Taste Blind? Core Concepts Receptors, nerve cell pathways, and taste areas of the brain are involved in sensing tastes. People differ in their response to taste sensations. A correlation is a relationship between two variables that tend to occur together. Some characteristics associated with the nervous system and behavior can be inherited. Class time required: Three forty minute class periods (an additional class period if Parts 6 and 7 are done). Teacher Provides: For each student 1 copy Taste Blind? student handout. Only copy pages you will have students complete. Parts 6 and 7 are optional extensions for honors or advanced classes. 1 color copy of Taste and the Nervous System diagram sheet (page v). This may be laminated or put in a sheet protector if used for multiple classes. If needed, this may be duplicated in black/white. Small plastic bag labeled Control Paper containing several pieces of control taste paper. Order 144116 control test paper from Wards. Small plastic bag labeled PTC Paper containing several strips of PTC taste paper. Order 144116 PTC test paper from Wards NOTE: Some students find the PTC taste very unpleasant. Consider providing small cups of water or pieces of hard candy to help students get rid of the lingering PTC taste. Optional a small piece of washed raw broccoli for each student to taste. Many students have never tasted raw broccoli. Follow appropriate food safety procedures and do not force students to try the raw broccoli. For the class: Four signs placed in different areas of the room: Tasters who like uncooked broccoli Tasters who do not like uncooked Non tasters who like uncooked Non tasters who do not like uncooked i
For Part 5: PTC Inheritance, each student team will need: Simulated paper gel (see page vi). Cut out and use a cotton swab to apply tiny spots of 1% phenolphthalein in the locations shown on the right. Tube or cup of labeled DNA Stain containing 1/8 teaspoon of Arm & Hammer washing soda. Plastic plate (6 or 7 inch) or tray large enough to fit the simulated paper gel. Plastic stirrer Access to water Suggested Class Procedure: 1. Distribute to each student one copy of Taste Blind? Read the paragraph in the box in Part 1: Are you taste blind. 2. Distribute a bag of control paper and PTC paper to each student. Ask students to read and follow the directions to determine if they are PTC tasters or non-tasters. 3. Count the number of students who are tasters. Count the number of students who are nontasters when indicated in the instructions. Write this information on the board. 4. Provide an opportunity for students to share their answers for the question 8 in Part 1. 5. Distribute copies of Taste and the Nervous System diagram sheet to each student. 6. Students work individually or with a partner to complete Part 2: PTC Tasting and the Nervous System. 7. Ask several students to share their answers to the questions in Part 2. 8. Students work individually or with a partner to complete Part 3: Taste Receptors and Evolution. This may be done as homework or in class. 9. Ask several students to share their answers to the questions in Part 3. 10. For Part 4: PTC Tasting and Broccoli Tasting, consider giving students an opportunity to taste small samples of uncooked broccoli. Students answer question 1 by circling the picture that best describes them. Read the research question and the definition for correlation aloud to the class. 11. Allow time for each student to write a hypothesis in question 2. ii
12. Students move to stand by the sign that best describes them. Some students may have difficulty deciding whether they like or do not like broccoli. You may need to encourage these students to select the one that best fits them. 13. Count and record the number of students on each sign. Record these in large numbers on each of the signs. 14. Students work individually or with a partner to complete Part 4: PTC Tasting and Broccoli Tasting. 15. Ask several students to share their answers to the questions 6, 7, and 8 in Part 4. 16. Part 5: PTC Inheritance requires prior instruction on pedigree analysis. If students have not done pedigrees, step them through the process by asking the following questions: Which individuals have the tt genotype? Hint: They would be non-tasters. Write tt for these individuals. Which individuals have at least one T gene? Hint: They would be tasters. Write a T for these individuals. Which individuals with a T gene had to get a t gene from one of their parents? Hint: They would have one parent who had the tt genotype. Add a t after the T for these individuals. Which individuals with a T gene had a t gene that they passed on to a tt offspring? Add a t after the T for these individuals. Write a? after the T for the all of the remaining individuals. 17. Distribute lab materials need for students to complete Part 5. Each student team will need: Simulated paper gel (see page vi) that you have spotted with phenolphthalein. Tube or cup of labeled DNA stain containing 1/8 teaspoon of Arm & Hammer washing soda. White plastic plate (6 or 7 inch) or white foam meat tray large enough to fit the simulated paper gel. Plastic stirrer Access to water Note: Parts 6 and 7 are optional. They were designed for honors or advanced classes. 18. Part 6: PTC Population Genetics and Evolution applies Hardy-Weinberg conditions to explore the evolution of PTC tasting. Students should work in small groups to brainstorm ideas for a scenario in which the seven conditions are NOT met. 19. Have several groups share their ideas for a scenario in which evolution of a population with only homozygous dominant tasters. iii
