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. Our final example is a homework problem that addresses some of these issues.
People who are homozygous for have two copies of a certain base-pair deletion mutation in a gene known as CCR5 are known to be largely resistant to HIV infection. CCR5 is the main co-receptor molecule that allows the virus to attach to certain white blood cells and enter them, establishing an infection; Jones et al.
In a study of random Caucasians of childbearing age in the United States, individuals were found to be homozygotes free of this deletion Glass et al. Assume that the U. Caucasian population is at H-W equilibrium at this locus. Susan is a Caucasian American woman at increased risk of HIV infection because she has multiple sex partners. What is the probability that Susan has little reason to worry about HIV infection i.
What is the predicted frequency of U. How many heterozygotes would be expected in this sample? Before HIV appeared, would you have expected the population to have been at H-W equilibrium at this locus? Why or why not? State your assumptions. In the absence of effective HIV treatments, what would you expect to happen to the allele frequencies over time? How would you expect the allele frequencies to change over time once effective HIV treatment was in use? How would your answer change if the HIV treatment was effective only for people past child-bearing age?
This assumes that the deletion was selection-neutral. With the appearance of HIV and in the absence of effective treatment, we would expect that the frequency of people without the homozygous deletion p 2 and 2pq should decrease, shifting the population out of H-W eq. The advent of an effective treatment should move the allele frequency distribution back toward equilibrium.
A treatment that has an effect only after childbearing age would have effects on the allele distribution similar to no treatment because, for a mutation to have a natural selection effect, it must affect reproductive success. Put simply, a population is in H-W eq if the genotype frequencies are the same in each generation.
This equilibrium requires a set of conditions that ensure that there are no unaccounted forces that would change the allele frequencies. For such a population, then, the genotype frequencies in the current or the next generation can be computed from the current allele frequencies. Focusing on this level of understanding for students — as well as avoiding confusing misstatements and flawed problems — is the primary key to effective teaching about Hardy-Weinberg equilibrium.
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What Is Hardy-Weinberg Equilibrium? Requirements for Hardy-Weinberg Equilibrium. Five Example Problems. End Notes. Article Navigation. Research Article October 01 Smith ; Mike U. This Site. Google Scholar. John T. Baldwin John T. The American Biology Teacher 77 8 : — Get Permissions.
Cite Icon Cite. Natural selection is not occurring. Adzhubei, I. Predicting functional effect of human missense mutations using PolyPhen Chapter Alfonso, R.
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The mid p-value in exact tests for Hardy-Weinberg equilibrium. Haeussler, M. The UCSC genome browser database: update. Hamosh, A. Online mendelian inheritance in man OMIM , a knowledgebase of human genes and genetic disorders. Karczewski, K. Variation across , human exomes and genomes reveals the spectrum of loss-of-function intolerance across human protein-coding genes. Landrum, M. ClinVar: public archive of interpretations of clinically relevant variants.
Lek, M. Evolution Introduction. Life History Evolution. Mutations Are the Raw Materials of Evolution. Speciation: The Origin of New Species. Avian Egg Coloration and Visual Ecology. The Ecology of Avian Brood Parasitism. The Maintenance of Species Diversity.
Neutral Theory of Species Diversity. Population Genomics. Semelparity and Iteroparity. Geographic Mosaics of Coevolution. Comparative Genomics. Cybertaxonomy and Ecology. Ecological Opportunity: Trigger of Adaptive Radiation. Evidence for Meat-Eating by Early Humans. Resource Partitioning and Why It Matters. The Evolution of Aging. Citation: Andrews, C. Nature Education Knowledge 3 10 The Hardy-Weinberg theorem characterizes the distributions of genotype frequencies in populations that are not evolving, and is thus the fundamental null model for population genetics.
Aa Aa Aa. Basic Mendelian Genetics. The Hardy-Weinberg Equilibrium. Figure 3: Wilhelm Weinberg. Evolutionary Implications of the Hardy-Weinberg Theorem. Rare alleles are found primarily in heterozygotes, as they must be, given that q 2 is much smaller than 2 pq when q is near zero, and p 2 is much smaller than 2 pq when p is near zero.
The second point takes on particular significance if we consider the potential for natural selection to influence the frequencies of new mutations. If a population conforms to all other Hardy-Weinberg assumptions, selection will eventually fix an advantageous allele in the population such that all individuals are homozygous for that allele.
The initial increase in frequency of a rare, advantageous, dominant allele is more rapid than that of a rare, advantageous, recessive allele. A new dominant mutation, however, is immediately visible to natural selection because its effect on fitness is seen in heterozygotes. Thus, although Hardy demonstrated that dominance alone does not change allele frequencies at a locus, the dominance relationships among alleles can have substantial influence on evolutionary trajectories.
Figure 4: A plot of Hardy-Weinberg equilibrium genotype frequencies p to the 2, 2pq, q to the 2 as a function of allele frequencies p and q. The Hardy-Weinberg equilibrium can be disturbed by a number of forces, including mutations, natural selection, nonrandom mating, genetic drift, and gene flow. For instance, mutations disrupt the equilibrium of allele frequencies by introducing new alleles into a population. Similarly, natural selection and nonrandom mating disrupt the Hardy-Weinberg equilibrium because they result in changes in gene frequencies.
This occurs because certain alleles help or harm the reproductive success of the organisms that carry them. Another factor that can upset this equilibrium is genetic drift, which occurs when allele frequencies grow higher or lower by chance and typically takes place in small populations.
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