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Lecture 3

BIOLOGY 1M03 Lecture Notes - Lecture 3: Tas2R38, Balancing Selection, Genotype

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Jon Stone

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PTC (Phenothiocarbamide) chemical compound, with taste duality (bitter/tasteless) depends on
ones genetic makeup
o Ability to taste is Dominant Genetic Trait (just need one allele in loci to express trait to
exhibit "taster phenotype) -TAS2R38 gene on chromosome 7 has been implicated in the
ability to taste PTC
Balancing Selection has maintained genes for 'tasting' and 'non tasting' at the PTC
locus on chromosome 7
Alleles subjected to balancing selection are not favored, and maintained at
similar frequencies across populations
o Phenotype, physiological traits of an organism; Genotype, organisms Genetic Makeup. Ratio
of tasters to non tasters respectively 3:1 (ranges based on ethnicity, from less than 60% in
some racial groups to 100% in others)
Hardy Weinberg Equation
o Allows us to calculate allelic or genotypic frequencies for a population. A population is in
Hardy Weinberg equilibrium if these frequencies are constant from one generation to the
If frequencies in equilibrium, implies that population is not evolving
When observe change in genetic frequencies, and population isn't in equilibrium,
evolution is occurring
Hardy Weinberg equilibrium theory states that a population will remain in
equilibrium unless at least one of the following, acts on the population:
Genetic Drift- chance fluctuations in gene frequency especially in small
Gene Flow- transfer of alleles into and out of population due to migration
Mutation-new mutations cause changes in frequency of alleles
Non-random Mating(Sexual selection)- mate selection eliminates random
Natural Selection- differential survival and reproductive success of certain
alleles increases their frequency
Unlikely that natural population could satisfy all of the conditions; Infinitety large
population size, no migration, no mutation, and no natural selection. Most
populations violate at least one of these criterion and are evolving. (Hardy Weinberg
Principle useful tool for establishing null hypotheses when want to test whether
evolution is occurring in population)
In order to do this, consider two possible forms of
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