20. Part 7: Population Genetics and PTC Tasting asks students to use the Hardy-Weinberg equation to calculate gene and genotype frequencies. After using the data provided, you may wish to ask students to apply what they learned to collect and analyze data from their class and/or other classes. Additional Teacher or Student Resources: PTC: Genes and Bitter Taste at http://learn.genetics.utah.edu/content/begin/traits/ptc/ Scientists Find Bitter Taste Gene at http://faculty.washington.edu/chudler/bitter.html A Word about PTC Safety at http://teach.genetics.utah.edu/content/begin/traits/tree.html Scroll to the bottom of the page. iv
Taste and the Nervous System v
For use in PTC Inheritance activity Print on cardstock paper and cut out the simulated gels. Use a cotton swab to apply spots of 1% phenolphthalein to the cardstock paper, as shown in the teacher information. 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 6
Taste Blind? Teacher answer key Part 1: Are you PTC taste blind? You might have heard of red green colorblindness, but I bet you haven t heard of taste blindness. Just as there are people who can t tell the difference between the colors red and green, there are people who can t taste a certain type of bitter flavor. And just like color blindness, it s genetic. 1. You have two bags of taste paper Control Paper and PTC Paper. Touch one of the paper strips from the bag labeled Control Paper to the tip of your tongue. Touch one of the paper strips from the bag labeled PTC Taste Paper to the tip of your tongue. 2. If both papers just taste like paper, you are a non-taster for PTC. If one tastes bitter and the other just tastes like paper, you are a taster. 3. Are you a taster or a non-taster? 4. How many students in your class are tasters? 5. How many students in your class are non-tasters? Percentage of tasters = number of tasters/total number in class X 100 6. What percentage of the students in your class are tasters? 7. What percentage of the students in your class are non-tasters? 8. How might the nervous system of a taster be different from the nervous system of a nontaster? List at least two possible differences. Student answers will vary. Encourage a diversity of answers and have students share their answers. 1
Part 2: PTC Tasting and the Nervous System 1. Below is a description of what happens in the body when people taste bitter substances such as PTC. For each of the statements below, write the letter of the picture from the Taste and Your Nervous System diagram sheet that best illustrates the statement. B A C D F E The tongue is covered with bumps called papillae. Each papilla contains many microscope taste buds. When PTC molecules enter the mouth, they dissolve in saliva and enter the taste buds through a pore. Each taste bud is filled with taste cells the cells that can tell when PTC is dissolved in a person s saliva (spit). The tips of the taste cells are covered with bitter taste receptor proteins. The dominant taster gene (T) makes a receptor protein that has the correct shape to match with the PTC molecules. People who are tasters have PTC receptors that fit with PTC molecules. When a PTC molecule fits into the receptor proteins, it will excite that taste cell and cause it to send an impulse (electrical signal) to other cells in the nervous system. Nerves conduct the impulses from the taste cells to relay areas and then the taste center of the brain. The taste center is the part of the brain that is responsible for the conscious sensation of BITTER! 2. Apply what you learned to make a drawing to illustrate the information below. Non tasters have a version of the taste receptor that cannot detect PTC because the PTC molecule will not fit into it. Your Drawing 3. Some people gradually or suddenly lose their ability to taste some foods or all foods. Explain two possible changes in the nervous system that could result in a change in the ability to taste. Change in shape of receptors, damage or loss of taste buds, damage to the nerves that conduct impulses to the brain, damage to the brain relay area or brain taste areas, change in ph of mouth, stroke, etc. Encourage students to share their answers. 2
Part 3: Taste Receptors and Evolution PTC is not found in nature, but people who can taste PTC are likely to taste other bitter substances that occur naturally. These bitter substances share the potential of being toxic. Some plants produce bitter tasting toxic compounds in order to protect themselves from being eaten. Human s ability to taste bitter flavors offered a survival advantage by protecting ancient people from poisonous plants. Upon further testing, it was found that about 25 percent of the population is unable to taste PTC while 75 percent finds it bitter. In the family studies that followed, taste blindness was found to be inherited as a recessive trait. Because animal studies showed that apes are different from humans in that all of them are PTC tasters, human non tasters are thought to have somehow appeared later in evolution. A bitter taste sensation triggers unlearned gaping, tongue thrusting, and oral ejection behaviors in human newborns and many animal species. People who taste a bitter food also may learn to avoid that food in the future. Today, bitter taste sensitivity may have harmful consequences for human health by causing people to avoid bitter tasting vegetables or medicines, some of which might lower the risk of cancer and heart disease. 