Gene Combo 59 40- to 1 2 50-minute sessions ACTIVITY OVERVIEW I N V E S T I O N I G AT SUMMARY Students use a coin-tossing simulation to model the pattern of inheritance exhibited by many single-gene traits, including the critter tail-color characteristic. They relate this model to the hypotheses they developed in Activity 58, Creature Features. The activity provides them with a framework within which to interpret Mendel s results (presented in Activity 60, Mendel, First Geneticist ) and their own seed-germination results (analyzed in Activity 62, Analyzing Genetic Data. ) KEY CONCEPTS AND PROCESS SKILLS 1. Hypotheses are based on evidence and can be revised in light of new evidence. 2. Creating models is one way to understand and communicate scientific information. 3. Sexual reproduction involves the union of two sex cells and produces unique individuals that show a combination of traits inherited from both parents. 4. The ratio of dominant to recessive traits in the third generation of a purebred cross provides an important clue about gene behavior. A statistically random process determines which allele each parent transfers to the offspring. KEY VOCABULARY allele dominant fertilization gene hypothesis inherit model probability random recessive Teacher s Guide D-65
Activity 59 Gene Combo MATERIALS AND ADVANCE PREPARATION For the teacher 1 Transparency 59.1, The Coin-Tossing Model 1 Transparency 59.2, Gene Combo Totals * 1 overhead projector For each pair of students * 2 pennies 1 Student Sheet 59.1 Gene Combo Results 1 small cup (optional) *Not supplied in kit Gather pennies and prepare the transparencies and student sheets. TEACHING SUMMARY Getting Started 1. Introduce the coin-tossing model. 2. Introduce chance and probability. Doing the Activity 3. Students toss coins to model how genes are passed from parent to offspring. 4. Collect and display the groups data and discuss Analysis Questions 1 and 2. Follow-Up 5. The class discusses the outcomes of the coin-tossing model. Extension Students post their results on the SALI page of the SEPUP website and compare their results with results from other classes. INTEGRATIONS Mathematics This activity, Gene Combo, deals with the topic of probability, which is often introduced in late elementary or early middle school. Probabilities can be expressed as ratios, fractions, percents, or decimals, and thus involve these topics as well. D-66 Science and Life Issues
Gene Combo Activity 59 BACKGROUND INFORMATION Probability Provided that sample size is adequate, experimental coin tossing corresponds reliably to theoretical predictions, and is therefore a very good model for Mendelian genetics. However, this correspondence does not itself verify the hypotheses or principles of Mendelian genetics. Actual data from breeding organisms are still to come in this unit, in the reading about Mendel s experiments that follows, and with the data on germinated seeds gathered by students in Activity 62, Analyzing Genetic Data. Ratios vs. Fractions Fractions are used to compare a part to the whole, while ratios are commonly used to compare two parts of a whole to each other. In the diagram shown here the shaded part of the circle represents 3/4 of the whole and the unshaded part represents 1/4 of the whole. The ratio of the shaded part to the unshaded part can be represented by 3/4 : 1/4. This can be simplified to 3:1. Students often confuse ratios and fractions. Pie graphs such as this one can be used to help them understand the difference between these two ways of expressing relationships among parts of a whole. Conventions for Allele Notation In this activity, students are instructed to notate the blue-tail allele as uppercase T and the orange-tail allele as lowercase t. This is because it has already been established that blue tail is dominant to orange tail. However, it is also acceptable to use B for blue and b for orange, since blue is the dominant trait, or any other notation that is clearly defined. Genes, Characteristics, Traits, and Alleles Genetics terminology can be confusing for students. A characteristic refers to one observable or measurable feature of an organism. Students studied a number of characteristics of humans in Activity 54, Investigating Human Traits. Each version of a characteristic is called a trait. A characteristic can be caused by one gene or by Teacher s Guide D-67
Activity 59 Gene Combo many genes. Each gene can exist in a number of different versions, or alleles. The critter tail-color characteristic in this activity models the pattern typical of classic Mendelian inheritance. A single gene codes for the tail-color characteristic. The gene has only two possible alleles and only two tail-color traits (blue and orange) exist. The blue-tail trait is dominant; only one allele for blue tail color is needed for the tail to be blue. D-68 Science and Life Issues
