Genetic Variation

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  • Created by: Megan
  • Created on: 08-03-14 13:35

Hardy-Weinberg Principle

...to calculate allele frequency in populations...

  • p = the frequency of the dominant allele 
  • q = the frequency of the recessive allele

Therefore p+q = 1 as everyone in the population must have a combination of these alleles

  • The frequency of the homozygous dominant genotype is p2
  • The frequency of the homozygous recessive genetype is q2
  • The frequency of the heterozygous genotype is 2pq - you can inherit either allele from each parent

Therefore p2 + q2 + 2pq = 1 as everyone in population has 1 of these 3 genotypes

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Types of Variation

Continuous - the quantitative differences between phenotypes and there is a wide range of variation within the population with no distinct categories

  • No distinct categories
  • Tends to be quantitative (numerical) data
  • Polygenic
  • Influenced by the environment

Discontinuous - qualitative differences between phenotypes and they fall into clearly distinguishable categories with no intermediates

  • Distinct categories
  • Tend to be qualitative
  • controlled by few genes
  • unaffected by the environment
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Genetic Drift

...the change in frequency of an allele in a population due to random sampling...

Instead of the environment affecting which individuals survive and breed, chance dictates which alleles are passed on.

Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation, especially in a small population - natural selection and genetic drift work alongside each other to drive evolution

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Natural Selection

  • New environmental factor affects the survival rate of a phenotype before reproduction
  • Organisms better adapted to the environment survive, reproduce, pass on their alleles/genes so the allele frequency of the advantageous gene increases
  • Change frequencies of alleles in gene pool/phenotype in population

Disruptive Selection:- the selection pressure acts against individuals in the middle of the curve and the result is a biomodal or two peaked curve. Example:- Darwin's Finches - big beaks for big seeds, small beaks for small seeds. Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds and hence were maladaptive.

Directional Selection:- one extreme of the charactersitic experiences selection against it. The result is that the distribution shifts towards the other extreme. Example - Giraffe Necks - individuals with short necks could not reach many leaves to feed on. So the distribution of neck length shifted to favour individuals with long necks

Stabilising Selection:- favours 'average' organisms so organisms with extreme forms of characteristics or mutations are selected against. Example - The heaviest and lightest babies have a higher mortality rate than average weight babies

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