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| 14.1 Nonrandom Mating 1. Human mating is nonrandom. People choose their mates for many reasons and frequently they choose people with traits similar to themselves. 2. At the population level, human mating is nonrandom because individuals do not contribute equally to the next generation (ie, some people have more children than others). 3. For some traits, such as blood types, mating can be considered random and allele frequencies are often in Hardy-Weinberg equilibrium. 4. Populations that practice marriage between related members (consanguinity) experience increases in the proportion of homozygotes at the expense of heterozygotes. 14.2 MigrationHistorical Clues 1. Migration disrupts gene frequencies by introducing new alleles into a population, although it may take many generations for different ethnic groups to intermarry. 2. We can make inferences about the directions in which ancient people traveled by looking at the distribution of allele frequencies in modern populations. Geographical and Linguistic Clues 1. Clines are changes in allele frequencies between neighboring populations. 2. Geographical barriers and language clues often result in clines with gradual or abrupt changes in allele frequencies. 14.3 Genetic Drift 1. Genetic drift occurs when a subset of a population has different gene frequencies, often reflecting a physical separation. The Founder Effect 1. The founder effect occurs when a few individuals leave a community to start a new settlement. 2. The resulting population may, by chance, lack some alleles present in the original population, or have high frequencies of others. Population Bottlenecks 1. In a population bottleneck, many members of a population die, and only a few individuals contribute genetically to the next generation. A population bottleneck may also occur if a small number of individuals colonize an island. 14.4 Mutation 1. Mutation alters gene frequencies by introducing new alleles. 2. Heterozygotes and mutations maintain the frequencies of deleterious alleles in populations, even if homozygotes die. 3. Genetic load is the collection of deleterious alleles in a population. 14.5 Natural SelectionTuberculosis Ups and Downs and Ups 1. Differential survival based on phenotype is called natural selection. 2. The bacteria causing tuberculosis (TB) was first identified by Robert Koch in 1882. 3. As a result of natural selection operating on bacterial and human populations the effect of TB on human health evolved from outbreaks of an acute systemic infection to an increasingly rare chronic lung infection. 4. Starting in the late 1980s s there has been a resurgence of TB as a result of natural selection and mutation. One in seven TB cases is resistant to several drugs and 5% of these patients die. Evolving HIV 1. HIV is a retrovirus and viral mutations accumulate rapidly in its RNA genome. 2. During the course of an HIV infections environmental conditions select for viral variants that can multiply rapidly and evade the immune system. Balanced Polymorphism 1. Balanced polymorphisms exist in populations when heterozygotes carrying a disease-causing allele enjoy a health advantage. 2. The recessive disease causing alleles for sickle cell disease, G6PD deficiency, PKU, Tay-Sachs disease, and Cystic fibrosis represent balanced polymorphisms since they are associated with conferring resistance to infectious diseases in the heterozygous condition. 3. A changing environment may make previously selected alleles a disadvantage, such as in diabetes mellitus and P-glycoprotein. 14.6 Gene Genealogy 1. Frequencies of different mutations in different populations provide information on the natural history of alleles. PKU Revisited 1. Mutational analysis in various populations indicates that PKU originated more than once, and that genetic drift, balanced polymorphism, and perhaps mutation have influenced its prevalence. CF Revisited 1. Studies of allele prevalence place the origin of CF farther back than previously thought. |