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The Hardy-Weinberg Principle

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BIOL 201
Elaine Power

The Hardy-Weinberg principle  Allele frequency → p (A) + q (a) = 1 (total gene pool)  Genotype frequency p2 (AA) + 2pq (Aa) + q2 (aa) = 1 (gene pool)  Allele frequency must be constant / population must be stable [EXAM] o Large population  Prevents large swings in frequencies  Anomalies and chance variation less significant o Random mating  Equal chance of alleles being passed on o No mutations / no immigration/emmigration / no natural selection  Prevents addition or removal of new alleles  Baseline by which to judge whether allele frequency of population's gene pool has changed  Gene pool: all the alleles in a population Selection and change of allele frequency Natural selection  New environmental factor affects survival rate of phenotype before reproduction o //otherwise population may become extinct  Organisms better adapted to the environment survive, reproduce, pass on their alleles/genes  Allele frequency of the advantageous gene increases  Changes frequencies of alleles in gene pool / phenotype in population  Population becomes adapted to environment Stabilising selection  Natural selection favours "average" organisms best adapted to that environment  Organisms with extreme forms of characteristics/mutations are selected against  Heaviest and lightest babies have highest mortality o Less likely to survive, reproduce, pass on their alleles  [Graph] Normal distribution curve with thinner bell-shaped curve Directional selection  Natural selection favours organisms with one extreme form of a characteristic  Pesticide resistance (warfarin - poison used to kill rats) o Resistant rats / need a lot of vitamin K / stabilising selection o New environmental effect: warfarin / kills normal rats o Resistant rats survived, reproduced, pass on resistance gene o New population forms by directional selection  Antibiotic resistance (penicillin resistance) o Resistant bacteria / unnecessary enzymes / selected against o New environmental factor: penicillin / kills normal bacteria o Resistant bacteria survived, reproduced, passed on resistance gene  [Graph] bell-shaped curve shifted to the right Disruptive selection  Natural selection favours organism with two extreme forms of a characteristic  Balanced polymorphism: equilibrium of non-carriers and carriers of a characteristic caused by natural selection  Sickle-cell anaemia o Abnormal Hb makes red blood cells sickle-shaped / stick in capillaries o People homozygous for this recessive allele die before reproducing o People heterozygous for the allele should be at
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