Human Genetics
Mariam55Chapter 14 Constant Allele Frequencies
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Introduction to allele frequencies
population
interbreeding group of same species in given geographical area
gene pool
collection of all alleles in members of the population
population genetics
study of the genetics of a population and how the alleles vary with time
gene flow
movement of alleles between populations when people migrate and mate
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Allele frequencies in a population
counts both chromosomes of each individual
allele frequencies affect the genotype frequencies
frequency of homozygotes/heterozygotes in the population
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Phenotype Frequencies
Frequency of a trait varies in different populations
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Microevolution in populations
changing allelic frequencies in populations
factors that change genotypic frequencies
nonrandom mating
migration
genetic drift
mutation
natural selection
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Macroevolution of a species
formation of new species over time
enough microevolutionary changes have occurred
prevent individuals in a population in producing fertile offspring with others
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Hardy-Weinberg Equation
allele frequencies predict genotypic frequencies
genotypic predicts phenotypic frequencies
population of diploid, sexually-reproducing species
disproves assumption that
dominant traits would become more common
recessive traits would become rarer
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proportion of
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Hardy-Weinberg Equation p = allele frequency of one allele q = allele frequency of a second allele
All of the allele frequencies together equals 1
All of the genotype frequencies together equals 1
Frequencies for each homozygote
Frequency for heterozygotes
p + q = 1
p2 + 2pq + q2 = 1
p2 and q2
2pq
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Source of the Hardy-Weinberg Equation
Figure 14.3
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Hardy-Weinberg Equilibrium
allele and genotypic frequencies do not change from one generation to the next
this gene is in Hardy-Weinberg equilibrium
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Applying the Hardy-Weinberg Equation
determine carrier probability
in autosomal recessive diseases
genotype of affected is known, homozygous recessive
frequency of homozygous recessive = q2 so √q2 = q
frequency of allele p = 1-q
heterozygote is 2(1-q)q
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Predictive counseling
What is the probability that two unrelated Caucasians will have an affected child?
Probability both are carriers = 1/23 x 1/23 = 1/529
Probability that their child has CF = ¼
if they are carriers
Probability = 1/529 x 1/4 = 1/2116
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Calculating risk of X-linked traits
for females with two X chromosomes
standard Hardy-Weinberg
p2 + 2pq + q2 = 1
for males with one X chromosome
allele frequency is phenotypic frequency
p + q = 1
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Figure 14.6
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DNA in Hardy-Weinberg equilibrium
not just protein-coding genes
equilibrium is unusual for protein-encoding genes that affect phenotype
portions of genome do not influence phenotype
not subject to selection
repeated DNA segments
transposons
CNVs
microsatellites
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DNA Profiling of noncoding DNA
detects differences in repeat copy number
calculates probability of identity
profiles can occur in two sources of DNA by chance
valuable in excluding a suspect
cannot include a suspect
considered along with other types of evidence
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Comparing DNA Profiles
Jeffreys used his technique to demonstrate that Dolly was truly a clone of the 6-year old ewe that donated her nucleus
Figure 14.9
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CODIS
Probability that any two individuals have same thirteen markers is 1 in 250 trillion Figure 14.10
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Population statistics to interpret DNA profiles
polymorphic repeats in different chromosomes
number of copies of a repeat = allele
probabilities based on observed frequency
population-specific probabilities
product rule to calculate probability of allele combination
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Challenges to DNA Profiling
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Recent examples of large-scale disasters
World Trade Center attack (2001)
Indian Ocean Tsunami (2004)
Hurricane Katrina (2005)
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Chapter 15 Changing Allele Frequencies
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Allele frequencies in populations
allele frequency
population estimate of specific allele
e.g., number of sickle HbS alleles in a population
all alleles add to 1
multiple alleles for same gene, HbS + HbN + Hb? = 1
each individual has two copies
Hardy-Weinberg equilibrium
allele frequencies from generation to generation
no change in proportions of allele frequencies
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Allele frequencies in equilibrium
Hardy-Weinberg monitors allele frequencies
alleles contribute to genotypes
genotypes contribute to phenotypes
stable populations = stable allele frequencies
adaptation to the environment
alleles needed to survive/reproduce
result of evolution = no further change needed
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Allele frequencies out of equilibrium
population conditions can change allele frequencies
change in alleles change in phenotypes
nonrandom mating
migration
genetic drift
mutation
natural selection
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Nonrandom Mating
random mating
individuals of any genotype are likely to mate with any other genotype
genotype phenotype
nonrandom mating
individuals with specific genotype are more likely to mate with others of specific genotype
genotype phenotype
traits that influence mate choice
physical appearance
