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60 40 1 50 -m i n ut e s e ss io G - to ns Mendel, First Geneticist REA DI N ACTIVIT Y OVERVIEW SUMMARY A reading describes Mendel’s experiments with pea plants. Students relate the rules discovered by Mendel in his analysis of pea plant crosses to their findings about critter genes. The reading introduces the idea that basic concepts discovered in working with one type of organism (for example, pea plants) can often be generalized to other organisms or groups (such as humans). Students also consider the scientific approaches employed by Mendel in planning and analyzing his experiments. KEY CONCEPTS AND PROCESS SKILLS 1. Breeding experiments can provide information about the behavior of genes. 2. 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. 3. Tracing the history of science demonstrates how individuals contributed to the development of modern scientific ideas, and reveals important interactions between science and society. 4. An allele is one of the two or more forms of a gene present in a population. 5. By virtue of his novel approach to biology, Mendel is considered by many as the founder of the field of genetics. 6. Scientists (such as Mendel) use many approaches to solve biological problems, including careful observation, experimentation, and mathematical analysis. Mathematical analysis applies to many areas of biology. 7. Mendel studied a number of traits in order to develop his general principles of heredity. Teacher’s Guide D-81 Activity 60 • Mendel, First Geneticist KEY VOCABULARY allele probability characteristic random dominant recessive gene/genetics sexual reproduction offspring trait MATERIALS AND ADVANCE PREPARATION For the teacher Transparency 60.1, “Mendel’s Results” 12 transparent blue plastic disks 12 transparent orange plastic disks * 1 overhead projector *Not supplied in kit TEACHING SUMMARY Getting Started 1. Introduce breeding and Mendel. Doing the Activity 2. Students read about and discuss Mendel’s life. 3. The class discusses Mendel’s experiments and results. Follow-Up 4. The class discusses Mendel’s pattern of inheritance and models Mendelian inheritance of the critter tail-color traits. 5. Students respond to the Analysis Questions. INTEGRATIONS Social Studies and History of Science This activity can be integrated with a study of other events of the time, including historical events in science (such as Darwin’s publication of Origin of Species, and the events in Activity 37, “The History of the Germ Theory of Disease”). The importance D-82 Science and Life Issues Mendel, First Geneticist • Activity 60 of plant and animal breeding efforts from prehistoric times to the present can also be explored. BACKGROUND INFORMATION Gregor Mendel Before Mendel, people’s ideas about heredity were similar to some of the hypotheses students used in the critter simulation. People knew that pure-breeding lines of some livestock and crops could be developed to reliably produce offspring with desired characteristics. They also knew that other traits were difficult to maintain by breeding, but did not know that some traits are caused by many genes or are only partially determined by genes. Although he had training in science, Mendel (like all scientists at the time) did not know of the cellular basis of fertilization, the process of cell division, or the existence of chromosomes. Yet Mendel was able to analyze the results of his experiments and propose a theory of heredity that began the formal field of genetics. One of his most important insights, which should be emphasized to students, was his realization that he should use large numbers of crosses and offspring, and apply the mathematics of probability to his results. Mendel’s work received very little recognition until several years after his death when three scientists working independently in 1900 simultaneously rediscovered the laws of inheritance. They found that these laws explained the inheritance of many traits in many species, although not all traits follow the simple one-gene patterns exhibited by the pea traits reported by Mendel. (In fact, the inheritance of most traits is much more complicated, involving in some cases many genes, multiple alleles for some genes, and environmental factors in determining organisms’ characteristics.) Mendel’s theory of inheritance is now the starting point for most genetics courses and is central to the development and understanding of the theory of evolution by natural selection. Mendel was educated in the natural sciences and worked as a teacher until he became abbot of the monastery in 1868. He communicated with others interested in agriculture and science, and belonged to scientific organizations. He followed up his experiments with peas with investigations of other plants and with experiments on breeding and keeping bees. He was also known for his work in meteorology. Students may be interested to know that despite Mendel’s sound preparation in science, he twice failed the tests to obtain official teacher certification. One of his biographers Teacher’s Guide D-83 Activity 60 • Mendel, First Geneticist has speculated that his failures were due to a nervous disposition in stressful situations; others have suggested that the testers disliked the unorthodox ideas about earth and life sciences expressed by Mendel. This reading gives you a chance to emphasize with your students an interesting lesson of history: people with great and challenging ideas are sometimes overlooked during their lifetimes. Darwin suffered too, but far less than did Mendel. Intellectual supporters flocked to Darwin’s side immediately after he published Origin of Species. By the time of