bio 207 lec 13
What is population genetics?
The branch of genetics that studies the genetic makeup of groups (populations) and how this changes over time due to evolutionary forces.
What is a Mendelian population?
A group of interbreeding sexually reproducing individuals that share a common gene pool.
Why is genetic variation considered the basis of all evolution?
Because heritable variation in traits allows natural selection to act — individuals with advantageous traits survive and reproduce more, passing those traits to the next generation.
What is an example of this in nature?
Darwin’s finches: ancestral finch populations had variation in beak size and shape, which allowed some birds to survive better under changing food sources (e.g., seeds vs. insects).
Why is genetic variation important in populations?
It provides the raw material for evolution — populations with more variation are better able to adapt to environmental changes.
Why are the wolves of Isle Royale an example of a Mendelian population?
Because they are a group of interbreeding, sexually reproducing individuals that share a common gene pool, allowing for the study of population-level genetics.
What is the origin story of the Isle Royale wolf population?
In 1949, a single breeding pair of wolves crossed frozen Lake Superior.
By 1959, there were 20 wolves.
By 1980, the population grew to 50 wolves.
What are the two components of genetic structure in a population?
Genotypic frequencies (how often each genotype occurs)
Allelic frequencies (how often each allele occurs)
How is genotypic frequency calculated?
f(AA)= number of AA individuals/ N
f(Aa)= number of Aa individuals/N
f(aa)= number of aa individuals/ N
✅ These should always add up to 1 (or 100%)
Why are there always fewer alleles than genotypes in a population?
Because genotypes are combinations of alleles — different arrangements of a smaller number of alleles create more genotypic possibilities.
What are the two ways to calculate allele frequencies?
A. By counting the number of alleles directly (e.g., from individual genotypes)
B. By using the frequencies of genotypes
✅ Both methods should give the same result, and allele frequencies always add up to 1.
How do you calculate allelic frequency from numbers of genotypes?
frequency of an allele = number of copies of that allele (N)/ number of copies of alleles at the locus (2N)
or
p= 2NAA + nAa/ 2N
q= 2naa + nAa/ 2N
p+q =1
How do you calculate allele frequencies from genotype frequencies?
p= f(A) = f(AA) + 1/2f(Aa)
q= f(a)= f(aa)+ 1/2f(Aa)
p+q =1
What if there are more than two alleles at a locus (e.g., A₁, A₂, A₃)?
genotype freq:
p= f(A1) = f(A1A1) + 1/2f(A1A2) + 1/2f(A1A3)
fist term is the homo, 2nd and 3rd terms are the hetero
repeat for A2 and A3
genotype allele (similar):
p, q,r
p= f(A1)= ( 2nA1A1 + n(A1A2) + n(A1A3) )/ 2N
Why are X-linked allele frequency calculations different?
Because females have 2 X chromosomes and males only have 1, so you must calculate separately by sex.
How do you calculate allele frequencies for X-linked loci (from counts)?
f(X1)= (2nx1x1 + nx1x2 + nx1y)/ 2nfemales + nmales
same for X2
Males contribute 1 X; females contribute 2 Xs.
How do you calculate X-linked allele frequencies from genotype frequencies?
f(x1) = f(x1x1) + 1/2f(x1x2) + f(x1y)
same for x2
Males are hemizygous, so their alleles are fully counted (not halved).
What does the Hardy-Weinberg Law describe?
It shows how allele and genotype frequencies remain constant from one generation to the next in an ideal population.
What are the assumptions of Hardy-Weinberg equilibrium?
Large population
Random mating
No mutation
No migration
No natural selection
What are the Hardy-Weinberg predictions for a locus with two alleles (A and a)?
f(AA) = p2
f(Aa) = 2pq
f(aa) = q2
where:
p= f(A), q= f(a), and p + q =1
✅ After one generation of random mating, the population reaches Hardy-Weinberg equilibrium.
Who developed the Hardy-Weinberg Law, and when?
G.H. Hardy and Wilhelm Weinberg independently formulated the law in 1908.
What does the Hardy-Weinberg Law describe?
It's a mathematical model that evaluates the effect of reproduction on allelic and genotypic frequencies in a population under ideal conditions (no evolution occurring).
What are the two main predictions of the Hardy-Weinberg Law?
Allelic frequencies remain constant from generation to generation — no evolution occurs.
Genotypic frequencies stabilize after just one generation of random mating, and will not change in future generations if conditions remain ideal.
What does the Hardy-Weinberg Law state about reproduction and allele frequencies?
If the assumptions are met, then reproduction alone does not change allelic or genotypic frequencies in a population.
What happens to genotypic frequencies after one generation of random mating?
They will stabilize and reach the Hardy-Weinberg proportions:
𝑝2 = frequency of AA
2𝑝𝑞 = frequency of Aa
𝑞2= frequency of aa
What does it mean if a population is in Hardy-Weinberg equilibrium?
The genotype frequencies match the predicted values of
p2 + 2pq + q2=1, and there is no evolution occurring at that locus.
Do individuals and gametes share the same allelic frequencies?
Yes — the allelic frequencies in individuals equal the frequencies in gametes.
