Chapter 18 - Population Genetics

• In 1983, David Lace confessed to the murder, but with Hodgson already convicted, the police refused to believe Lace.
• David Lace committed suicide in 1988, long before the DNA evidence implicated him.
• The DNA-based analysis used to exonerate Mr. Hodgson and hundreds of other wrongly convicted prisoners was dependent on population genetic analysis.
• The principles of population genetics are at the heart of many questions facing society today.
• Actual populations of organisms in nature are finite rather than infinite in size.
• The Hardy–Weinberg law tells us that allele frequencies remain the same from one generation to the next in an infinitely large population.
• Linkage disequilibrium is the outcome of the fact that new mutations arise on a single haplotype.
• Linkage disequilibrium will decay over time because of recombination.
• Population genetics analyzes the amount and distribution of genetic variation in populations and the forces that control this variation.
• Population genetics has its roots in the early 1900s, when geneticists began to study how Mendel’s laws could be extended to understand genetic variation within whole populations of organisms.
• Mendel’s laws explain how genes are passed from parent to offspring in known pedigrees, but these laws are insufficient to understand the transmission of genes from one generation to the next in natural populations, in which not all individuals produce offspring and not all offspring survive.
• The new haplotypes can have unique properties that alter protein function.
• Recombination is a critical force sculpting patterns of genetic variation in populations.
• Migration can add genetic variation to a population via gene flow from another population of the same species.
• If the association between the alleles at two loci is nonrandom, then the loci are said to be in linkage disequilibrium (LD).
• Mutation is the ultimate source of all genetic variation.
• If there is a random relationship between the alleles at the two loci, then the frequency of any haplotype will be the product of the frequencies of the two alleles that compose that haplotype.
• In the case of linked loci, alleles are not gained or lost; rather, recombination creates new haplotypes.
• Recombination can create variation that takes the form of new haplotypes.
• The result has been a revolution in our understanding of genetic variation in populations.
• One measure of Darwinian fitness is the number of offspring that an individual has, symbolized as W.
• Darwinian fitness refers to the ability to survive and reproduce and considers both viability and fecundity.
• Selection is the process by which individuals with certain heritable features are more likely to survive and reproduce than are other individuals that lack these features.
• Relative fitness is the fitness of an individual relative to some other individual, usually the most fit individual in the population, symbolized as w.
• Natural selection is the process by which populations change over time as the environment favors features that enhance the ability to survive and reproduce.
• Absolute fitness is the number of offspring that an individual has, and it is also the number of alleles at a locus that an individual contributes to the gene pool.
• The concept of fitness applies to genotypes as well as to individuals.
• Genetic drift is a stronger force in small populations than in large ones.
• Population size is a key factor affecting genetic variation in populations.
• The relative fitness of an individual is calculated by subtracting the fitness of the most fit individual in the population from 1.
• Most new neutral mutations are lost from populations by genetic drift.
• In population genetics, a locus is simply a location in the genome; it can be a single nucleotide site or a stretch of many nucleotides.
• If the mutation rate and the number of nucleotide differences between two sequences are known, then the time since their divergence can be estimated.
• The number of observed mutations per genome per generation provides an estimate of the rate.
• For humans, the mutation rate is approximately 1 x 10^-3, meaning that on average, one mutation occurs in one DNA copy per thousand years.
• Geneticists can estimate mutation rates by starting with a single homozygous individual and following the pedigree of its descendants for several generations.
• With 3 billion bp in our genome, that adds up to a total of about 3 million differences between the set of chromosomes inherited from a person’s mother and the set inherited from a person’s father for non-inbred individuals.
• Levels of nucleotide diversity at synonymous and silent sites in some different organisms are shown in Figure 18-15.
• Children of marriages of first cousins show about a twofold higher frequency of disorders as compared to children of unrelated parents.
• When there is inbreeding in a population, the random-mating assumption of Hardy–Weinberg will be violated.