BILD 3 Lecture Notes - Lecture 9: Genotype Frequency, Allele Frequency, Heterozygote Advantage
13 Jun 2018
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BILD 3 Lecture 9
4/20/2018
• S=selection coefficient (e.g. percentage of a genotype, e.g. aa, that dies)
o Relative fitness = 1-s
o 1 is the highest level of fitness
o directional selection: changes mean, variation goes down
o multiply relative fitness by original population # before selection to get the
number present after selection has occurred
o to alulate the populatio’s fitess: (genotype frequency)(genotype relative
fitness)
o how does the reduction of the frequency of the homozygous recessive allele
ipat the populatio’s oeall fitess fo the seod geeatio?
▪ Calculate the new allele frequencies (p and q) after selection occurs 1
time around
▪ The allele frequencies go down for the homozygous recessive, and the
genotype frequencies shift in concurrence.
• Allele frequenies ae p’s ad ’s
• Genotype frequencies are calculated from p2+2pq+q2
(homozygous dominant, heterozygous, homozygous recessive)
▪ Use new genotype frequencies to calculate a new mean fitness of the
population after another round of breeding
• Heterozygote advantage
o For some traits, heterozygote genotypes have the highest fitness because they
can have a different phenotype for either homozygous genotypes
o Stabilizing selection
o Natural selection can maintain both alleles at such a locus, even if both
homozygotes are selected against
o Meas does’t hage, aiatio deeases
o Ex: malaria and sickle-cell disease: heterozygotes that carry one sickle cell allele
ae poteted fo seee alaia, ad do’t get the full-blown symptoms of
sickle cell disease
o AA= s1 and aa=s2
o S1 does’t eessaily = “2
• Biological species concept: group whose members have the potential to interbreed in
nature and produce viable, fertile offspring
• Prezygotic barriers: individuals never even mate
o Physial isolatio a’t get ea eah othe eause of a aie
o Habitat isolation: mate in different places, e.g. water vs land
o Behavioral isolation: different ways of recognizing mates
▪ E.g. lacewings: males produce low frequency sound to attract female and
the thee’s a outship i espose. Matig does not occur unless the
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