1. What change might have caused the appearance of human non-tasters? A mutation in the gene for PTC tasting or the PTC receptor gene. 2. What might be an evolutionary advantage to being a bitter taster? The bitter taste warns people that a plant or substance might be toxic. 3. What might be an evolutionary disadvantage of being a bitter taster? The bitter taste might cause people to avoid a healthy diet or needed medicines. 4. How could scientists and the medical and food industries use an understanding of the biology of bitter taste sensations to help improve human health? They may find ways to make healthy foods or medicines taste better. 3
Part 4: PTC Tasting and Broccoli Tasting 1. Do you like broccoli? Or, are you a picky eater who does not like broccoli? Circle the statement that best describes you. I like uncooked broccoli It does not have a bitter taste I don t like uncooked broccoli It has a bitter taste Research Question: Is there a correlation between the ability to taste PTC and the avoidance of (not liking) broccoli? Correlation: a relationship between two variables which tend to occur together. 2. Hypothesis: If there is a correlation between PTC tasting and avoidance of broccoli, then People who can taste PTC will not like broccoli. OR People who cannot taste PTC will like broccoli. 3. Your teacher has prepared 4 signs. Stand by the sign that best describes you. Tasters who like uncooked broccoli Tasters who do not like uncooked broccoli Non tasters who like uncooked broccoli Non tasters who do not like uncooked broccoli 4
4. Count the number of people at each sign. Then organize this information by completing the data table below. Correlation of PTC Tasting and Liking Broccoli Variables Number of People PTC Tasters Non-tasters Like uncooked broccoli Do not like uncooked broccoli Like uncooked broccoli Do not like uncooked broccoli 5. Prepare a bar graph to illustrate the data in your data table. Label the vertical axis and the four bars. Title: Do NOT accept The Effect of PTC Taste on Liking Broccoli Number of People 5
6. What conclusions can you make based on the data in your data table? Student answers will vary depending on class data. 7. Do the results of your experiment support your hypothesis? Explain why or why not. Student answers will vary depending on their hypothesis and the class data. 8. Describe one change you could make to improve this experiment. Increase the number of students (sample size). 6
Part 5: PTC Inheritance There are two versions of the PTC tasting gene a dominant T allele and a recessive t allele. The T allele codes for the production of a taste receptor on the tongue that works. The t allele codes for the production of a taste receptor that does not work. Persons with one or two dominant T alleles (TT or Tt genotypes) have the "taster" phenotype. Persons with two recessive alleles (tt genotype) have the "non-taster" phenotype. The "non-taster" phenotype is therefore described as a "recessive" trait. The pedigree below shows the pattern of inheritance for PTC tasting in for a family. Squares are males, circles are females. People with the "Taster" phenotype are indicated by white symbols People with the "Non-taster phenotype are indicated by gray symbols tt T? 1 2 Tt Tt Tt T? 3 4 5 6 T? tt T? T? 7 8 9 10 1. Write the genotype inside the symbol for each person in the pedigree. If you cannot be certain whether the person is TT or Tt, write T?. 2. Write the numbers (shown under each symbol) for each of the individuals who: Are homozygous recessive 1 and 8 Are homozygous dominant none for certain Are heterozygous 3, 4, and 5 Homozygous = has two identical alleles Heterozygous = has two different alleles Could be either homozygous dominant or heterozygous 2, 6, 7, 9, and 10 7
Genetic testing can be used to determine a person s genotype for the PTC tasting gene. A scientist: Collected DNA samples from each member of the family (1-10) shown in the pedigree on the previous page. Used a special laboratory technique called PCR (polymerase chain reaction) to make copies of the PTC alleles for each family member. Placed these PTC gene allele copies into different wells (1-10) of an electrophoresis gel. Put the gel into a gel electrophoresis chamber and turned on the electrical current which caused the small gene pieces moved further in the electrophoresis gel than the large gene pieces. Your lab kit contains a simulated paper gel that is like the one that the scientist made. The gene pieces (which are made of DNA) are not visible unless the gel is stained. You will need to stain the DNA (the PTC gene alleles) to make it visible. 1. Add just enough water to the plate to completely cover the bottom of the white plate or tray. 2. Add the entire contents of the tube of DNA Stain to the water in the plate. This stain will bind to the DNA pieces (PTC genes) and turn them pink. 3. Use a stirrer to mix the stain until it is completely dissolved. 4. Place the simulated paper gel into the stain in the plate. 5. Draw the pattern of bands on the drawing of the gel below. Direction of DNA Movement 1 2 3 4 5 6 7 8 9 10 Wells with DNA samples 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 0O 8
