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Population Genetics: Hardy-Weinberg Principle AP Biology 2007-2008 5 Agents of evolutionary change Mutation Gene Flow Genetic Drift AP Biology Non-random mating Selection Mutation: A permanent, heritable change in the nucleotide sequence in a gene or a chromosome Gene flow: The transfer of alleles of genes from one population to another Non-random mating: Any mating system in which males are not randomly assigned to females. Genetic drift: The process of change in the genetic composition of a population due to chance or random events rather than by natural selection, resulting in changes in allele frequencies over time. Selection: Natural selection chooses the fittest to survive. AP Biology Populations & gene pools Concepts A population is a localized group of interbreeding individuals Gene pool is collection of alleles in the population remember difference between alleles & genes! Allele frequency is how common is that allele in the population how many A vs. a in whole population AP Biology Evolution of populations Evolution = change in allele frequencies in a population Hypothetical ?: what conditions would cause allele frequencies to not change in a population? REMOVE all agents of evolutionary change 1. very large population size (no genetic drift) 2. no migration (no gene flow in or out) 3. no mutation (no genetic change) 4. random mating (no sexual selection) 5. no natural selection (everyone is equally fit) AP Biology Hardy-Weinberg equilibrium Hypothetical, non-evolving population preserves allele frequencies Serves as a model (null hypothesis) – States that both allele and genotype frequencies in a population remain constant from generation to generation unless specific disturbing influences are introduced. Those disturbing influences include non-random mating, mutations, selection (from nature), limited population size/genetic drift, and gene flow. It is important to understand that outside the lab, one or more of these "disturbing influences" are always in effect. Hardy–Weinberg equilibrium is impossible in nature. Genetic equilibrium is an ideal state that provides a baseline against which to measure change. AP Biology Natural populations rarely fit into H-W equilibrium H-W is a useful model to measure the forces acting on a population and as a measure of evolutionary change G.H. Hardy mathematician AP Biology W. Weinberg physician Hardy-Weinberg equation Counting Alleles assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q frequencies must add to 1 (100%), so: p+q=1 BB AP Biology Bb bb Hardy-Weinberg equation Counting Individuals frequency of homozygous dominant: p x p = p2 frequency of homozygous recessive: q x q = q2 frequency of heterozygotes: (p x q) + (q x p) = 2pq frequencies of all individuals must add to 1 (100%), so: p2 + 2pq + q2 = 1 BB AP Biology Bb bb H-W formulas Alleles: p+q=1 B Individuals: p2 + 2pq + q2 = 1 BB BB AP Biology b Bb Bb bb bb Using Hardy-Weinberg equation population: 100 cats 84 black, 16 white How many of each genotype? p2=.36 BB q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 2pq=.48 Bb q2=.16 bb Must assume population is in H-W equilibrium! AP Biology What are the genotype frequencies? Using Hardy-Weinberg equation p2=.36 Assuming H-W equilibrium 2pq=.48 q2=.16 BB Bb bb p2=.20 =.74 BB 2pq=.64 2pq=.10 Bb q2=.16 bb Null hypothesis Sampled data How do you explain the data? AP Biology Application of H-W principle Sickle cell anemia inherit a mutation in gene coding for hemoglobin oxygen-carrying blood protein recessive allele = HbHb normal allele = HB low oxygen levels causes RBC to sickle breakdown of RBC clogging small blood vessels damage to organs AP Biology often lethal Sickle cell frequency High frequency of heterozygotes 1 in 5 in Central Africans = HBHb unusual for allele with severe detrimental effects in homozygotes 1 in 100 = HbHb usually die before reproductive age Why is the Hs allele maintained at such high levels in African populations? Suggests some selective advantage of being heterozygous… AP Biology Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells Malaria 1 2 AP Biology 3 Heterozygote Advantage In tropical Africa, where malaria is common: homozygous dominant (normal) die or reduced reproduction from malaria: HBHB homozygous recessive die or reduced reproduction from sickle cell anemia: HbHb heterozygote carriers are relatively free of both: HBHb survive & reproduce more, more common in population Hypothesis: In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. AP Biology Frequency of sickle cell allele & distribution of malaria