🧠 Why? Mendelian inheritance ensures each allele is passed to 50% of gametes on average.
How are genotype frequencies derived in a population Punnett square?
Sperm allele frequencies: p and q
Egg allele frequencies: p and q
Use multiplication:
-
𝑝2= AA
2pq = Aa
𝑞2= aa
What does Hardy-Weinberg equilibrium imply about evolution?
That the population is not evolving, because allele frequencies remain constant. evolution results from changes to allelic
frequencies.
implication of H-W --> Does reproduction alone cause evolution?
No — without mutation, selection, or migration, reproduction does not change allele frequencies.
What determines the most common genotype at a locus? (implication of HW)
If p = q = 0.5 → Aa (heterozygote) is most frequent
If one allele is more common → the corresponding homozygote becomes most frequent
In a population in Hardy-Weinberg equilibrium, the
genotypic frequencies are determined by allelic frequencies
What if a population doesn’t match Hardy-Weinberg proportions? (4th implication of HW)
It indicates that one or more assumptions have been violated, which may suggest evolution is occurring.
What gene affects the ability to taste PTC?
The TAS2R38 gene — it has two common alleles:
Taster allele (dominant)- bitter
Non-taster allele (recessive)
What genetic variation explains PTC tasting?
Specific SNPs (single nucleotide polymorphisms) cause amino acid changes in the taste receptor, altering sensitivity to bitterness.
Is there selective pressure on the ability to taste PTC?
No, these taste receptor alleles are in Hardy Weinberg Equilibrium!
PTC (phenylthiocarbamide) is a non-toxic chemical that is not naturally found in the environment, so there is no current selective pressure favoring tasters or non-tasters.
Why study PTC tasting if it’s not naturally selected?
Because it’s a simple, well-understood example of:
Mendelian inheritance
Allelic variation in human populations
A model for how traits might be affected by natural selection if a similar compound were present in nature
What does population variation show about PTC tasting?
100% of Indigenous Americans are tasters
~50% of Aboriginal Australians are tasters
Women, Asians, and African-Americans are more likely to be “super-tasters”
Does non-random mating (HW violation) affect allele frequencies?
No — it affects genotypic frequencies, not allelic frequencies.
What is positive assortative mating?
When individuals prefer mates with similar traits (e.g., tall × tall).
What is negative assortative mating?
When individuals prefer dissimilar traits (e.g., tall × short).
What is inbreeding?
A form of positive assortative mating where individuals mate with close relatives.
what is outcrossing?
preferential
mating between unrelated
individuals
What is the genetic consequence of inbreeding?
It increases homozygosity and decreases heterozygosity, raising the risk of recessive disorders. This is harmful because the increase in homozygotes boosts the probability that deleterious and lethal recessive alleles will combine to produce a harmful trait.
What evolutionary forces can change allele frequencies? (HW violation)
Mutation
Migration (gene flow)
Genetic drift
Natural selection
How do mutations affect allele frequencies?
They introduce new alleles, shifting p and q over time.
Forward mutation: allele A → a
Reverse mutation: a → A
Eventually reach a mutation equilibrium
where did/ does genetic variation ultimatley arise from
mutations
How does migration (gene flow) affect allele frequencies?
Migration introduces alleles from outside populations, altering the gene pool (gene flow).
What is genetic drift?
Random fluctuations in allele frequencies due to small population size, like flipping a coin only a few times.
It causes sampling error.
sampling error
deviation from an expected
ratio due to limited sample size
What does Mendelian inheritance say about gamete production?
That an individual will produce gametes with each of their two alleles in equal proportion — about 50:50.
Why might gamete allele ratios deviate from 50:50 in real populations?
Because this 50:50 ratio is only accurate when many gametes are produced — in small populations, random chance can cause deviations.
What is this effect called when allele frequencies shift due to chance in small populations?
Genetic drift — a random change in allele frequencies due to sampling error, especially in small populations.
What is natural selection?
differential reproduction of certain
genotypes. A process where individuals with advantageous (adaptive) traits reproduce more, increasing the frequency of adaptive alleles.
Why is natural selection unique among evolutionary forces?
It is the only force that promotes adaptation to the environment.
For example polar bears have adapted to live in extreme Arctic
environments
Genotype Number
LMLM 182
LMLN 172
LNLN 44
calcualte genotypic frequency
.457, .432, .111
Genotype Number
LMLM 182
LMLN 172
LNLN 44
calculate allelic freq
M= .673, N=.327
64.95% of people tasted bitter = 63 people (TT, or Tt)
35.05% of people tasted filter paper = 34 people (tt)
n=97
how many people are expted to be super testers (TT), how many Tt and how many tt?
f(tt) = q2 = 34/97 = 0.3505
q = 0.592 = allele frequency for t in BIOL 207 population!
Because p + q = 1
p = 1 - q
p = 0.408
f(TT) = p2 = 0.1665
f(Tt) = 2pq = 0.4831
f(tt) = q2 = 0.3505
Meaning:
• Approximately 16/97 people are expected to be TT “super tasters” – if the filter paper was intensely/strongly bitter, you are
probably a ”super taster”!
• Approximately 47/97 people are expected to be Tt