Transcript of Population genetics with qs
- Population Genetics
- The Gene Pool
- Members of a species can interbreed & produce fertile
offspring
- Species have a shared gene pool
- Gene pool all of the alleles of all individuals in a
population
- The Gene Pool
- Different species do NOT exchange genes by interbreeding
- Different species that interbreed often produce sterile or less
viable offspring e.g. Mule
- Populations
- A group of the same species living in an area
- No two individuals are exactly alike (variations)
- More Fit individuals survive & pass on their traits
- Speciation
- One species may split into 2 or more species
- A species may evolve into a new species
- Requires very long periods of time
- Modern Evolutionary Thought
- Modern Synthesis Theory
- Combines Darwinian selection and Mendelian inheritance
- Population genetics - study of genetic variation within a
population
- Emphasis on quantitative characters
- Modern Synthesis Theory
- 1940s comprehensive theory of evolution (Modern Synthesis
Theory)
- Introduced by Fisher & Wright
- Until then , many did not accept that Darwins theory of natural
selection could drive evolution
S. Wright A. Fisher
- Modern Synthesis Theory
- Todays theory on evolution
- Recognizes that GENES are responsible for the inheritance of
characteristics
- Recognizes that POPULATIONS , not individuals, evolve due to
natural selection & genetic drift
- Recognizes that SPECIATION usually is due to the gradual
accumulation of small genetic changes
- Microevolution
- Changes occur in gene pools due to mutation, natural selection,
genetic drift, etc.
- Gene pool changes cause more VARIATION in individuals in the
population
- This process is called MICROEVOLUTION
- Example: Bacteria becoming unaffected by antibiotics
(resistant)
- Allele Frequencies Define Gene Pools As there are 1000 copies
of the genes for color, the allele frequencies are (in both males
and females): 320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8
(80%) R 160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r
500 flowering plants 480 red flowers 20 white flowers 320 RR 160 Rr
20 rr
- Species & Populations
- Population - a localized group of individuals of the same
species.
- Species - a group of populations whose individuals have the
ability to breed and produce fertile offspring.
- Individuals near a population center are, on average, more
closely related to one another than to members of other
populations.
- Gene Pools
- A populations gene pool is the total of all genes in the
population at any one time.
- If all members of a population are homozygous for a particular
allele, then the allele is fixed in the gene pool.
- The Hardy-Weinberg Theorem
- Used to describe a non-evolving population.
- Shuffling of alleles by meiosis and random fertilization have
no effect on the overall gene pool.
- Natural populations are NOT expected to actually be in
Hardy-Weinberg equilibrium .
- The Hardy-Weinberg Theorem
- Deviation from Hardy-Weinberg equilibrium usually results in
evolution
- Understanding a non-evolving population, helps us to understand
how evolution occurs
.
- Assumptions of the H-W Theorem
- Large population size - small populations can have chance
fluctuations in allele frequencies ( e.g. , fire, storm).
- No migration - immigrants can change the frequency of an allele
by bringing in new alleles to a population.
- No net mutations - if alleles change from one to another, this
will change the frequency of those alleles
- Assumptions of the H-W Theorem
- Random mating - if certain traits are more desirable, then
individuals with those traits will be selected and this will not
allow for random mixing of alleles.
- No natural selection - if some individuals survive and
reproduce at a higher rate than others, then their offspring will
carry those genes and the frequency will change for the next
generation.
- Hardy-Weinberg Equilibrium The gene pool of a non-evolving
population remains constant over multiple generations; i.e. , the
allele frequency does not change over generations of time. The
Hardy-Weinberg Equation: 1.0 = p 2 + 2 pq + q 2 where p 2 =
frequency of AA genotype; 2 pq = frequency of Aa plus aA genotype;
q 2 = frequency of aa genotype
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- But we know that evolution does occur within populations .
- Evolution within a species/population = microevolution.
- Microevolution refers to changes in allele frequencies in a
gene pool from generation to generation. Represents a gradual
change in a population.
- Causes of microevolution :
- Natural selection (1 & 2 are most important)
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- Genetic drift = the alteration of the gene pool of a small
population due to chance.
- Two factors may cause genetic drift:
- Bottleneck effect may lead to reduced genetic variability
following some large disturbance that removes a large portion of
the population. The surviving population often does not represent
the allele frequency in the original population.
- Founder effect may lead to reduced variability when a few
individuals from a large population colonize an isolated
habitat.
- *Yes, I realize that this is not really a cheetah.
- 2) Natural selection As previously stated, differential success
in reproduction based on heritable traits results in selected
alleles being passed to relatively more offspring (Darwinian
inheritance). The only agent that results in adaptation to
environment. 3) Gene flow -is genetic exchange due to the migration
of fertile individuals or gametes between populations.
- 4) Mutation Mutation is a change in an organisms DNA and is
represented by changing alleles. Mutations can be transmitted in
gametes to offspring, and immediately affect the composition of the
gene pool. The original source of variation.
- Genetic Variation, the Substrate for Natural Selection Genetic
(heritable) variation within and between populations: exists both
as what we can see ( e.g. , eye color) and what we cannot see (
e.g. , blood type). Not all variation is heritable. Environment
also can alter an individuals phenotype [ e.g. , the hydrangea we
saw before, and Map butterflies (color changes are due to seasonal
difference in hormones)].