Remember that the gene for non-taster is recessive. That means that individuals 1 and 8 on the pedigree and the simulated gel are homozygous recessive and have the tt genotype. 6. Shorter DNA pieces move further in the gel than longer DNA pieces. According to the gel, which piece is shorter the T allele (taster) or the t allele (non-taster)? Circle one. Hint: Refer to both the pedigree and the electrophoresis gel. T = taster allele t = non-taster allele Support your answer with information from the electrophoresis gel and the pedigree. The T allele for PTC tasting is shorter because it goes further in the gel than the T allele. 7. According to the gel, which individuals (1-10) are homozygous dominant for the PTC gene? 2, 6 and 9 Support your answer with information from the electrophoresis gel and the pedigree. These individuals have only one short DNA band. 8. According to the gel, which individuals (1-10) are heterozygous for the PTC gene? 3, 4, 5, 7, 10 Support your answer with information from the electrophoresis gel and the pedigree. These individuals have two different kinds of DNA bands both long and short. 9. If couple 5 and 6 have another child, what is the probability that the child could be a nontaster? Support your answer with an explanation or a Punnett square diagram. 0 % probability because individual 6 is TT or homozygous dominant. All of this person s children will get at least one T gene so all will be tasters. 9
10. If person 8 and a person who is heterozygous for the PTC trait have a child, what is the probability that the child could be a non-taster? Support your answer with an explanation or a Punnett square diagram. 50 % probability because individual 8 is tt or homozygous recessive and could only contribute a t allele. The person who is heterozygous could contribute either a T allele or a little t allele. Want to learn more about the genetics of PTC tasting? Explore Online Mendelian Inheritance in Man: PTC http://omim.org/entry/171200. 10
Part 6: PTC Population Genetics and Evolution Scientists who study population genetics and evolution have proposed that evolution will not occur in a population when these seven conditions are met: Mutations are not occurring Natural selection is not occurring The population is infinitely large All members of the population breed All mating is totally random All members produce the same number of offspring There is no migration into or out of the population However, because it is highly unlikely that any of these seven conditions will happen in the real world, evolution is the inevitable result. In other words, evolution will occur when any of the seven conditions are not met. Describe a fictitious scenario or write a science fiction story that explains the rapid evolution of a human population with ONLY homozygous dominant (TT) tasters. Student answers will vary but may include: Non-taster genes are more likely to mutate to form taster genes Natural selection--people who are tasters survive better than non-tasters Natural selection People who are tasters are more likely to reproduce The population is very small genetic drift Tasters more likely to breed than non-tasters Tasters prefer breeding with other tasters or do not breed with non-tasters Tasters have more offspring than non-tasters Tasters chase non-tasters out of the area Or, a creative student might suggest gene testing and only allowing homozygous tasters to reproduce. 11
Part 7: PTC Population Genetics Students counted the number of tasters and non-tasters in the biology classes at a school. They found that there were 128 tasters and 72 non-tasters in these classes. Frequency of trait = number of student with trait/total number of students 1. Calculate the frequency of tasters in the classes = 128 / 200 = 0.64 2. Calculate the frequency of non-tasters in the classes = 72 / 200 = 0.36 Hardy-Weinberg equations: p 2 + 2pq + q 2 = 1 p + q = 1 p = frequency of dominant allele q = frequency of recessive allele p 2 = frequency of homozygous dominant individuals q 2 = frequency of homozygous recessive individuals 2pq = frequency of heterozygous individuals 3. What is q 2 (the frequency of non-tasters in the classes)? 0.36 Explain how you arrived at your answer. This was already calculated for question 2. 4. What is q (the frequency of the non-taster allele in the classes)? 0.6 Explain how you arrived at your answer. q 2 =.36 the square root of q 2 = q q = the frequency of homozygous recessive people (non-tasters)=.6 12
5. What is p (the frequency of the taster allele in the classes)? 0.4 Explain how you arrived at your answer. p + q = 1 p = 1-q = 1 -.6 = 0.4 6. What is p 2 (the frequency of homozygous tasters in the classes)? 0.16 Explain how you arrived at your answer. p 2 = 0.4 2 = 0.16 7. What is 2pq (the frequency of heterozygous tasters in the classes)? 0.48 Explain how you arrived at your answer. 2pq =2 X 0.4 X 0.6= 0.48 8. What is p 2 + 2pq (the frequency of tasters in the classes)? 0.64 Explain how you arrived at your answer. 2pq + p 2 = 0.16 +0.48 = 0.64 9. Explain how is it possible to have more tasters than non-tasters in the classes when the allele frequency for non-tasters is higher than the allele frequency for tasters. Because some of the non-taster alleles are hidden in the heterozygotes or alternative explanations. 13