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Powerpoint Lecture Outline
Human Genetics
Concepts and Applications
Eighth Edition
Ricki Lewis
Prepared by
Dubear Kroening
University of
Wisconsin-Fox Valley
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Chapter 14
When Allele Frequencies Stay
Constant
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Population
• Is an interbreeding group of the same species
within a given geographical area
• Gene pool  the collection of all alleles in the
members of the population
• Population genetics  the study of the genetics
of a population and how the alleles vary with time
• Gene Flow  alleles can move between
populations when individuals migrate and mate
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Allele Frequencies
Allelic
Frequency
# of particular allele
total # of alleles in the population
• Count both chromosomes of each individual
• Allele frequencies affect the genotype
frequencies
– The frequency of each type of
homozygote and heterozygote in the
population
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Phenotype Frequencies
• Frequency of a trait varies in different populations
Table 14.1
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Microevolution
• Genetic change due to changing allelic frequencies in
populations
• Allelic frequencies can change when:
– Nonrandom mating
– Gene flow
– Genetic drift
– Mutation
– Natural selection (unequal reproductive success)
• Macroevolution
– The formation of new species
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Microevolution
• When the relative frequencies of alleles
in a population change over a number
of generations, evolution is occurring on
its smallest scale (microevolution)
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There are several potential causes of microevolution
• Genetic drift is a change in a gene pool due to chance
(smaller populations)
– Genetic drift can lead to the founder Effect and cause the
bottleneck effect
• Gene flow can change a gene pool due to the
movement of genes into or out of a population
• Mutation changes alleles
• Nonrandom mating
• Natural selection leads to differential reproductive
success
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Genetic Drift
• Natural selection is not the only source of
evolutionary change.
• The smaller a population is, the farther the
results may be from what the laws of probability
predict. This kind of random change in allele
frequency is called genetic drift.
• How does genetic drift take place?
– In small populations, individuals that carry a particular
allele may leave more descendants than other
individuals do, just by chance.
– Over time, a series of chance occurrences of this type
can cause an allele to become common in a
population.
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Genetic Drift
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
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Hardy-Weinberg Equilibrium:
• If a population’s gene pool
remains constant, then the
population will not evolve.
(Hardy-Weinberg Equilibrium)
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Hardy-Weinberg Equilibrium
• Developed by mathematicians
• A condition in which allele frequencies
remain constant
• Used algebra to explain how allele
frequencies predicts genotype and
phenotype frequencies in equilibrium
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Hardy-Weinberg principle
• The Hardy-Weinberg principle states that
allele frequencies in a population will
remain constant unless one or more
factors cause those frequencies to
change.
• The situation in which allele frequencies
remain constant is called genetic
equilibrium (juh-net-ik ee-kwih-lib-ree-um).
• If the allele frequencies do not change, the
population will not evolve.
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Five conditions are required for
Hardy-Weinberg equilibrium
Evolution v/s Equilibrium
• Five conditions are
required to
maintain genetic
equilibrium from
generation to
generation
• The population is very
large
• The population is isolated
• Mutations do not alter the
gene pool
• Mating is random
• All individuals are equal
in reproductive success
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Hardy-Weinberg Equation:
• Used to calculate the frequency of
alleles
p2 + 2pq + q2 = 1
• Frequency of WW + Frequency of Ww + Frequency of
ww = 1
• The combined frequencies of all
alleles must be 100%
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Hardy-Weinberg Equilibrium
p = allele frequency of one allele
q = allele frequency of a second allele
p+q=1
All of the allele frequencies together equals 1
or the whole collection of alleles
p2 + 2pq + q2 = 1
All of the genotype frequencies
together equals 1
p2 and q2
genotype frequencies for each homozygote
2pq
genotype frequency for heterozygotes
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Figure 14.3
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Hardy-Weinberg Equilibrium
Generation 1
p allele frequency of D normal finger length = .7
q allele frequency of d short middle finger = .3
Genotype frequencies
DD
p2 = (.7)2 = .49
