Bio 110
Lecture 012 Population Genetics Student Notes 012sr2
Beavers
1. Population Genetics
a. Studying organisms as ______________________________.
2. POPULATION GENETICS
a. POPULATIONS: A group of interbreeding individuals of the same species that inhabit the same
space at the same time.
b. POPULATION GENETICS: The study of the frequencies of different genotypes in populations
and the changes in those frequencies that result from patterns of mating (inbreeding or assortive
mating), natural selection, mutation, migration, and random chance.
c. Or: POPULATION GENETICS: scientific discipline that studies what happens in whole
populations at the genetic level.
d. GENE POOL: the sum total of all alleles carried in all members of a population.
e. Mating patterns have different effects on different genotypes in the population.
i. Inbreeding
ii. Assortive
iii. Migration
iv. Mutation (or recombination)
v. Natural selection
vi. Random fluctuation in actual reproduction rates
3. DESCRIBING POPULATIONS
a. The genetic makeup of a population changes over time due to:
1. New alleles arise by ____________________________.
2. Mutations are introduced to populations by _________________________.
3. Pre-existing mutations can disappear in populations – how?
b. Microevolution: alterations of a population's gene pool.
TERMS USED TO DESCRIBE POPULATIONS:
c. PHENOTYPE FREQUENCY: The proportion of individuals in a population that express a
particular phenotype.
i. Friendly vs. Fussy: Friendly is dominant. Total population=20. 4 are fussy.
ii. % of population that is fussy? _______________
iii. % of population that is friendly? ____________
d. GENOTYPE FREQUENCY: The proportion of total individuals in a population that are of a
particular genotype.
i. Same population using molecular analysis. 12 homozygous dominant for friendly.
ii. % FF? ________
iii. % Ff? ________
iv. % ff? _________
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Bio 110
Lecture 012 Population Genetics Student Notes 012sr2
Beavers
e. ALLELE FREQUENCY: A measure of the commonness of an allele in a population; the
proportion of all copies of a gene in a population that are of a given allele type.
i. How many total alleles are present in this population?___________
ii. Do both homozygotes and heterozygotes contribute to the frequency of an allele? ___
iii. What is the frequency of the F allele? ______
iv. What is the frequency of the f allele? _______
v. The frequency of alleles should always sum to: ____
f. The requirement of a _________________________ between phenotypic and genotypic
variation limits investigation of population genetics. Example: plants that are 5'8''. What gene
determines that exact height?
4. GENE POOL ALLELE NUMBERS VARY
a. Each diploid individual contributes ONLY two alleles for each gene of a particular trait. There
can be more than two possible alleles making up a particular trait.
b. ABO blood groups have 3 different alleles that create 4 phenotypes and 6 genotypes.
Alleles: _____ ______ _____
Phenotypes ______ ______ ______ ______
Genotypes _______ _______ _______ _______ ________ ________
c. The human CFTR gene has >1900 disease causing alleles.
d. Population geneticists have developed analytical models for estimating the genetic and
phenotypic variations of a population OVER TIME.
5. HARDY-WEINBERG LAW – A tool for prediction and estimation!
a. Named for the two men who independently developed it in 1908.
b. HW equilibrium states that sexual reproduction does not cause a constant reduction in genetic
variation in each generation, but instead IN THE ABSENCE OF DISTURBING FORCES, the
amount of variation remains constant.
c. Hardy-Weinberg: used as a model for understanding allele, genotype, and phenotype frequencies
for a SINGLE-GENE TRAIT in a genetically stable population.
d. Why bother with prediction?
i. Population management – do we make carrier screening standard of care? Is a 1/25
carrier risk in Caucasians high enough to offer carrier screening?
ii. Potential incidence in a population of disease? (Sickle cell, Tay-Sach's)
iii. How common are carriers in a certain population?
iv. Healthcare burden to society if a high prediction of affected offspring?
6. HARDY-WEINBERG (HW) LAW – Tool for Prediction starting with Assumptions about the
Population:
a. The population includes a VERY LARGE, virtually infinite number of individuals.
b. Individuals mate at random (they do not choose each other by genotype!)
c. No new mutations appear in the gene pool!
d. No migration (in or out) of population.
e. No natural selection: in other words, there are no genotype-dependent differences in the ability
to survive to reproductive age and transmit genes to the next generation.
f. These assumptions permit the mathematical derivation of an equation for ________________
genotype frequencies for a population of diploid individuals.
g. Of course: no actual population _______________________________________________.
