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Analysis Questions Allele Frequencies And Sickle Cell Anemia Lab Pdf

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Sickle cell disease is a group of disorders that affects hemoglobin , the molecule in red blood cells that delivers oxygen to cells throughout the body.

Sickle cell disease

Mike U. Smith, John T. The American Biology Teacher 1 October ; 77 8 : — This article is focused on the meaning of H-W eq and its application, rather than mathematical manipulation.

Typical textbook problems are critiqued, and a model problem is presented. One of the more difficult topics for introductory biology students to understand and for teachers to teach is the Hardy-Weinberg equilibrium H-W eq principle. More problematic than lack of manipulative skill 1 is the difficulty of understanding why the principle is true and understanding how the principle applies to specific populations or, more importantly, the value of its application.

The H-W eq principle is, of course, the cornerstone of introductory population genetics and is therefore an important part of understanding evolution, as is recognized in most science standards. Students who demonstrate understanding can use mathematical representations to support explanations of how natural selection may lead to increases and decreases of specific traits in populations over time. Given its importance and the well-recognized difficulty in teaching and learning it in accord with these standards, we here explain basic concepts of H-W eq, emphasizing distinctions that are sometimes ignored at the cost of coherent understanding.

In general, a system is said to be in equilibrium if all competing influences are balanced. In the body, for example, we speak of homeostasis as the ability to maintain the internal equilibrium regardless of changes in the environment e.

Definition: A population is in Hardy-Weinberg equilibrium if the genotype frequencies and allele frequencies are the same in each generation at birth. Consider the simplest situation of a monogenic Mendelian trait: a pair of alleles, one dominant A and the other recessive a, within a population of n individuals. These would be the allele frequencies if there are aa and AA individuals or aa, Aa, and AA. If a finite population is at H-W eq, however, both the genotype and the allele frequencies will be essentially the same in subsequent generations.

Below, we will demonstrate that a population is in H-W eq if the following conditions hold with respect to a particular gene :. Fully random mating: each pair from the population is equally likely to breed. This is not the case when females of a species often prefer males with certain traits. Examples originally identified by Darwin include peacock feather displays, antlers in deer, and the manes of lions. Observation 1. As long as a population satisfies biological conditions 1—5, the allele frequencies p and q are the same in each generation.

Why is this so? Conditions 1 and 2 guarantee that there is no change in the allele frequencies between the birth and maturity of the next generation; there are no unaccounted forces that would change the allele frequencies i. Conditions 3 and 4 guarantee that at birth, the pool of alleles in the next generation is the same as in the current generation; mating just reshuffles the alleles; the allele frequencies remain the same.

The population needs to be infinite to guarantee that the frequencies remain exactly p and q. The probabilities p and q represent the averages over many trials, so it will only be approximate in a particular trial on a finite population. Likewise, the other assumptions are rarely, if ever, true of a given population e.

Thus, H-W eq is largely a theoretical state, like a frictionless plane, an absolute vacuum, or travel at the speed of light. As with those concepts in physics, it nevertheless plays a fundamental conceptual role in biology and is a valuable tool for understanding evolution. Also, for evolution-neutral mutations, the population is often close enough to equilibrium to provide a tool for comparing their frequencies against the frequencies of linked genes of interest to determine how close the latter are to H-W eq Chen, Building on the work of other biologists and mathematicians, in Wilhelm Weinberg — , a German obstetrician-gynecologist, and G.

Hardy — , a leading mathematician of his day, independently demonstrated the conditions required for genotype equilibrium Figure 1. In a famous lecture earlier that same year, R. Punnett Figure 2 had combined Mendelian genetics with natural selection Edwards, After the talk, Udny Yule Figure 3 , one of the founders of modern statistics, asked whether a dominant—recessive allele pair would not eventually achieve a ratio Yule, Punnett's response, though not entirely apt, was a suggestion that a dominant allele should eventually drive the recessive out which is not the case.