Gamete
frequencies
Dd
2pq = 2 (.7)(.3) = .42
.49
Frequency D gamete = .7
.21
dd
q2 = (.3)2= .09
.21
.09
frequency d gamete = .3
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Hardy-Weinberg Equilibrium
Figure 14.4
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Hardy-Weinberg Equilibrium
Figure 14.4
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Table 14.2
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The Hardy-Weinberg equation is useful in public health
science
• Public health scientists use the HardyWeinberg equation to estimate
frequencies of disease-causing alleles in
the human population
– Example: phenylketonuria (PKU)
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The evolution of antibiotic resistance in bacteria is a serious public
health concern
• The excessive use of antibiotics is
leading to the evolution of antibioticresistant bacteria
– Example:
Mycobacterium
tuberculosis
– MRSA
Figure 13.22
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Adaptive change results when natural selection upsets
genetic equilibrium
• Natural selection results in the
accumulation of traits that adapt a
population to its environment
– If the environment should change, natural
selection would favor traits adapted to the
new conditions
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VARIATION AND NATURAL SELECTION
Variation is extensive in most populations
• Phenotypic variation may be
environmental or genetic in origin
– But only genetic changes result in
evolutionary adaptation
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How natural selection affects variation
• Natural selection tends to reduce
variability in populations
– The diploid condition preserves variation by
“hiding” recessive alleles
– Balanced polymorphism may result from the
heterozygote advantage
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Applying Hardy-Weinberg Equilibrium
• Used to determine carrier probability
• Homozygous recessive used to
determine frequency of allele in
population (phenotype is genotype)
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Calculating the Carrier Frequency of
an Autosomal Recessive
Figure 14.5
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Table 14.3
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Calculating Risk with X-linked Traits
Females:
p2 + 2pq + q2 = 1
Males:
p+q=1
All of the women
in the population
All of the men
in the population
Hemophilia is X-linked and occurs in 1 in 10,000 males
p = 1/10,000 = .0001
therefore
q = .9999
Carrier females
= 2pq = 2 (.0001) (.9999)
= .0002
1 in 5000 are carriers
Affected females = p2 = (.0001) 2 1 /100 million women
= .00000001
will have hemophilia
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Calculating Carrier Frequency for
X-linked Traits
Figure 14.6
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DNA Profiling
• Hardy-Weinberg equilibrium applies to portions
of the genome that do not affect phenotype
• They are not subject to natural selection
• Short repeated segments that are not protein
encoding, distributed all over the genome
• Detects differences in repeat copy number
• Calculates probability that certain combinations
can occur in two sources of DNA
• Requires molecular techniques and population
studies
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Table 14.4
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Comparing DNA Repeats
Figure 14.7
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DNA Profiling
• Developed in 1980s
• Identifies individuals
• Used in forensics, agriculture, paternity
testing, and historical investigations
• DNA can be obtained from many
sources
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Figure 14.8
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DNA Identification
Figure 14.9
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A Sneeze Identifies Art Thief
Table 14.6
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DNA Profiling Techniques
•
(1) A blood sample is collected from
suspect
•
(2) White blood cells release DNA
•
(3) Restriction enzymes cut DNA
•
(4) Electrophoresis aligns fragments by
size
•
(5) Pattern of DNA fragments transferred to
a nylon sheet
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DNA Profiling Techniques
•
(6) Exposed to radioactive probes
•
(7) Probes bind to DNA
•
(8) Sheet placed against X ray film
•
(9) Pattern of bands constitutes DNA profile
•
(10) Identify individuals
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Box Figure 14.1
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Comparing DNA Sequences
Figure 14.10
Figure 14.10
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DNA Profiles
• Databases are essential for statistical
analysis
• Nuclear and mitochondrial DNA can be used
• Very small amounts of DNA can be used
• Recent examples of large scale analysis
– World Trade Center victims
– 2004 Asian tsunami disaster
– Hurrican Katrina
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Genetic Privacy
• DNA dragnets
• Health care concerns
• Health Insurance Portability and
Accountability Act (HIPAA)
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