Prof Duncan Shaw

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Lecture 43

• Prof Duncan Shaw

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Alleles Fitness

• Fitness means the relative ability of organisms
  to survive and pass on genes
• Alleles can affect fitness
• In most cases not at all (neutral)
• Sometimes to decrease it (deleterious)
• Very rarely to increase it (advantageous)
• The frequency of alleles in the whole population
  affects the health of the population, so its
  important to know about population genetics

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Calculating Allele Frequencies

• We have a gene with alleles A and a
• Count number of individuals in population with
  each genotype
• 300 AA
• 500 Aa
• 200 aa
• Allele frequency of A is (2x300 500)/2000
  0.55
• Allele frequency of a is (500 2x200)/2000 0.45

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Allele Genotype Frequencies

• In previous example, we had 300 AA, 500 Aa, and
  200 aa individuals
• Genotype frequencies are therefore
• 0.3 AA
• 0.5 Aa
• 0.2 aa
• Allele frequencies and genotype frequencies are
  related, but not the same thing

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The Hardy-Weinberg Law

• Frequencies of alleles A and a are p and q,
  respectively (so p q 1)
• Calculation involving a Punnet square shows that
  genotype frequencies will be
• AA p2
• Aa 2pq
• aa q2
• Also, p2 2pq q2 1
• These frequencies stay the same over time, if
  population is large, randomly mating, and alleles
  have same fitness
• Such a population is in equilibrium

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Recessive alleles carrier frequencies

• For a rare allele, heterozygotes (called
  carriers if the allele is recessive) are much
  more frequent than homozygotes 2pq gtgtq2
• Cystic fibrosis is caused by a recessive allele
  and affects 1/1700 Caucasian newborns
• q2 1/1700 0.00059
• ? q 0.024
• ? p 1 - q 0.976
• ? 2pq 0.047
• About 1 in 21 Caucasians is a carrier for CF

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X - linked genes

• Males only have 1 copy of each gene on the X
  chromosome (hemizygous), from mother
• Therefore, for X-linked genes in males, genotype
  frequency is the same as allele frequency
• For rare X-linked recessive alleles, more males
  than females will be affected
• Example X linked colour blindness affects 1/20
  males
• q 0.05
• q2 0.0025
• About 1/400 colour-blind females

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Population Evolution

• Changes in the gene pool resulting a species
  adapting to its environment
• Dependent on genetic variation
• Driven by natural selection - differences in
  fitness make better adapted individuals more
  likely to pass on their genes
• Can be described in terms of allele frequencies
  in the population

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Factors that change allele frequencies

• Mutation - formation of new alleles, leading to
  new capabilities of organism
• Migration - movement of individuals between
  populations
• Natural selection - different abilities of
  organisms to survive and reproduce
• Genetic drift - in small populations, random
  changes in allele frequency

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Heterozygote advantage

• Sickle-cell anaemia is caused by a recessive
  allele - homozygotes have reduced fitness
  (without modern medical care), so it should
  disappear
• But heterozygotes have increased fitness - dont
  have anaemia, but are more resistant to malaria
  than individuals without allele
• Distribution of sickle-cell anaemia in the world
  is similar to that of the malaria parasite,
  Falciparum

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