his death over 100 years ago, evolution by natural selection had achieved broad acceptance. Mendel’s work was an important part of this. Developments in microscopy and studies of the cell around the turn of the century allowed the scientific community a chance to understand the deep significance and rigor of his results. These advances led to natural selection getting the final cornerstone it needed—the particulate nature of inheritance. Mendel didn’t live to see his work recognized, let alone placed at the foundation of modern biology. But modern molecular biology and genetic engineering are based on his groundbreaking research. REFERENCES Orel, Vitezslav. Gregor Mendel: The First Geneticist. New York: Oxford University Press, 1996. D-84 Science and Life Issues Mendel, First Geneticist • Activity 60 TEACHING SUGGESTIONS GETTING STARTED Introduce breeding and Mendel. Initiate a discussion of breeding, using dogs, cats, or other familiar organisms. For example, you might ask, What is dog breeding? Why do people breed dogs, cats, and other animals? Breeding refers to the process of selecting two parent individuals to mate (in the case of animals) or cross (in the case of plants) in order to produce a desired outcome or improvement in the offspring. This might be done to produce a dog or cat with a desired appearance or behavior, or a cow that produces more milk. Introduce the idea that people have been breeding animals and crops for thousands of years and that breeding efforts have been key to developing high-yield crops and transforming societies from hunter-gatherers to farmers. Tell students that they will read about how an Austrian monk made a breakthrough in genetics by breeding and carefully studying pea plants. DOING THE ACTIVIT Y 2. his environment that were favorable to his research. The reading suggests that a thoughtful, methodical approach characterized Mendel’s research. It also suggests patience and a willingness to invest in a long-term effort to obtain the data he needed. In addition, students should note that Mendel, though also a monk, had training in science and mathematics that helped to prepare him to form and explore his ideas. His insights into genetics also suggest that he was creative and daring in his analysis of his results. 3. The class discusses Mendel’s experiments and results. After students read the first few paragraphs of Part Two, again pause to discuss the Stopping to Think questions. You may want to mention that modern geneticists have also used organisms such as fruit flies, with a 2-week generation time, and yeast, with a generation time of hours, to study the behavior of genes. Explain that Mendel made sure he had purebred plants. In other words, his original stock of greenseeded peas never produced yellow-seeded offspring, and vice versa. Students should recognize Students read about and discuss Mendel’s that this is equivalent to the critter-breeding exper- life. iment, in which the blue-tailed critter Skye was This reading has two major purposes: 1) to give students an historical perspective on the development of one of the major concepts in the biological sciences, and 2) to provide data from Mendel’s experiments breeding pea plants, which students can compare to their own work with the critter model. from an island containing only blue-tailed critters, and the orange-tailed Poppy was from an island containing only orange-tailed critters. The rest of the critter scenario neatly parallel’s Mendel’s experiments as well. Emphasize the very large numbers of crosses and the many seeds his crosses produced for analysis. Without a large sample size, the 3:1 Plan to stop after students read Part One and discuss ratio of offspring might not have been conclusive, the first Stopping to Think question with the class. or even apparent, as students may have observed Discuss some of the characteristics of Mendel and with their results from Activity 59, “Gene Combo.” Teacher’s Guide D-85 Activity 60 • Mendel, First Geneticist Before students answer the questions, review how mines which gene each Generation Two parent to calculate ratios and clarify the relationship of (such as Ocean and Lucy) donates to the offspring. ratios to fractions. Figure 1 is intended to help illus- Work through the four different combinations pos- trate two different ways of expressing the number of sible in the third generation (blue-blue, blue- yellow- and green-seeded plants. A ratio expresses orange, orange-blue, orange-orange). Discuss how the relative proportion of one group to another, in this works out to the 3:1 ratio of blue:orange tails in this case 3 yellow:1 green or 1 green:3 yellow. Frac- the story. In Activity 61, “Gene Squares,” students tions express the relative proportion of one group to will use Punnett squares to further investigate this the total, as in 3/4 yellow, 1/4 green. basic model of inheritance. FOLLOW–UP 4. Discuss the inferences Mendel made from his results and how he used the actual numbers of dif- The class discusses Mendel’s pattern of ferent kinds of pea plants to explain and model how inheritance and models Mendelian the results arose. Use this example to help students inheritance of the critter tail-color traits. see the difference between the results or evidence of Use Stopping to Think 3 to prompt a review of the pattern of inheritance described by Mendel, commonly referred to as Mendelian inheritance. In this pattern, a particular characteristic (such as flower color) has two different versions, called traits (such as purple vs. white). If an individual has alleles for both versions of the trait, the dominant version appears, while the recessive version is completely masked. (Such