Punnett later asked his friend Hardy about this question, prompting the analysis we now describe. Let A and a represent the two possible alleles of a simple Mendelian trait; let p and q represent the frequencies of A and a, respectively, in the parent generation. Hardy and Weinberg argued that if every pair of individuals is equally likely to mate condition 6 , then the frequencies of the three possible genotypes at birth can be determined by thinking of a Punnett square but labeling the rows and columns with allele frequencies instead of alleles Figure 4.

This Punnett square demonstrates the crucial H-W insight : Under fully random mating, the frequency of AA homozygotes in the next generation is p 2 , that of heterozygotes is 2pq, and that of aa homozygotes is q 2. Now we can deduce the Hardy-Weinberg Equilibrium Principle: Consider a population satisfying biological conditions 1—6. If, in a certain generation, the allele frequencies are p and q and the genotype frequencies are p 2 , 2pq, q 2 , then both the genotype and allele frequencies remain the same for as many generations as conditions 1—6 continue to hold.

Here is why this principle holds. When a population satisfies conditions 1—5, Observation 1 ensures that allele frequencies will remain unchanged in every succeeding generation that satisfies those conditions. Applying condition 6 and the crucial H-W insight, in each generation, after, 6 the genotype frequencies are p 2 , 2pq, and q 2. This is the genius of the H-W principle : after one generation of fully random mating, both the genotype and allele frequencies are fixed until one of conditions 1—6 is violated.

To summarize, 1 allele frequencies can always be computed from the genotype frequencies in the same generation if all genotypes can be identified , but not vice versa; and 2 if the population is in H-W equilibrium, genotype frequencies in the current or the next generation can be computed from the current allele frequencies.

These two equations are widely used in biology teaching, but all too often they are used as a mathematics exercise that does not promote understanding. Equation 1 is true for any monogenic Mendelian trait because there are only two outcomes. Squaring Equation 1 yields. So, Equation 2 simply follows mathematically from Equation 1. There is no assumption about random mating and no other biological assumption in the step from Equation 1 to Equation 2.

Interestingly, using only these formulae, we can determine whether H-W eq exists in a single generation of a population by determining whether the genotype distribution matches that predicted from the allele distribution, but this requires that both the allele frequencies and the genotype frequencies are known. Hardy provided an ingenious way to determine whether H-W eq exists in a single generation, given only the genotype frequencies.

For a short account of Hardy's proof in modern language accessible to advanced students, along with several other proofs of the H-W principle, see Baldwin, The following textbook problems are built on the assumption that, if a population is in H-W eq which is often a dubious assumption , then it is possible to calculate the allele frequencies from the frequency of the homozygous recessives which can be found by observation.

The data below demonstrate the frequency of tasters and non-tasters in an isolated population at H-W eq. The allele for non-tasters is recessive. How many of the tasters in the population are heterozygous for tasting? An acceptable answer would be any number in the range of —, depending on how the students rounded the variables in the H-W equation. This is a standard H-W eq problem. This yields the number of heterozygotes as 0.

You sampled individuals and determined that could detect the bitter taste of PTC and 65 could not. Calculate the following frequencies. The frequency of the recessive allele. The frequency of the dominant allele. The frequency of the heterozygous individuals.

Both problems are focused on making calculations that students can do without understanding what H-W eq is. In Problem 1, H-W eq is explicitly assumed so that the problem is technically correct.

But it doesn't say what chemical was being tasted presumably PTC , so it doesn't ask students whether H-W eq conditions could be met for this trait. And students can solve Problem 2 only by assuming H-W eq, which is not justified. The ability to taste PTC phenylthiocarbamide , a bitter substance that cannot be tasted by some individuals, is frequently used in H-W eq problems, likely because it is assumed to be selected neither for nor against, given that PTC does not occur in nature.

