BIOEE 1780 Lecture 25: Lecture 25
7 May 2018
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Population Genetics: the study of the distribution of alleles within populations and the mechanisms that
can cause allele frequencies to change over time
Genetic Locus: the specific location of a gene or piece of DNA sequence on a chromosome
• When mutations modify the sequence at a locus, they generate new alleles.
Theorems: mathematical statements that have been proven based on previously established theorems
and axioms.
• Use deductive reasoning
• Show that a statement necessarily follows from a series of statements or hypotheses (the proof)
• Ex.) Hardy-Weinberg equilibrium
• Not the same as theories
o Theories: explanations supported by substantial empirical evidence; the explanations are
tentative, but weighted by the quantity of evidence that supports them
Hardy Weinberg Assumptions
• Population is infinitely large.
o If a population is finite, allele frequencies can change randomly from generation to generation
due to chance (i.e. genetic drift).
o Very large populations behave similarly to the model.
• No selection
• No mutation
• No migration
• Random mating
• (Diploid, sexually reproducing organisms)
These assumptions are never met in natural or laboratory populations.
Additive Allele: an allele that yields twice the phenotypic effect when 2 copies are present at a given
locus than occurs when only one copy is present
• Not influenced by the presence of other alleles
• No dominance
• Affects phenotype, making it particularly vulnerable to selection
The rate of adaptation depends upon whether the selected allele is dominant, recessive, codominant,
or overdominant.
When the positively selected allele is dominant and has just arisen…
• Population is only A2 initially
• Mutation arises in the heterozygous state, producing an A1A2 genotype
o A1A2 has an advantage
o A1A2 increases in frequency until A1A1 individuals appear
• A1A1 and A1A2 have the same fitness. No selection is pushing the A1 allele to fixation.
o Some A2A2 individuals still exist in the population.
o These A2 alleles exist because they're sheltered in heterozygous individuals.
When the positively selected allele is recessive and has just arisen…
• Population is only A2 initially
• Mutation arises in the heterozygous state, producing an A1A2 genotype
Document Summary
Population genetics: the study of the distribution of alleles within populations and the mechanisms that can cause allele frequencies to change over time. Genetic locus: the specific location of a gene or piece of dna sequence on a chromosome: when mutations modify the sequence at a locus, they generate new alleles. Theorems: mathematical statements that have been proven based on previously established theorems and axioms: use deductive reasoning, ex. ) Hardy-weinberg equilibrium: not the same as theories. Show that a statement necessarily follows from a series of statements or hypotheses (the proof: theories: explanations supported by substantial empirical evidence; the explanations are tentative, but weighted by the quantity of evidence that supports them. Hardy weinberg assumptions: population is infinitely large. These assumptions are never met in natural or laboratory populations. The rate of adaptation depends upon whether the selected allele is dominant, recessive, codominant, or overdominant.
