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| 13.1 The Importance of Knowing Allele Frequencies 1. Population genetics concerns the study of allele frequencies in a population. 2. Genes in a population comprise the gene pool. 3. Gene flow is movement of alleles between populations. 4. Microevolution reflects changes in gene frequencies in populations, which can be traced using the Hardy-Weinberg equation. 5. Allele frequencies are affected by mutation, migration, genetic drift, selection, and non-random mating. 13.2 When Allele Frequencies Stay Constant-Hardy Weinberg Equilibrium 1. In Hardy-Weinberg equilibrium, gene frequencies stay relatively constant. 2. Hardy-Weinberg equilibrium occurs in large populations where mating is random and there is no migration, mutation, or natural selection. 3. For a population in Hardy-Weinberg equilibrium where frequencies are known for two alleles, the proportion of homozygous dominants equals the square of the frequency of the dominant allele (p2); the proportion of homozygous recessives equals the square of the frequency of the recessive allele (q2); proportion of heterozygotes equals 2pq. 4. Allele frequencies can be calculated from the proportions of genotypes (either p or q). The frequency of the recessive allele (q) equals the proportion of homozygous recessives plus 1/2 that of the carriers. The frequency of the dominant allele equals the proportion of homozygous dominants plus 1/2 that of the carriers. 13.3 Practical Applications of Hardy-Weinberg Equilibrium 1. In real life situations, the value of q2 can be derived by knowing the frequency of affected individuals in a population. The Hardy-Weinberg equation can then be used to predict the likelihood of someone being a carrier. 2. For X-linked recessive traits, the frequency of the recessive allele in males is q and in females it is q2. 3. For very rare inherited disorders, p approaches 1, so the carrier frequency is 2q (approximately twice the frequency of the disease-causing allele). 13.4 DNA Fingerprinting-A Practical Test of Hardy-Weinberg Assumptions 1. Polymorphisms are DNA sequences that vary among individuals with a frequency of at least 0.01. 2. Polymorphisms that alter restriction enzyme cutting sites (RFLPs) can be used to distinguish among individuals. DNA Patterns Distinguish Individuals 1. DNA fingerprints are based on differences in the numbers of DNA repeats. 2. Polyacrylamide gel electrophoresis is used to separate pieces of DNA, and distinguish among individuals on the basis of the sizes of certain DNA regions. Population Statistics Are Used to Interpret DNA Fingerprints 1. Population statistics derived from databases are applied to DNA fingerprinting results to determine the probability that the same pattern is in two individuals. A limitation of the method is that databases may not adequately represent real human populations. 2. Developing narrower ethnic databases and considering historical and social factors may help develop more realistic population statistics. DNA Fingerprinting to Identify World Trade Center Victims 1. PCR analysis of short tandem repeats (STRs) can be used to obtain a unique fingerprint pattern. 2. STR analysis was used to identify victims in the World Trade Center attack on Sept. 11, 2001. |