ethnic or religious preferences
intelligence and shared interests
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Nonrandom mating alters allele frequencies
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genotype phenotype
mating more often contributes more alleles to population
e.g., Mongolian DNA in China
mating less often removes alleles from population
e.g., family planning for Tay-Sachs dz
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Examples of nonrandom mating
consanguinity
marriage between relatives that share a recent ancestor (“blood” relative)
increases proportion of homozygotes in a population
endogamy
marriage within the same community
increases likelihood of shared ancestors
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Migration
individuals migrate and move alleles
addition or removal of alleles alters frequencies
geographical or language barriers
few isolated populations
introduction of new alleles
genetic admixture
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Migration alters allele frequencies
clines
allele frequencies change from neighbor population
immigrants introduce alleles, emigrants remove
e.g., prevalence of galactokinase deficiency
autosomal recessive disease that causes blindness
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introduction of galactokinase deficiency follows the migration of transient populations of Roma people throughout Europe
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Genetic Drift
random fluctuations in allele frequencies
generation to generation, happen by chance
favorable reproduction
random and unpredictable
effects can be accelerated when
population becomes very small
sampling changes allele frequencies
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Genetic drift alters allele frequencies
genetic drift happens all the time
random mating, random mutation, etc.
usually evens out over generations
small populations enhance effect of genetic drift
random events are more common, more obvious
natural disaster (bottleneck effect)
migration (founder effect)
geographical barrier (founder effect)
behavior (founder effect)
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Founder effect on genetic drift
samples larger population
small group leaves to found new population
new population relies on alleles of founders
different allele frequencies than the original population
lack some alleles, and/or high frequency of others
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The U.S. was founded by a small group of British citizens with extraordinary behavior: independence, disdain for authority, and self-righteousness.
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Bottleneck effect on genetic drift
large population drastically reduced in size
natural disaster, war, illness, etc.
reduction is NOT due to genotype/selection
population size rebounds with survivors
descendants of limited number of alleles
new population with restricted gene pool
loss of genetic diversity
less ability to adapt to change
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Mutation
mutations create new genetic variation
convert one allele to another
introduces new alleles
genetic variation arises from
mutation
crossing over/independent assortment
duplications/copy number variants (CNVs)
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Mutations alter allele frequencies
harmful alleles eliminated by natural selection
if they affect reproduction
harmful recessive alleles remain in population
maintained in heterozygotes
reintroduced by mutations
genetic load
all recessive deleterious alleles in a population
mutations are rare and most are silent
small effect on reproduction and/or survival
more effect in small population size or selection
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Natural Selection
differential survival and/or reproduction
individuals with a particular genotype/phenotype
changes allele frequencies
negative selection against a trait
removes negative alleles from population
positive selection for a trait
maintains positive alleles in population
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Artificial Selection
controlled breeding
intent of concentrating particular phenotype
e.g., crops, pets
nonintentional breeding of particular phenotype
e.g., antibiotic resistance in bacteria
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Balanced Polymorphism
natural selection can remove harmful recessive alleles through homozygotes
alleles replaced by new mutation
alleles persist in heterozygotes
harmful recessive condition can be prevalent
heterozygote has health/reproductive advantage
heterozygote advantage maintains recessive, disease-causing allele in a population
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Balanced polymorphism maintains allele
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heterozygote advantage reduces mortality to other diseases
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Heterozygote advantage: Sickle Cell trait
carrier of sickle cell hemoglobin
resistant to malaria, or develop very mild cases
sickle cell trait blood is thicker than normal
blocks parasite infection
prevents parasite from growing
blocks spread of infection
other hemoglobinopathies have similar protection
alpha thalassemia and hemoglobin C block the parasites
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Heterozygote advantage: cystic fibrosis
carrier of abnormal CF chloride channel protein
resistance to diarrheal diseases
cholera toxin opens normal chloride channels
produces severe diarrhea and dehydration
abnormal channels do not open/do not transport
typhoid fever requires functional CFTR for bacteria to enter the cell
abnormal channel prevents infection
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Forces that Change Allele Frequencies
Figure 15.16
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Eugenics manipulates population alleles
control of human reproduction
intent to change a population’s genetic structure
positive eugenics promotes reproduction among those considered superior
negative eugenics interferes with reproduction of those judged inferior
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A Chronology of Eugenics-Related Events
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