Gene Combo Activity 59 TEACHING SUGGESTIONS GETTING STARTED 1. Introduce the coin-tossing model. Ask students, What conclusions were you able to draw by the end of Activity 58, Creature Features? In particular, ask, How many genes for the tail-color characteristic do you think each critter has? Tell students that they will investigate a model for the behavior of genes that assumes that each parent has two versions of the gene for tail color and that only one version from each parent is transferred to each offspring. Introduce the word allele, which first appears in the Student Book in Activity 60, Mendel, First Geneticist. An allele is a version of a gene. In this activity, tail color is determined by two different alleles; one provides information resulting in a blue tail and the other provides information resulting in an orange tail. A coin-tossing simulation will be used to model a random process for determining which of the two alleles a parent gives an offspring. Teacher s Note: In the coin-tossing model for this activity, the term version of a gene is used in place of allele. As students develop an understanding of the need for a different term, you will introduce the term allele. 2. Introduce chance and probability. Begin a discussion of chance, probability, and randomness by asking students what the chances are of picking an ace of hearts from a deck of cards. Students should suggest 1 out of 52. This is correct as long as the deck is a normal deck of cards and as long as the choice of cards is random (each card is equally likely to be chosen; there is no bias). Use this example to operationally define the terms random and probability. You may want to contrast this with a non-random example of probability, such as an upcoming sporting event. Ask if the winner will be determined randomly. The answer is no; instead, the outcome will depend at least partially on the preparation, talent, and ultimate performance of the rival teams. Contrast this with another situation, such as the selection of a winning raffle ticket, which is a random process. Encourage students to apply these concepts to the outcome of tossing a coin by asking, What are the chances that a coin toss will result in heads (vs. tails)? Students will probably say 50-50. The odds are equal for heads or tails because the process is random. Tell students they will use the outcomes of coin tosses (heads or tails) to simulate the random transfer of genes from parents to offspring. They will then compare the results of the random simulation to the results of the critter breeding to see if this random model fits the results. They will assume that Ocean and Lucy are one breeding pair chosen from Generation Two; i.e., they are offspring of Skye and Poppy and both have blue tails. DOING THE ACTIVITY 3. Students toss coins to model how genes are passed from parent to offspring. Review the model presented on page D-30 in the Student Book and on Transparency 59.1. The model is reproduced below. Each side of the coin represents a single version of the gene, and each parent contributes one version. The model assumes that Ocean and Lucy each contain one copy of each version of the gene, just as the coins contain one Teacher s Guide D-69
Activity 59 Gene Combo side representing each version. This is justified by the fact that their parents were Skye and Poppy, who each came from an island where all critters had tails of the same color. Each member of Ocean and Lucy s generation must have an allele for blue tail color, but also must have an allele for orange tail color (since that is the only one Poppy can have contributed); this is summarized in part (c) of the model. Part (d) tells how to interpret results, based upon the assumptions of the model. The Coin-Tossing Model a. The outcome of a coin toss (heads or tails) represents the one version of a tail-color gene that is contained in the sex cell (sperm or egg) contributed by a parent critter. Tails represents the blue version and heads represents the orange version. b. A future offspring critter receives a version of the tail-color gene from each of its two parents when fertilization occurs. c. Each side of the coin represents one of the two versions of the tail-color gene carried by each Generation Two critter, such as Ocean and Lucy. d. Blue tail color is dominant to orange tail color. This means that if a critter has at least one copy of the blue version of the gene, its tail is blue. A critter has an orange tail only if it has no blue versions of the tail-color gene. their coin tosses are not completely random when done by hand. Be sure to review the genetic shorthand of representing dominant and recessive traits as upper and lower case letters. Note that any letter can be used, as long as the upper and lower case of the same letter are used for the different forms of the gene. Often the letter chosen is the first letter of either the characteristic (tail color, T/t) or the dominant trait for the characteristic (blue, B/b). Here, T/t is arbitrarily used. Depending on your student population, you may want to provide students with some guidance on how to construct the table required for Procedure Step 6 before they begin the activity. If your students are proficient at constructing data tables, you may wish to assess them on the Organizing Data element of the DESIGNING AND CONDUCTING INVESTIGATIONS (DCI) variable. A level 3 response is shown in Table 1 below. Allow the students about 15 minutes to collect their data and provide you with their results. Typical student results follow in Table 1. Note that the ratio of the data calculated is 2.3:1; this simulation relies on having a large sample size. Therefore, it is important to stress the difference between the class results and those of pairs of students. Be prepared to ask students why a larger sample size is more scientifically valid. Table 1: Gene Combo Sample Results Gene Combo No. of Times Tail Color Totals Use a transparency copy of Student Sheet 59.1, Gene Combo Results, to demonstrate the procedure of the simulation on the overhead projector TT Tt 5 6 blue blue 14 blue before handing out supplies. You may wish to make small cups available to students if they suspect that tt tt 3 6 blue orange 6 orange D-70 Science and Life Issues
