Hardy Weinberg Analysis worksheet

Lab 9. Hardy Weinberg Analysis The Hardy-Weinberg theorem was named after G. H. Hardy and Wilhelm Weinberg after they demonstrated this mathematic principle first when looking at genetics. This theorem states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences once a population is in “HardyWeinberg Equilibrium.77 The equation is comparing 2 alleles in the gene pool, and it is p2 + 2pq + q2 = 1 .Where homozygous dominant genotype is represented by p2, homozygous recessive is q2, and heterozygous is 2pq. But, this equation can only accurately calculate the genotypic frequencies if the population follows certain assumptions: 1. Natural selection is not occurring, 2. Neither mutation nor migration is introducing new alleles into the population, 3. Population size is infinite, 4. Random mating occurs. The Hardy-Weinberg theorem constitutes a null model for the discipline of population genetics, and is fundamental to the study of evolution. After PTC was discovered the geneticists determined that the ability to taste this compound was influenced by genetics. Specifically, the PTC gene, TAS2R38, was discovered in 2003. There are two common forms of this gene, and multiple rare forms. One of the common forms is the tasting allele, and the other is the non-tasting allele. But since there are two alleles for every gene, many have different forms of tasting. For example, someone with a heterozygous gene could taste the bitterness, but not to the extent of the homozygous dominant (tasting) gene. Although this compound is not found in nature, but this ability to taste correlates to the ability to taste other bitter substances that do occur naturally. In fact, studies have shown that PTC tasters may be more sensitive to foods in the cabbage family, like broccoli. Or even why some are more susceptible to smoking than others. Lab Exercise Complete both the case study and the Hardy-Weinberg/PTC taste activity. Answer all questions including the pre-lab and post-lab questions. NAME Student Guide DATE Carolina BioKits™ Human Genetics of Taste Background You may wonder why we experience an unpleasant, bitter taste when eating certain foods. Does a pleasing taste, evolutionarily speaking, mean that the food is good for us and that unpleasant tasting food is bad? This question is difficult to answer for many reasons, including the fact that the development of new foods has greatly outpaced the speed of human evolution. Another question arises; which came first, the ability to taste bitter molecules or the prevalence of bitter molecules in plants? It is likely that plants have adapted to produce bitter tasting molecules that prevent animals from eating them. If an animal finds the taste of a plant displeasing, then it is unlikely to return to that plant for a second meal. The same general rules apply to toxins in plants. If an animal becomes sick from eating a particular plant, it is also unlikely to return for more. One explanation for the development of bitter taste perception is the avoidance of these poisonous plants; often, toxic compounds in plants are bitter. Selection pressure for the ability to distinguish between toxic and nontoxic plants may have influenced the allele frequencies for many of the human genes known to code for bitter taste receptors. How do we know if something is bitter? The act of tasting something bitter can be summarized with a two-step process. First a molecule binds to a specific receptor protein on the tongue. The binding of the molecule to the receptor generates a signal that is sent from the cell to the brain, allowing you to perceive a bitter taste sensation. The receptors in your taste buds are so specific that they can detect even slight differences between molecules. The binding strength between the taste molecule and the receptor is determined by the shape of the receptor; and a tightly bound molecule produces a strong taste sensation. The shape of the receptor is influenced by genetic factors. One type of taste paper used in the following activities contains phenylthiocarbamide (PTC). PTC is nontoxic, but chemically it resembles alkaloid compounds found in some poisonous plants. The ability to taste PTC is controlled by the “PTC gene,” TAS2R38. There are two common forms (or alleles) of the PTC gene, and at least five rare forms. Each person possesses two copies of the gene, one inherited from the mother and one from the father. Specific combinations of alleles determine whether a person finds PTC extremely bitter; slightly bitter, or tasteless. In this activity, we will focus on the two common alleles, designated T and t. The allele that confers the ability to taste PTC is considered dominant. Even among PTC tasters the ability to taste the chemical depends on its concentration in solution. The length of time the PTC is exposed to the tongue and the degree of mixture with the taster’s saliva also contribute to taste sensation and strength. In these activities, you will investigate the appearance of genetically controlled traits in a sample population. You will test your ability and the ability of your classmates to taste various substances. Afterward, you will calculate the genetic frequencies of your class for the TAS2R38 alleles. You will use the following taste papers during this exploration of human genetics of taste. Control Taste Paper This paper serves as a control. Any taste sensation that you have from this untreated paper is a result of the paper itself and is not the result of any chemicals added to the paper. Thiourea Taste Paper Thiourea is the name for a molecule with -N-C=S within its structure. Many chemicals with this structure will bind to the bitter taste receptor regulated by the TAS2R38 alleles. Tasters perceive thiourea as bitter. PTC Taste Paper Phenylthiocarbamide (PTC) is a type of thiourea. Tasters perceive PTC as bitter. Sodium Benzoate Taste Paper It is not fully known how the ability to taste sodium benzoate is regulated. The most common taste reactions to sodium benzoate are sweet, salty, bitter, and tasteless. ©2012 Carolina Biological Supply Company/Printed in USA. CAR@UNA S-1 Hardy-Weinberg Analysis Carolina BioKits™: Human Genetics of Taste Student Guide Genetic studies are often carried out