Hardy Weinberg Equilibrium

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Hardy Weinberg Equilibrium

• Chapter 23
• Collect movie questions
• Assign Outline 23 and 24 by next Friday

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Populations Evolve

• Natural selection acts on individuals
• Differential survival survival of the fittest
• Differential reproductive success the most fit
  have the most offspring
• Populations Evolve
• Population changes over time
• Best/most fit traits increase within the
  population

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Individuals do not evolve

• Example from yesterdays lab

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Variation

• A requirement for natural selection! Why?
• Natural selection requires variation
• There must be differences
• Some individuals must be more fit than others

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Where does variation come from?

• Mutation creates variation
• New mutations are constantly appearing
• Mutations change DNA sequence
• Changes in amino acid sequence
• Changes protein
• Changes structure
• Changes function
• Changes in protein may change phenotype and
  fitness!

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Sexual Reproduction

• Sexual reproduction increases variation
• One ancestor has lots of descendants
• Recombination from cross over
• Offspring have new traits/new phenotypes
• Recombines alleles into new arrangements for
  every offspring!

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How do we measure changes in populations?

• Measure the change in allele frequency (allele
  type of gene)
• All of the genes and alleles in a population
  the gene pool
• Factors that affect the frequency of alleles in a
  population
• Natural selection
• Genetic Drift The founder effect, the Bottleneck
  effect
• Gene Flow

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1. Natural Selection

• Natural selection is driven by the environment
• A changing environment
• Climate change
• Food availability
• New predators or diseases
• The most fit alleles will increase in frequency
  within the population

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2. Genetic Drift

• Changes in gene frequency from one generation to
  the next due to chance
• Examples
• The Founder effect A small group of the
  population splinters off and starts a new colony
• Bottleneck Famine, Flood etc. reduces the
  population to a small number, and then the
  population expands back/grows.

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3. Founder Effect

• When a new population is started by only a few
  individuals.
• Any examples??
• Some alleles may be at high frequency by chance,
  others may be missing altogether.
• This alters the gene frequency in the new gene
  pool

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Example Blood types
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Bottleneck

• When large population is drastically reduced and
  then bounces back.
• Any examples??
• All cheetahs share a small number of
  allelesalmost like identical twins!
• Two bottlenecks
• Ice Age 10,000 years ago,
• Last 100 years poaching
• These are AT RISK populationswhy?

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3. Gene Flow

• Individuals moving from one area to another
  migration
• New alleles move into the gene pool
• Reduces the difference between populations

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Hardy-Weinberg Equilibrium

• What would a hypothetical, non-evolving
  population look like?
• This is useful for comparison as a model
• Most natural populations ARE NOT in Hardy
  Weinberg equilibrium
• Their model is useful to compare/measure, and if
  the population is not in H-W equilibrium, then it
  is evolving!

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Hardy Weinberg Equilibrium

• A non-evolving population (at equilibrium) occurs
  IF
• The population is very large (no genetic drift)
• There is no Migration (in or out)
• No mutation
• Random Mating (no competition for mates)
• No natural selection
• How will we measure changes in the population?

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Hardy Weinberg Theorem

• USED TO CALCULATE ALLELE FREQUENCY in a
  population.
• Frequency of Dominant Allele in a population is
  p
• Frequency of recessive allele q
• We can calculate the expected frequency of
  individuals with
• Homozygous dominant
• Homozygous recessive
• Heterozygous individuals

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Hardy weinberg theorem

• P2 2pq q2 1
• pfrequency of dominant allele
• qfrequency of recessive

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Example Problem 1

• What is the allele frequency in this population
• Allele for Red flower R (completely dominant)
• Allele for white flower r
• RR 320 flowers, Rr 160, rr 20

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Example Problem 2

• What is the frequency of the PKU allele in a
  population if
• 1 in 10,000 babies in the US are born with PKU.
• PKU is a recessive disease
• What is the frequency of non-PKU allele?
• What is the frequency of heterozygotes?