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Bio 110
Lecture 012 Population Genetics Student Notes 012sr2
Beavers
7. HARDY-WEINBERG LAW
a. Hardy-Weinberg law defines the relationships between genotype and allele frequencies within a
generation and from one generation to the next.
i. Example: Genotype: Aa – Allele frequencies: A = 0.03, a = 0.07
b. The Hardy-Weinberg equilibrium is the stable frequency distribution of genotypes A/A, A/a,
and a/a, in the proportions of p2, 2pq, and q2 respectively (where p and q are the frequencies of
alleles A and a), that is a consequence of random mating in the absence of mutation, migration,
natural selection, or random drift.
8. Hardy Weinberg
a. A simple binomial equation that describes the precise relationships between allele, genotype and
phenotype frequencies.
b.
p2 + 2pq + q2 = 1
c. In population genetics – the Punnett square no longer represents one mating. The Punnett square
represents GAMETE frequencies produced by the whole population.
d. First – determine allele frequency in your defined population.
i. Define genotype.
ii. Count individuals with each genotype.
iii. Consider total alleles represented by genotypes (remember each individual has TWO!)
iv. Divide by total alleles (number of individuals multiplied by two).
9. PREDICTING THE FREQUENCY OF ALBINISM
a. The Hardy Weinberg equation enables us to use information on genotype and allele frequencies
to predict the genotype frequency of the next generation.
Question: in the next generation of a certain population, how many will be albinos?
b. Suppose for example that in a population of 100,000, consider people carrying the recessive
allele for albinism (a) - there are
i. 100 aa albinos
ii. 1800 Aa heterozygous carriers.
c. What will be the frequency of heterozygous carriers in the next generation?
d. COMPUTE THE ALLELE FREQUENCIES IN THE PARENT POPULATION.
e. Total alleles: __________________________
f. _________________________ AA individuals
g. _________________________Aa individuals
h. _________________________ aa individuals
i. Frequency of A allele:
j. Frequency of the “a” allele:
k. The HW equation for the albino gene in this population is:
p2 + 2pq + q2 = 1
l. To find what the frequency of heterozygous carriers will be in the next generation of 100,000
people will be:
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Bio 110
Lecture 012 Population Genetics Student Notes 012sr2
Beavers
_______________________________ Aa individuals
_______________________________ AA individuals
_______________________________ aa individuals
m. This example shows that in one generation, the genotypic frequencies have changed.
n. Frequencies of genotypes have changed, but not allele frequencies. Why? Conservation of allele
proportions. Population must remain LARGE – no alleles lost or gained.
10. CAUSES OF ALLELE FREQUENCY CHANGES
a. What Changes Allele Frequencies?
i. Migration, mutation, genetic drift, non-random mating, fitness and natural selection.
b. MUTATION: The process that produces a gene or a chromosome set differing from that of the
wild type. Heritable alterations in DNA sequence.
c. MUTATION RATE (μ): probability that a copy of an allele changes to some other allelic form in
one generation.
d. The ultimate source of variation is _______________________: occur rarely and by chance.
Mutations can be _______________________, ______________________ or helpful.
11. SOURCES OF VARIATION
a. MIGRATION: any form of the introduction of genes from one population into another.
b. The change in allele frequency is proportional to the difference in frequency between the
recipient population and the average of the donor population.
c. Unlike spontaneous mutation rate, the migration rate (m) can be large.
d. If the difference in allele frequency between the donor and the recipient population is large, the
change in frequency may be substantial.
12. CHANGES TO ALLELE FREQUENCY: Genetic Drift
a. GENETIC DRIFT: Unpredictable, chance fluctuations in allele frequency that have a neutral
affect on fitness of a population.
b. A change in allele frequencies as a consequence of the RANDOMNESS of inheritance from one
generation to the next.
c. FIXED: describes a population in which no further changes in allele frequency can occur (unless
a mutation or migration was introduced).
13. CHANGES TO ALLELE FREQUENCY: A type of genetic drift: Founder Effect
a. FOUNDER EFFECT: variation of genetic drift, occurring when a few individuals separate from
a larger population and establish a new one that is isolated from the original population, resulting
in altered allele frequencies in the new population.
b. Example: Ellis-van Creveld syndrome.
14. POPULATION BOTTLENECK – an event that leads to a founder population.
a. Population bottleneck: Occurs when a large proportion of individuals in a population die, often
because of environmental disturbances. This results in survivors who are essentially equivalent
to a _____________________ population.
b. Northern elephant seal hunted almost to extinction in the 1890’s. 20 survived at one point.
c. Dominant males typically monopolize breeding.
d. Number has increased to 30,000, but all are almost genetically identical.