Gene Combo Activity 59 4. Collect and display the groups data and discuss Analysis Questions 1 and 2. Collect the groups summary data and use them to complete Transparency 59.2, Gene Combo Totals. Students should copy the class totals into their science notebooks. Then work with the students to perform the calculations required in Analysis Questions 1 and 2. You may need to guide the class through the questions. If you wish them to have further practice with this type of problem, you can have each pair repeat the procedure for the data gathered in their groups. Discuss the idea that with 20 coin tosses, as used in this activity, you would expect to get heads-heads, heads-tails, tails-heads, and tails-tails each about 1/4 of the time, or 5 times out of 20. However, note that heads-tails and tails-heads are essentially the same outcome for the critters (resulting in one each of the blue and orange alleles), so these two results can be added. Also make the point that theoretical predictions and actual outcomes are not identical, as their data clearly demonstrate. Most groups will not have a perfect 3:1 ratio of blue:orange tails. Students should see that some groups get a higher ratio, while others get a lower ratio, than the theoretical ratio of 3:1. The whole class s results will usually be closer to 3:1 than many of the individual group results, but still might be nearer to 2.5:1 or 3.5:1 than 3:1. FOLLOW UP 5. The class discusses the outcomes of the coin-tossing model. Review the basic principles of probability and heredity as illustrated by this activity. Ask, Why did you toss a coin to model gene behavior? The coin toss models the fact that there is a 50-50 chance of the parent passing a blue vs. an orange allele to the offspring. Analysis Questions 1 3 are difficult for most students to complete without help. They are best done as a whole class discussion or in small groups followed by class discussion. Discuss the ratios produced by the model. Have students compare the different groups results with the total results. Emphasize that the larger the group the closer results should be to the predicted 3:1 ratio. Discuss the relationship between the colored-disk model used in the previous activity and the cointossing model. In this case, the coin toss simulates a random process for determining which allele each parent gives to its offspring. What were the assumptions built into this model? Each organism has exactly two pieces of genetic information (alleles) for tail color. In addition, every sex cell produced by a second-generation critter (such as Ocean or Lucy) has a 50% or 1/2 chance of having a blue tail-color gene, and a 50% or 1/2 chance of having an orange tail-color gene. Question 4 prompts students to relate the coin-tossing model to the Generation Three critter data from Activity 58, Creature Features. Ask students to compare the model in this activity to Hypotheses A, B, and C. Which one was being modeled? Students should suggest Hypothesis C, based on the equal genetic contribution by each parent and the concept of a single dominant gene overwhelming a single recessive gene. The coin-tossing model adds the concept of a random mechanism for determining which gene is contributed by each parent. Teacher s Guide D-71
Activity 59 Gene Combo Tell students that Hypothesis C is the one that represents the modern understanding of heredity. Exactly how it was discovered, and how that understanding relates to sexual reproduction, will be discussed in the next few activities. The thought process the students have gone through in thinking about alternate hypotheses is similar to the kind of thinking scientists often engage in when trying to solve a problem. Question 5 provides an opportunity to confront the common misconception that the dominant trait is the more common one. It can be scored with the U N D E R S TA N D I N G C O N C E P T S (UC) scoring guide and used as a baseline assessment of students understanding of the concept of dominant and recessive traits. Class discussion should uncover the fact that dominance refers only to which trait is found in an individual who has both types of alleles. A striking example of a human trait that is rare, although dominant, is polydactyly, or extra digits on the hand. When asked, most students will assume that polydactyly is a recessive trait. The Marfan syndrome is another example of a trait that is dominant, but rare. This is also an opportunity to introduce the term recessive to describe a trait that is observed only when two alleles for the trait are present. A recessive trait is essentially masked, or hidden, by a dominant trait. their prior misconceptions and construct an accurate foundation on which they can build, both within this unit and in future courses. Activities 60 and 61 provide evidence to support the hypothesis built into the coin-tossing model; in Activity 61 students will use Punnett squares to explore the random nature of Mendelian inheritance in another way. Teacher s Note: The terms heterozygous and homozygous will be formally introduced in Activity 61. Extension Students post their results on the SALI page of the SEPUP website and compare their results with results from other classes. This provides a larger sample size. Instructions for posting your classes results are provided on the SALI page of the SEPUP website. At the end of the activity, review the coin-tossing model and discuss new terms and their meaning in the context of the critter tail-color model. Show students how new vocabulary terms can help them express ideas by inserting the word allele in place of versions of the tail-color gene on Transparency 59.1. Genetics concepts can be difficult for students; the development of these activities is intended to address D-72 Science and Life Issues
