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14
Population Genetics
and Evolution
Population Genetics
• Population genetics
involves the application of
genetic principles to entire
populations of organisms
• Population = group of organisms of the same
species living in the same geographical area
• Subpopulation = small population unit
• Gene pool = all alleles in population
Population Genetics
• Genotype frequency = proportion of
individuals in a population with a
specific genotype
• Allele frequency = proportion of
alleles in a population
Hardy-Weinberg Principle
• Hardy Weinberg (HW) Principle analyzes
the factors which may affect the
frequencies of alleles in a population
• HW principle uses a simple equation to
calculate allelic frequencies:
P = frequency (f) of all alleles = 1
p = (f) dominant allele
q = (f) recessive allele p + q =1
Hardy Weinberg Principle
HW Principle states that allelic frequencies
will remain constant over time if the
following conditions are met:
• Mating is random
• Allelic frequencies are the same in males
and females
• All genotypes have equivalent viability
and fertility
Hardy Weinberg Principle
• Mutation does not occur
• Migration into the population is absent
• Population is large so that allelic
variations do not occur by chance
These idealized conditions are never met,
but HW principle permits analysis of
mechanisms responsible for changes in
allelic frequencies
Hardy Weinberg Principle
• HW equation can be used to calculate
genotype frequencies in a population:
pp = homozygous dominant genotype
qq = recessive genotype
pq = heterozygous genotype
Since p+q = 1,
binomial expansion:
p2 + 2pq + q2 = 1
Hardy Weinberg Principle
p2 = pp = frequency of homozygous
dominant genotype
2pq = frequency of heterozygous
genotype
q2 = frequency of recessive genotype
• Fixed allele: frequency = 1
• Lost allele: frequency = 0
• Rare alleles mostly heterozygous
Hardy Weinberg Principle
• Phenotype = genotype = q2 in recessive
individuals; q2 is used to calculate the
frequencies of the homozygous dominant
(p2) and heterozygous individuals (2pq)
• HW equation can be used to determine the
frequency of recessive disease alleles in a
population and carrier frequency
Hardy-Weinberg Principle
•
Hardy-Weinberg frequencies can be
extended to multiple alleles:
-- Frequency of any homozygote=square
of allele frequency
-- Frequency of any heterozygote = 2 X
product of allele frequencies
Hardy-Weinberg Principle
• X-linked genes are a special case because
males have only one X-chromosome
• Genotype frequencies among males are the
same as allele frequencies:
Frequency of H males = p
Frequency of h males = q
DNA Typing
• DNA typing involves the use of
polymorphisms to link individuals with
tissue samples
• Highly polymorphic genes are used in
DNA typing
• Polymorphic alleles may differ in
frequency among subpopulations =
population substructure
• DNA exclusions are definitive
Allelic Variation
• Allelic variation may result from differences
in the number of units repeated in tandem =
simple tandem repeat polymorphism (STRP)
• STRPs can be used to map DNA since they
generate RFLPs which can be detected by
Southern blot analysis
Genetic Inbreeding
• Inbreeding means mating
between relatives
• Inbreeding results in an excess of
homozygotes compared with random mating
• In most species, inbreeding is harmful due to
rare recessive alleles that would not
otherwise become homozygous
Genetics and Evolution
• Evolution refers to changes in the gene
pool resulting from mutations which
produce phenotypic changes subject to
the forces of natural selection
• Natural selection refers to environmental
interaction with phenotypic variants to
select for those with reproductive
advantage
Evolution
Processes which result in the
formation of new species include:
• Mutation: origin of new genotypes
• Migration: movement among subpopulations
• Natural selection: results in adaptation
• Random genetic drift: changes
in allele frequencies
Natural Selection
• Natural selection is the driving force of
adaptive evolution and is a consequence
of the hereditary differences among
organisms and their ability to survive in
the surrounding environment
• Adaptation: progressive genetic
improvement in populations due to natural
selection
Natural Selection
Natural selection depends on the following
principles:
• More organisms are produced than can
survive and reproduce
• Organisms differ in their ability to survive
and reproduce, based on genotypic
differences
• The genotypes that promote survival are
favored and are reproduced
Fitness
• Fitness is the relative ability of genotypes
to survive and reproduce
• Relative fitness measures the comparative
contribution of each parental genotype to
the pool of offspring genotypes in each
generation
• Selection coefficient refers to selective
disadvantage of genotype
Allelic Selection
• Frequency of very common or rare alleles
changes very slowly
• Selection for or against very rare
recessive alleles is inefficient
• Lethal genotypes are not passed on to the
next generation and their frequency
decreases over time
• Frequency of favored genotypes
increases over time
Selection in Diploids
• Frequency of favored
dominant allele changes
slowly if allele is common
• Frequency of favored recessive allele
changes slowly if the allele is rare
• Rare alleles are found most frequently in
heterozygotes and when favored allele is
dominant recessive alleles in heterozygotes
are not exposed to natural selection
Selection in Diploids
• Selection for or against a recessive allele
is very inefficient when the recessive
allele is rare
• This accounts for the persistence of rare
recessive disease causing alleles in the
human population in heterozygote
carriers, even if they are lethal when
homozygous
Selection in Diploids
• Selection can be balanced by new
mutations
• New mutations often generate harmful
alleles and prevent their elimination from
the population by natural selection
• Some changes in allele frequency are
random due to genetic drift among
subpopulations
Heterozygote Superiority
• Heterozygote superiority = fitness
(measurement of viability and fertility) of
heterozygote is greater than both
homozygotes
• In sickle cell anemia overdominance of the
heterozygote carriers is observed because
they are more resistant to malaria
Maternal Inheritance
• Maternal inheritance refers to the
transmission of genes only through the
female
• In higher animals, mitochondrial DNA
shows maternal inheritance
• Mitochondria are maternally inherited
because the egg is the major contributor
of cytoplasm to the zygote
Maternal Inheritance
• Males do no not transmit mitochondria
DNA to offspring
• Some rare genetic disorders are the result
of mutations in mitochondrial DNA and are
transmitted from mother to all offspring
• Recombination does not occur in
mitochondrial DNA making it a good
genetic marker for human ancestry
Phylogenetic Tree
• Phylogenetic Tree represents a
depiction of the lines of descent
connecting mitochondrial DNA
inheritance patterns
• mtDNA maternally inherited
• Greatest mtDNA diversity in African
populations