1 Quiz 02 Results Name Quiz 02a Attempt Score 1.05215 Attempts 162 (Total of 166 attempts for this assessment) Graded Attempts 162 Question 1: Multiple Choice Average Score 0.31481 points At the start of an experiment, a 1 M aqueous solution of NaCl is separated from an equal volume of a 1 M aqueous solution of glucose by a membrane that is permeable to water but not to NaCl or glucose. After a while, you would expect the volume of the glucose solution to have ... Correct Percent Answered stayed the same. 30.864% increased. 3.704% decreased. 62.963% increased, then decreased. 0.617% decreased, then increased. 1.852% Unanswered 0% Question 2: Multiple Choice Average Score 0.30864 points This diagram (Sweat Gland.jpg) shows a typical sweat gland in human skin. The secretory coil fills the lumen with a NaCl solution that is isotonic with the blood plasma. As this solution moves upward through the reabsorptive duct, NaCl is reabsorbed into the blood, such that the sweat that is secreted onto the skin surface is hypotonic. Assume that along its entire length, the walls of the sweat gland consist of a single layer of epithelial cells joined by tight junctions. Which one of the following statements about this system is false? Correct Percent Answered One would not expect to find aquaporin channels in the membranes of reabsorptive epithelial cells. 7.407% One would expect intracellular [Na+] to be lower than extracellular [Na+] in both the secretory and reabsorptive epithelial cells. 28.395% CFTR Cl- channels are located in the apical membranes of secretory epithelial cells. 2.469% In reabsorptive epithelial cells, one would expect to find Na+/K+ pumps only in the apical membrane. 61.728% Unanswered 0% Question 3: Multiple Choice Average Score 0.42901 points2 Cystic fibrosis (CF) is an autosomal recessive genetic disease. For almost all people born with the disease, both parents were carriers, i.e., both had a normal (N) and a mutant (n) gene for the CFTR. As shown in the attached diagram (CF Punnett Square.gif), about one-quarter of the children of such parents would have CF. Currently, the median lifespan of a person with CF is about 33 years. Before the recent advent of modern medicine, however, most people with CF would have died as children, long before they reached reproductive age. So, one might imagine that natural selection would have eliminated the mutant gene from human populations long ago. However, the gene is fairly common, with 1 out of 25 persons of European descent being heterozygous carriers (Nn). This has led to speculation that the mutant gene persists in human populations because, in the heterozygous condition, it confers some selective advantage. Which one of the following is the most likely candidate for such a selective advantage? NOTE: This question focuses on the most widely accepted hypothesis, disregarding recent evidence suggesting that the heterozygote condition may confer some protection from tuberculosis. Again, try to reason out the answer, rather than just looking it up. Correct Percent Answered The greater saltiness of sweat in the heterozygote would help rid the body of excess sodium. 0.617% Thickened mucus in lung airways of the heterozygote would trap more inhaled pathogens, reducing respiratory infections. 12.963% The heterozygous condition would decrease the risk of fatal dehydration in cases of bacterial infection of the intestine that causes severe diarrhea. 85.802% In the heterozygote, lower volumes of alkaline fluid secretion in the pancreas would render intestinal contents slightly more acidic, thus enhancing digestion of food. 0.617% Unanswered 0% Question 4: Essay Average Score 0 points Think about the evolutionary issues raised in question #3 of Quiz 2a. Speculate on why the incidence of heterozygous carriers is much lower in African Americans (1 out of 60) than in Americans of European ancestry (1 out of 25). For an enhanced learning experience, feel free to include recent findings on protection from tuberculosis if you wish, but it is not necessary. The attached paper (Poolman on Tuberculosis & CF.pdf) will give you some background on the tuberculosis story (start with the Introduction.) Given Answers The cystic fibrosis mutation causes a defective CFTR ion channel that prevents chloride ions from leaving the cell and being secreted into the lumen. Most noticeably, this causes abnormally thick mucus to be secreted. Cystic fibrosis, before modern medicine, often caused death before reproductive age, and so from an evolutionary perspective, the mutation should have been lost. The survival of the cystic fibrosis mutation in heterozygous carriers raises the question that the gene confers a selective advantage. In certain diseases such as cholera, the secretion of Cl-, Na+, other ions, and water into the lumen causes diahrrea that can eventually cause the death of the host. With the CFTR mutation, chloride ions are not able to be secreted and diahrrea is less likely to occur. So, it is thought that the selective advantage of the cystic fibrosis mutation is to reduce the efficacy of deadly diahrrea-causing diseases such as cholera. Most of the past cholera pandemics affected Europe and Asia, with only one Pandemic reaching into Africa. So, the rate of preservation of the "beneficial" CF mutation would be higher in European Americans compared to African Americans because the European ancestors were more exposed to cholera than the African. It is likely that the Europeans more often had to fight against dehydration where the Africans did not. Since you do die at a young age if you are a carrier, there needs to be an advantage since you are unlikely to3 increase fitness. This is the same with many things since the body puts the immediate survival over long term survival. The incidence of heterozygous carriers is probably much lower in African Americans in in Americans of European ancestry because the "adavantage" conferred by heterozygositic is context and environmentaly dependent. In African Americans, it probably was the case that there was no selective pressure for the allele, and thus overtime, the incidence of heterozygotes decrease. It is speculated that heterozygotes may have protection against tuberculosis, and this is the selective pressure thath as maintained the allele in relative high proportions in the European population. Tuberculosis seems like