BIO 370 Lecture Notes - Lecture 4: Genotype Frequency, Allele Frequency, Null Model
Lecture 4:
Critics on Mendel’s conclusions:
• Didn’t explain variation in continuously variable traits, controlled by multiple loci and so
consequences not understood.
• Didn’t explain frequency of observed discrete traits in nature. Early criticisms grounded
in transmission genetics than population genetics.
• Brachydactyly**: Punnett was able to advocate for Mendel’s idea by analyzing human
pedigrees, showed consistency. Showed difference b/w transmission genetics and
population genetics:
o Condition: malformation of the fingers. Inherited as a single locus and 2 alleles, in
which the malformation allele is dominant. Controlled by a single locus on an
autosome.
o Yule expected 3:1 ratio, but didn’t consider population as a whole.
o Population genetics: how and why traits change/remain the same through time
when summed across all possible parents and offspring, as affected by
evolutionary forces.
o Hardy developed mathematical way of proving how dominant alleles don’t just
replace recessive alleles over time.
o Transmission genetics: how traits through genetic factors get shared b/w specific
parents to produce specific offspring, and how they differ bc laws of segregation
and assortment.
o Molecular genetics: genes at chemical and cellular level, governed by
biochemical forces and principles.
• HW model: Null model for population genetics: what we would expect to find if nothing
happens in our system. It’s a prediction based on assumption that no evolution is taking
place. Any inconsistency with the model = evolution occurring, changes in genotype
frequency.
o HW equation allows us to predict allele frequencies from genotype frequencies in
absence of evolution.
o Allele frequency: p and q. p + q = 1
o P2 + 2pq + q2
o Assumptions: No natural selection, population infinitely large (no genetic drift),
no migration, random mating, no mutation.
1. Gamete pool approach: one big bucket of gametes we can reach in and
pull out to make new individuals.
o Null model for population genetics:
1. If allele frequencies change due to evolutionary forces, genotype
frequencies change from HW frequencies.
2. Genotype frequencies will return to HW frequencies in one generation if
those forces are removed = HW model represents a mixed equilibrium.
• Population genetics equilibrium types:
o Stable: returns to same point no matter how disturbed.
o Unstable: move away from stable point under slightest disturbance.
o Neutral: stabilizes at new location when disturbance stops.
o Mixed: combo of stable and neutral; if shifted, stays in new position so any value
of p on the graph is a neutral EQ. If pushed forward of backward on the side of
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Document Summary
Critics on mendel"s conclusions: didn"t explain variation in continuously variable traits, controlled by multiple loci and so consequences not understood, didn"t explain frequency of observed discrete traits in nature. Early criticisms grounded in transmission genetics than population genetics: brachydactyly**: punnett was able to advocate for mendel"s idea by analyzing human pedigrees, showed consistency. Showed difference b/w transmission genetics and population genetics: condition: malformation of the fingers. Inherited as a single locus and 2 alleles, in which the malformation allele is dominant. It"s a prediction based on assumption that no evolution is taking place. If pushed forward of backward on the side of half-pipe, it will rest at the bottom (graph looks like an upside down u, with p on the x-axis and y as heterozygote frequency). Neutral: **look at hw mixed example in discussion worksheet. Measures of fitness are relative: viability: living to reproductive age, fecundity: how many offspring are produced, ex.
