especially if you used that larger data set earlier in the investigation. Work with the entire class to complete the numbers in the top rows of Table 7. Then, have students compile the numbers in the bottom rows for comparison. You may wish to use this opportunity to have students create bar graphs of the allele frequencies for comparison. This is a good visual way to compare the data. CC_TG.indb 153 Crazy Chromosomes 179 153 8/1/14 11:28 AM Allele Frequency and Evolution Evaluate INVESTIGATION C2 Notes and Reflections Name ____________________________________________ Date ________________________ 1. What is evolution? Evolution is the change in the genetic makeup of a population over time. 2. How are the Hardy-Weinberg formulas used by scientists to determine if evolution is occurring? We can measure a change in the frequency of an allele between two separate populations to determine if the frequencies have changed between the two. If they have changed, then we can hypothesize that evolution is occurring. 3. What is natural selection and how is it related to evolution? Natural selection is the process by which organisms with favorable phenotypes are able to reproduce and pass the alleles for those phenotypes on to offspring. Natural selection is the driving force behind evolution. 4. What is meant by the term fitness, from an evolutionary point of view? Fitness measures the ability of an individual to survive, reproduce, and pass its alleles on to future generations. 5. In a population of 100 Crazy Creatures, 17 individuals have the star antenna shape, which is recessive to having a knob-shaped antenna. Calculate the allele frequency of the dominant and recessive alleles. Then, calculate the frequencies of homozygous dominant, heterozygous, and homozygous recessive individuals. Copyright © CPO Science Can be duplicated for classroom use C2 Allele Frequency and Evolution Crazy Chromosomes Question tt5 = answer 17 ÷ 100 = 0.17 t = 0.17 = 0.41 T = p + 0.41 = 1 p = 0.59 p 2 + 2 pq + q 2 = 1 TT = p 2 = 0.592 = 0.35 Tt = 2 pq = 2(0.59)(0.41) = 0.48 tt = 0.17 WRAPPING UP Have students reflect on what they learned from the investigation by answering the following question: We know it is important to study the evolution of genes at an individual level. Why is it also important to study the evolution of a species at the population level? 154 CC_TG.indb 154 180 8/1/14 11:28 AM