Argument Analysis Worksheet
Author (s): Jared Diamond Title: Curse and Blessing of the Ghetto Topic: How evolutionary theory can help us understand the high rate of TS among the Ashkenazim Question/Problem: “What is it about this one group of people that produces such an extraordinarily high risk of this disease” (Diamond, 2013, p. 201)? “Can history help explain why the Tay-Sachs gene in particular is so much more common in Ashkenazim than in their non-Jewish neighbors or in other Jews” (Diamond, 2013, p. 202)? Thesis: “[T]he accumulation [of GM2 ganglioside] provides heterozygotes with some protection against TB, and that that’s why the genes for Tay-Sachs, Gaucher’s, and Niemann-Pick disease reached high frequencies in the Ashkenazim” (Diamond, 2013, p. 204). “I’d say that the evidence points to some selective advantage of Tay-Sachs heterozygotes among the Ashkenazim, and that TB resistance is the only plausible hypothesis yet proposed” (Diamond, 2013, p. 204-5). Evidence in support of the thesis: Evidence to Support Thesis: Tay-Sachs is caused by a build-up of the fatty acid GM2 ganglioside in nerve cells. In most individuals, the enzyme hexosaminidase A breaks down GM2 ganglioside but individuals with Tay-Sachs have two faulty copies of the hexosaminidase A gene, meaning that they do not produce enough of this enzyme to prevent build-up of the fatty acid. Evidence to Support Thesis: Even though evolutionary theory predicts that genetic diseases will be selected out of a population, some common genetic diseases may persist because they have some advantages. “They kill or impair individuals who inherit two copies of the faulty gene, but they help those who receive only one defective gene by protecting them against other diseases” (Diamond, 2013, p. 204). Sickle-cell anemia is the best understood example of this. Sickle-cell anemia is most common in regions affected by malaria; individuals with just one copy of the mutant hemoglobin gene have greater resistance to malaria, which leads to selection for heterozygotes and increases the likelihood of two heterozygotes producing homozygous recessive offspring. “Natural selection sustains such genes because more heterozygotes than normal individuals survive to pass on their genes, and those extra gene copies offset the copies lost through the deaths of homozygotes” (p. 204). Evidence to Support Thesis: The results of a study from 1972 asking US Ashkenazic parents of TS children what their parents died of revealed that most died of usual causes of death and only 1 of 306 died of TB. This “suggested that Tay-Sachs heterozygotes might somehow have been protected against TB” (Diamond, 2013, p. 204). Evidence to Support Thesis: The history of the Jewish diaspora plays an important role. For nearly 2000 years, Jews have been migrating through much of the Middle East and Mediterranean, as well as into Eastern and Western Europe during the Middle Ages and most recently into the United States. The degree of anti-Semitism Jews experienced over this time varied but Jews were more often restricted to urban environments than their non-Jewish neighbors.
Evidence to Support Thesis: TB is most common in urban settings. People who live in cities have been historically more likely to contract TB. Tay-Sachs occurs at its highest rates in areas where TB has also occurred at high rates. This suggests a relationship between the two conditions. Evidence to Support Thesis: Ashkenazi Jews also have a higher rate of other diseases associated with excess build-up of GM2 ganglioside, Gaucher’s and Niemann-Pick, which suggests there has been strong selection pressure for greater fat accumulation among the Ashkenazim. Evidence to Support Thesis: Because of the unique historical conditions the Jews have experienced in Eastern Europe (isolation in urban centers and greater exposure to TB), it appears that there has been active selection for individuals who are heterozygous for hexosaminidase A production, i.e. they carry one good hexosaminidase A allele that produces enough of the enzyme to prevent too much accumulation of fatty acids in neurons but the faulty hexosaminidase A allele means they produce less of the enzyme than people with 2 copies of the good gene, which appears to provide some protection from TB.
Does the author engage any counterarguments or alternative explanations? If so, state the counterargument(s) and evidence used to support it. Diamond engages 3 clear counterarguments and refutes them.
1. A higher mutation rate among the Ashkenazim is responsible for the high rate of TS. New copies of the faulty hexoA gene appear due to mutation at a rate faster than existing copies disappear with TS deaths. While new copies do appear in all human populations, this is why the disease persists in all populations, it is a very rare mutation (only 1 in 400,000 births globally) and the 1 in 3,600 TS births among the Ashkenazim is dramatically higher than any natural mutation rate would produce.
2. The Ashkenazim interbred with some other population that had an uncommonly high rate of the disease. One author suggested that the Ashkenazim are not actually a Semitic people but are descendants of the Turkic Khazar tribe. There is no evidence to support this hypothesis.
3. The high rate of TS can be explained by founder effect and genetic drift. Perhaps a very small segment of an original population broke off to form the Ashkenazim and, by random chance, this group had a higher rate of this very rare allele. Over time, this allele would be very common in the population if they continued to have little gene flow with other populations. This explanation seems to be appropriate for the high rate of TS among the Pennsylvania Dutch but the genetic and population history of the Ashkenazim makes it unlikely genetic drift was at work. The Pennsylvania Dutch are a very small population descended from a very small population (just a few hundred). Founding populations of the Ashkenazim were significantly larger than this and were also very widespread, not geographically isolated as genetic drift requires.
What are the main themes from the course that this article illustrates? What was most interesting or compelling about this article to you? (3-5 sentences)
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