an individual is called a hybrid.) an experiment—the raw data and any calculations—and a conclusion or interpretation. Ask students to describe which aspects of the model are orderly and which are random. Emphasize that the process follows a set of rules Mendel was able to describe. These rules are the basis of the assumptions of the coin-tossing model used for Activity 59. The only random aspect is which of the two possible alleles is donated by each parent. When two individuals who have alleles for both Review the terms dominant and recessive. You may traits produce offspring, approximately 1/4 of the want to use the flower color cross (in Table 1 on offspring show the recessive trait and approximate- page D-36 in the Student Book) for a final review of ly 3/4 show the dominant trait. these concepts and of how to calculate ratios. This At this point, have students consider Mendel’s model as described on page D-37 in the Student Book. Use the blue and orange transparent disks to demonstrate the model on the overhead projector, and if possible have student volunteers participate. Place two blue disks for Skye and two orange disks will help students begin answering Analysis Questions 1 and 2. Analysis Question 1 can be used to assess the “Organizing Data” element of the DESIGNING AND C O N D U C T I N G I N V E S T I G AT I O N S (DCI) variable. A sample level 3 response is provided in Table 1. for Poppy. Each Generation Two critter gets a blue At this time, explain to students that the genetics disk from Skye and an orange one from Poppy. behind many human traits is far more complicated Activity 59 modeled the random process that deter- than Mendel’s model. Nevertheless, Mendel’s D-86 Science and Life Issues Mendel, First Geneticist • Activity 60 model provides a foundation necessary for under- that the two characteristics with the larger sam- standing some of the more complicated inheritance ple sizes—seed color and seed surface—have patterns that will be presented in later activities. ratios closer to the predicted value than the two 5. characteristics with smaller sample sizes.) Students respond to the Analysis Questions. 3. 1. seeded to yellow-seeded offspring. Explain why a TO ANALYSIS QUESTIONS 1:3 ratio of green-seeded plants to yellow-seeded Based on Mendel’s results, what trait for each pea characteristic is dominant? Make plants is the same as a fraction of 1/4 green-seeded DCI od plants. a table of the dominant and recessive alternatives A 1:3 ratio means that for every one green-seed- for each characteristic in Table 1. Add an extra col- ed plant there are (approximately) three yellow- umn to your table; you will use it to record your seeded plants. A ratio compares one group to answers to Question 2a. another, in this case green- to yellow-seeded plants. A fraction compares one group to the See Table 1 below. 2. Look at Figure 1, which shows the ratio of green- SUGGESTED ANSWERS a. total. In this case, the total number of plants is Calculate to the hundredths place the ratio of 4. For every four plants total, there are about dominant to recessive for each characteristic in the three yellow-seeded plants (3/4) and one green- third generation. Record the ratio for each charac- seeded plant (1/4). teristic in the table you prepared for Question 1. 4. Mendel performed his experiments on more charac- The ratios are shown in the fourth column of teristics than the four shown in Figure 1. Why was Table 1. it important for him to look at more than one char- b. acteristic? Why are the ratios not exactly 3:1? They are not exactly 3:1 because they are actual data from a small sample set, while the 3:1 ratio is predicted for a large set of results, based on the theoretical probability of each outcome. (Note By looking at more than one characteristic, Mendel determined that his observations were generalizable and not just a fluke. Note that similar results have been obtained with many Table 1 Characteristic Dominant Trait Recessive Trait Ratio Dominant:Recessive flower color purple white 705:224 = 3.15 : 1 seed color yellow green 6,022:2,001 = 3.01 : 1 seed surface smooth wrinkled 5,474:1,850 = 2.96 : 1 pod color green yellow 428:152 = 2.82 : 1 Teacher’s Guide D-87 Activity 60 • Mendel, First Geneticist 5. characteristics in many species of plants and math is used to compare populations. For exam- animals. ple, they might say that mathematics helps us Reflection: People often think of mathematics as important to physics and chemistry, but not to life science (biology). What is your opinion? D-88 calculate average pulse rates. (Some more sophisticated examples students will probably not think of are studies of the rates of biochemical reactions and the interpretation of X-ray dif- Challenge students to think of examples of fraction images to determine the structures of mathematics in life science. They are likely to proteins and nucleic acids. Both of these exam- think of measurement (of physiological indica- ples involve very sophisticated mathematical tors like pulse, blood pressure, and tempera- analysis.) Some mathematicians devote their ture), scale, and dosage. They may mention how careers to studying biological problems. Science and Life Issues Science and Life Issues Transparency 60.1 purple x white all purple 705 : 224 (purple:white) Original Cross (Generation One) Generation Two Offspring Generation Three Offspring Flower Color ©2006 The Regents of the University of California Seed Surface 6,022 : 2,001 (yellow:green) all yellow 5,474 : 1,850 (smooth:wrinkled) all smooth green x yellow wrinkled x smooth Seed Color 428 : 152 (green:yellow) all green green x yellow Pod Color Mendel’s Results Teacher’s Guide D-89