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Bio 110
Lecture 012 Population Genetics Student Notes 012sr2
Beavers
15. NON-RANDOM MATING
a. In some species, only a few dominant males gain reproductive access to females.
b. Fertilization typically follows some sort of contest among males.
c. In some species, mating is not random because one sex, usually the female, controls mate
selection and is quite picky. Males must display their virtues.
16. RANDOM and NON-RANDOM MATING
a. ENDOGAMY: Mating between individuals within a group or subgroup, rather than at random in
a population.
b. Non-random associations in the human species as a whole result in large differences in genotype
frequencies from group to group.
c. The human species combined does not follow HW equilibrium.
17. INBREEDING
a. INBREEDING: Mating between relatives.
b. ENFORCED OUTBREEDING: Deliberate avoidance of mating between relatives.
c. NEGATIVE INBREEDING: Preferential mating between individuals who are unrelated.
18. ASSORTATIVE MATING
a. POSITIVE ASSORTATIVE MATING: A situation in which like phenotypes mate more
commonly than expected by chance.
b. NEGATIVE ASSORTATIVE MATING: Preferential mating between phenotypically unlike
partners.
c. NOTE: Assortative mating is specific to a particular phenotype, inbreeding relates to the entire
genome.
19. FITNESS: VIABILITY AND REPRODUCTIVE SUCCESS
a. VIABILITY: The probability that a fertilized egg will survive and develop into an adult
organism.
b. The fitness of individuals who possess variations that help them to survive and reproduce is
relatively high. Those without adaptive variations have a fitness that is relatively low.
20. NATURAL SELECTION ACTS ON FITNESS TO ALTER ALLELE FREQUENCY
a. FITNESS: the relative advantage or disadvantage in reproduction that a particular genotype
provides to members of a population in comparison to alternative genotypes at the same locus.
b. Natural selection acts on differences in fitness to alter allele frequencies.
21. NATURAL SELECTION AT WORK TODAY
a. SELECTION: The production of different average numbers of offspring by different genotypes
in a population as a result of the different phenotypic properties of those genotypes.
b. NATURAL SELECTION: In nature, the process that progressively eliminates individuals whose
fitness is low and chooses individuals of high fitness to survive and become parents of the next
generation.
c. NATURAL SELECTION CAN LEAD TO PESTICIDE RESISTANCE.
d. Florida homeowners and roaches.
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Bio 110
Lecture 012 Population Genetics Student Notes 012sr2
Beavers
22. SELECTION
a. ARTIFICIAL SELECTION: The purposeful control of mating by choice of parents for the next
generation.
23. HETEROZYGOUS ADVANTAGE
a. Many recessive diseases persist in human populations at low but stable frequencies. Why?
b. Heterozygote Advantage: the situation where heterozygotes have a higher fitness than either
homozygote.
c. Sickle cell anemia example.
24. SOLVING A HW PROBLEM
a. Consider the inherited condition of phenylketonuria (PKU), AR, eliminates activity of the
enzyme that converts the amino acid phenylalanine to the amino acid tyrosine. This build up
interferes with normal brain development. Medical studies show that about 1/3600 Caucasians in
the U.S. have PKU. How can we figure out the allele frequency from this?
25. SOLVING A HW PROBLEM
a. A population called the “founder generation” consisting of 2000 AA individuals, 2000 Aa
individuals, and 6000 aa individuals is established on a remote island. Mating within this
population occurs at random, the three genotypes are selectively neutral, and mutations occur at a
negligible rate.
b. What are the frequencies of alleles A and a in the founder generation?
c. Is the founder generation at Hardy-Weinberg equilibrium?
d. What is the frequency of the A allele in the second generation (that is the generation subsequent
to the founder generation)?
e. What are the frequencies for the AA, Aa, and aa genotypes in the second generation.
f. Is the second generation at Hardy-Weinberg equilibrium?
g. What are the frequencies for the AA, Aa and aa genotypes in the third generation?
26. SOLVING A HW PROBLEM
a. Brachydactyly: a condition that causes people to have short, stubby fingers. If a group of BB
persons homozygous for the allele causing short, stubby digits were all to conceive children with
bb mates homozygous for the allele determining normal finger length, all the progeny will be the
Bb heterozygotes showing the brachydactyly phenotype.
b. Even though the phenotype and genotype frequencies have changed drastically, the allele
frequencies have NOT.
27. Will not cover
a. 4th edition: pg. 662-664. Will not cover pages 666 starting at Modifications to Hardy-Weinberg
through page 682. DNA analysis and 9/11 victim identification on page 681 is worth reading but
will not be on the exam.
b. 5th edition: pgs. 674-685 starting at Adding Selection to Hardy-Weinberg predictions.
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