Thus, the student is expected to deduce or more likely assume that the H-W conditions apply. Teachers and textbooks, however, rarely make this reasoning explicit, leaving students with the misperception that understanding PTC tasting is just a game or a puzzle that likely seems unimportant to them because it doesn't relate to their daily lives.

In fact, recent research has shown that the ability to taste PTC is strongly correlated with the ability to taste other bitter substances that do occur naturally, many of which are toxins.

Thus, it seems likely that the ability to taste bitter substances such as PTC is positively selected for. Sickle-cell anemia is an interesting genetic disease. Normal homozygous individuals SS have normal blood cells that are easily infected with the malarial parasite. Thus, many of these individuals become very ill from the parasite and many die. Individuals homozygous for the sickle-cell trait ss have red blood cells that readily collapse when deoxygenated.

Although malaria cannot grow in these red blood cells, individuals often die because of the genetic defect. However, individuals with the heterozygous condition Ss have some sickling of red blood cells, but generally not enough to cause mortality. Thus, heterozygotes tend to survive better than either of the homozygous conditions. The solution above, which assumes H-W eq and that natural selection is not occurring with regard to this gene, contradicts the statement of the problem, which notes selective pressures for one and against another of two blood-cell phenotypes.

Justify your answer. Because the members of the population that contract sickle-cell because they are homozygous recessive will likely die before reproducing, the frequency of alleles in the population is not stable. There is natural selection taking place. Although this problem instructs students to use the H-W equations, again the known effects of natural selection at this locus mean that H-W eq is impossible.

The problem therefore asks for what is, in fact, a meaningless calculation. Then it asks students to answer a question that demonstrates that the computation was meaningless but does not ask them to recognize that it was meaningless! The H-W eq-based frequencies are irrelevant.

A second issue that arises in problems about sickle-cell anemia is that two opposing selective pressures are at work — a positive selection for heterozygosity and a negative selection against affected homozygotes. Such a situation can produce balanced polymorphism equilibrium but not H-W eq, because the calculation to keep the genotype constant requires further parameters.

This makes sickle-cell anemia a poor choice for the context of most introductory-level H-W problems. A more conceptual shortcoming of all these problems is that there is no readily apparent value to the calculation. Who cares? How might such a calculation be used to answer a research question or be applied to a case that is at least interesting to the students?

When students see the utility of such calculations or find the case interesting, they are more likely to engage in this learning.

Sickle cell disease

Sickle cell disease SCD is a group of blood disorders typically inherited from a person's parents. The care of people with sickle cell disease may include infection prevention with vaccination and antibiotics , high fluid intake, folic acid supplementation, and pain medication. As of , about 4. Herrick in Beet and J. Signs of sickle cell disease usually begin in early childhood. The severity of symptoms can vary from person to person.

Mike U. Smith, John T. The American Biology Teacher 1 October ; 77 8 : — This article is focused on the meaning of H-W eq and its application, rather than mathematical manipulation. Typical textbook problems are critiqued, and a model problem is presented. One of the more difficult topics for introductory biology students to understand and for teachers to teach is the Hardy-Weinberg equilibrium H-W eq principle.

Robert O. Opoka, Christopher M. Ndugwa, Teresa S. Latham, Adam Lane, Heather A. Hume, Phillip Kasirye, James S. Hodges, Russell E. Ware, Chandy C.

sickle cell disease) and individuals who are homozygous for the normal hemoglobin E. In the absence of malaria, there is selection against the sickle cell allele. The film may be viewed in its entirety or paused at specific points to review content with students. questions pertaining to the information provided in the film.

Sickle cell disease

Makani, S. Ofori-Acquah, O. Nnodu, A. Wonkam, K.

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Ту, что работает в столовой. Бринкерхофф почувствовал, как его лицо заливается краской. Двадцатисемилетняя Кармен Хуэрта была поваром-кондитером в столовой АН Б. Бринкерхофф провел с ней наедине несколько приятных и, как ему казалось, тайных встреч в кладовке.

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