Gene Combo Activity 59 SUGGESTED ANSWERS TO ANALYSIS QUESTIONS 1. What is the ratio of blue-tailed to orangetailed critter pups? Use the class data to answer this question: a. Divide the number of blue-tailed offspring by the number of orange-tailed offspring. ratio of tail colors = number of blue-tailed offspring number of orange-tailed offspring When 20 coin tosses from each of at least 12 student pairs are combined, there is still a possibility that the full class s data will be ambiguous, with the calculated ratio falling around 2.5:1 or 3.5:1 instead of near 3:1. One way to improve results is to have the students toss more coins. Another option is to combine data from other sections of the course. (Both of these methods increase sample size.) If you have Internet access in the classroom, the best approach is to go to the SEPUP website and look up the data from multiple classes. See the instructions at the SALI page to post your results. b. Round this value to the nearest whole number. Then express it as a ratio by writing it like this: : 1 (whole number) Students are likely to get ratios between 2:1 and 4:1. c. Express this ratio as a pair of fractions, so that you can use them to complete the following sentence: About of the offspring have blue tails, and about of the offspring have orange tails. About 3/4 of the offspring have blue tails, and about 1/4 of the offspring have orange tails. d. Explain why the class obtained such a large ratio. For example, why isn t the ratio of blue to orange tails 1:1, that is, 1/2 blue and 1/2 orange? Blue tails are much more likely because three coin-toss combinations yield a blue tail, and only one gives orange. This is because blue is dominant only the blue-tail trait is observed as long as there is at least one allele for blue tail color present. 2. You and your partner are about to toss two coins 100 times. Predict about how many times the outcome would be: a. heads-heads about 25 times (1/4 probability on each toss) b. heads-tails about 25 times (1/4 probability on each toss) c. tails-heads about 25 times (1/4 probability on each toss) d. tails-tails about 25 times (1/4 probability on each toss) 3. How sure are you that you will get exactly the results you predicted for Question 3? Explain your answer. You cannot be sure you will get exactly those results. The answers for Question 2 are based on probability and are the most likely results. The real-world results are rarely exactly what is Teacher s Guide D-73
Activity 59 Gene Combo predicted theoretically, due to random variation in the set of observed coin tosses. Probability allows us to predict how likely each result is, but not the actual sets of results obtained. (If students are having difficulty with this idea, you might compare this with similar situations. For example, you would predict that a family of four children would have two girls and two boys. This is the most likely outcome, but certainly does not happen in all families with four children!) 4. Look back at Activity 58, Creature Features. Do the results of the coin-tossing model match the Generation Three critter data? Explain. The Generation Three critters were about 3/4 blue-tailed and 1/4 orange-tailed. So the results of the Gene Combo model are consistent with the Generation Three critter data. 5. Try to write your own definition of the phrase UC dominant trait as it is used in genetics. Hint: Does it mean that every time any pair of critters mates, most of the offspring will have blue tails? Why or why not? This question provides an opportunity to get a baseline assessment of students understanding of the concept of a dominant trait. They will have another chance to be assessed on a similar question later in the unit. A sample level 3 answer follows: A dominant trait is a trait that you can always observe if at least one allele for the trait is present. For example, the blue-tail trait is dominant and is observed even if an allele for another trait (orange tail) is present. This does not mean that every time a pair of critters mates most of the offspring will be blue-tailed. If both parents have orange tails, for example, then all their offspring will also have orange tails. Teacher s Note: Students may use the word gene interchangeably with allele at this time. This is not a serious error at this point. D-74 Science and Life Issues
The Coin-Tossing Model a. The outcome of a coin toss (heads or tails) represents the one version of a tail-color gene that is contained in the sex cell (sperm or egg) contributed by a parent critter. Tails represents the blue version and heads represents the orange version. b. A future offspring critter receives a version of the tail-color gene from each of its two parents when fertilization occurs. c. Each side of the coin represents one of the two versions of the tail-color gene carried by each Generation Two critter, such as Ocean and Lucy. d. Blue tail color is dominant to orange tail color. This means that if a critter has at least one copy of the blue version of the gene, its tail is blue. A critter has an orange tail only if it has no blue versions of the tailcolor gene. 2006 The Regents of the University of California Science and Life Issues Transparency 59.1 D-75
Gene Combo Totals Student Group Coin Tossing Model Results No. of Blue Tails No. of Orange Tails 2006 The Regents of the University of California Totals Science and Life Issues Transparency 59.2 D-77
Name Date Gene Combo Results Offspring Ocean s contribution (T or t?) Lucy s contribution (T or t?) Offspring s genes (TT, Tt, tt, or tt?) Offspring s tail color (blue or orange?) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 2006 The Regents of the University of California 15 16 17 18 19 20 Science and Life Issues Student Sheet 59.1 D-79