with controlled mating of genetically uniform stocks of plants and animals. However, selective breeding among humans is quite rare, so much of what we know about human heredity is based on frequency analysis. You will use a mathematical formula known as the Hardy-Weinberg equation to estimate the frequency of PTC tasting alleles present in your classroom population. For this activity, we will assume that there are only two alleles for the tasting trait, T (dominant) and t (recessive). (As noted, there are at least five forms that are relatively rare.) The equation p + q = 1 indicates that the proportion of p alleles plus the proportion of q alleles is 1. However; this gives only the allele frequencies in a population. To determine the genotype frequencies, the equation is expanded by squaring both sides to get p2 + 2pq + q2 = 1. In this equation, p2 represents the proportion of individuals with a homozygous dominant genotype, 2pq represents the proportion of individuals with a heterozygous genotype, and q2 represents the proportion of individuals with a homozygous recessive genotype. Hardy-Weinberg Equation p2 十 2pq + q2 = 1 where p2 represents the frequency of TT 2pq represents the frequency of Tt q2 represents the frequency of tt The Hardy-Weinberg equation is dependent on several conditions. If these conditions are met, the population’s allelic and genotypic frequencies will remain statistically constant over time, a condition referred to as Hardy-Weinberg equilibrium. These conditions are not entirely met in your classroom, but we can still use the equation to predict approximate genotypic frequencies. The conditions for Hardy-Weinberg equilibrium are as follows: 1. The population size is infinite or very large. 2. Mating within the population is random; i.e.r there is no mating preference for any specific phenotype over another. 3. There is no mutation occurring in the population. 4. There is no exchange of genetic information with other populations. There is no immigration or emigration of individuals. 5. There is no selection for one phenotype over another. All phenotypes have an equal chance of surviving and passing on their genes. ©2012 Carolina Biological Supply Company/Printed in USA. CAR@UHA S-2 Carolina BioKits™: Human Genetics of Taste Student Guide Pre-laboratory Questions 1. If the ability to taste thiourea were controlled by two alleles, one dominant (H) and one recessive (h), would there be any way to distinguish heterozygous (Hh) and homozygous dominant (HH) individuals without mating them or performing DNA analysis? Explain your answer. 2. Suppose that two heterozygous thiourea tasters have children. Use a Punnett square to predict the genotypes of their offspring. Use the notation H for the dominant (taster) allele and h for the recessive (nontaster) allele. FEMALE phenotypic ratio: genotypic ratio: Materials control taste paper PTC taste paper sodium benzoate taste paper thiourea taste paper paper towel Procedure 1. Using a pencil, draw lines to divide the paper towel into four separate regions. Label the four quadrants as follows. a. control taste paper b. PTC paper c. sodium benzoate taste paper d. thiourea taste paper 2. Start with the control taste paper. Place the untouched end of the control taste paper on your tongue. Move it around to be sure the sample mixes with your saliva and contacts numerous taste receptors. The control paper should be tasteless. 3. Discard the taste paper. ©2012 Carolina Biological Supply Company/Printed in USA. CAR@UNff s-3 Carolina BioKits™: Human Genetics of Taste Student Guide Data Table PTC Thiourea Sodium Benzoate Tasters Tasters Nontasters Tasters Nontasters Nontasters Sweet Salty Bitter Other Personal Class Totals % Class 4. Repeat the taste test for the other three taste papers. Each time, record your taste response in the Data Table. 5. Complete the Data Table by filling in the test results for the entire class population. 6. Using your class total population data, perform a Hardy-Weinberg Analysis to determine the allele frequency and genotype frequency, and the number of individuals possessing each genotype. 7. Answer the Laboratory Questions. Laboratory Questions 1. Which conditions of Hardy-Weinberg equilibrium might not have been met in this simulation? 2. If the ability to taste PTC and similar molecules were a selective advantage, meaning that PTC tasters were more likely to survive and produce offspring, would you expect the frequency of the dominant allele to change? 3. If the ability to taste sodium benzoate is genetically inherited, do you think that the ability to taste it is determined by two alleles, or more than two? Explain your answer. ©2012 Carolina Biological Supply Company/Printed in USA. CAR@UHA S-4 Case Study： Millie is a student at Pme Forest High School. Her high school science class learned about PTC tasting when her class learned about traits. As it turned out, she was not a taster. Millie then decided to get some PTC paper and have her family do the taste test. Surprisingly, everyone in her family is a taster; her mother, her father, both her brothers, even her grandparents, aunt, and uncle. Millie was quite perplexed. Is it possible that Millie cannot taste PTC when everyone else in her family can taste? Create a pedigree for Millie’s family starting with her grandparents. Note: Her aunt is her dad5s sister. Her uncle is her mom’s brother. Millie decided to take DNA samples from her family and run this through gel electrophoresis. This is what her gel looked like. D DDDDDDD Millie’s Brother (B2) Millie’s Brother (Bl) Millie (M) Millie’s Dad (D) Millie’s Mom (M) Since Millie is a nontaster, and her mom and dad are tasters, do their DNA patterns differ, if so how? What can you conclude from this data? Based on the DNA gel, can you assign which DNA band corresponds to the normal PTC gene (which you must have to taste PTC)? Which DNA band corresponds to the mutant PTC gene? Now seeing the DNA fragments of her parents, can you predict which PTC gene Millie got from her mom and dad? Show a Punnett square to help your reasoning. What percent chance of Millie’s parents having another baby who cannot taste PTC? How about one that is like B1, homozygous for the PTC taster gene? Or like B2, heterozygous?