- Variation within populations Most variations occur as
quantitative characters ( e.g. , height); i.e. , variation along a
continuum, usually indicating polygenic inheritance. Few variations
are discrete ( e.g. , red vs . white flower color). Polymorphism is
the existence of two or more forms of a character, in high
frequencies, within a population. Applies only to discrete
characters.
- Variation between populations Geographic variations are
differences between gene pools due to differences in environmental
factors. Natural selection may contribute to geographic variation.
It often occurs when populations are located in different areas,
but may also occur in populations with isolated individuals.
- Geographic variation between isolated populations of house
mice. Normally house mice are 2n = 40. However, chromosomes fused
in the mice in the example, so that the diploid number has gone
down.
- Cline , a type of geographic variation, is a graded variation
in individuals that correspond to gradual changes in the
environment. Example: Body size of North American birds tends to
increase with increasing latitude. Can you think of a reason for
the birds to evolve differently? Example: Height variation in
yarrow along an altitudinal gradient. Can you think of a reason for
the plants to evolve differently?
- Mutation and sexual recombination generate genetic variation a.
New alleles originate only by mutations (heritable only in gametes;
many kinds of mutations; mutations in functional gene products most
important). - In stable environments, mutations often result in
little or no benefit to an organism, or are often harmful. -
Mutations are more beneficial (rare) in changing environments.
(Example: HIV resistance to antiviral drugs.) b. Sexual
recombination is the source of most genetic differences between
individuals in a population. - Vast numbers of recombination
possibilities result in varying genetic make-up.
- Diploidy and balanced polymorphism preserve variation a.
Diploidy often hides genetic variation from selection in the form
of recessive alleles. Dominant alleles hide recessive alleles in
heterozygotes. b. Balanced polymorphism is the ability of natural
selection to maintain stable frequencies of at least two
phenotypes. Heterozygote advantage is one example of a balanced
polymorphism, where the heterozygote has greater survival and
reproductive success than either homozygote (Example: Sickle cell
anemia where heterozygotes are resistant to malaria).
- Frequency-dependent selection = survival of one phenotype
declines if that form becomes too common. (Example: Parasite-Host
relationship. Co-evolution occurs, so that if the host becomes
resistant, the parasite changes to infect the new host. Over the
time, the resistant phenotype declines and a new resistant
phenotype emerges.)
- Neutral variation is genetic variation that results in no
competitive advantage to any individual. - Example: human
fingerprints.
- A Closer Look: Natural Selection as the Mechanism of Adaptive
Evolution Evolutionary fitness - Not direct competition, but
instead the difference in reproductive success that is due to many
variables. Natural Selection can be defined in two ways: a.
Darwinian fitness - Contribution of an individual to the gene pool,
relative to the contributions of other individuals. And,
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- - Contribution of a genotype to the next generation, compared
to the contributions of alternative genotypes for the same
locus.
- Survival doesnt necessarily increase relative fitness; relative
fitness is zero (0) for a sterile plant or animal.
- Three ways (modes of selection) in which natural selection can
affect the contribution that a genotype makes to the next
generation.
- a. Directional selection favors individuals at one end of the
phenotypic range. Most common during times of environmental change
or when moving to new habitats.
- Directional selection
- Diversifying selection favors extreme over intermediate
phenotypes. - Occurs when environmental change favors an extreme
phenotype. Stabilizing selection favors intermediate over extreme
phenotypes. - Reduces variation and maintains the current average.
- Example = human birth weights.
- Diversifying selection
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- Natural selection maintains sexual reproduction
- -Sex generates genetic variation during meiosis and
fertilization.
- Generation-to-generation variation may be of greatest
importance to the continuation of sexual reproduction.
- Disadvantages to using sexual reproduction: Asexual
reproduction produces many more offspring.
- -The variation produced during meiosis greatly outweighs this
disadvantage, so sexual reproduction is here to stay.
- All asexual individuals are female (blue). With sex, offspring
= half female/half male. Because males dont reproduce, the overall
output is lower for sexual reproduction.
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- Sexual selection leads to differences between sexes
- a. Sexual dimorphism is the difference in appearance between
males and females of a species.
- Intrasexual selection is the direct competition between members
of the same sex for mates of the opposite sex.
- This gives rise to males most often having secondary sexual
equipment such as antlers that are used in competing for
females.
- -In intersexual selection (mate choice), one sex is choosy when
selecting a mate of the opposite sex.
- -This gives rise to often amazingly sophisticated secondary
sexual characteristics; e.g. , peacock feathers.
- Natural selection does not produce perfect organisms a.
Evolution is limited by historical constraints ( e.g. , humans have
back problems because our ancestors were 4-legged). b. Adaptations
are compromises. (Humans are athletic due to flexible limbs, which
often dislocate or suffer torn ligaments.) c. Not all evolution is
adaptive. Chance probably plays a huge role in evolution and not
all changes are for the best. d. Selection edits existing
variations. New alleles cannot arise as needed, but most develop
from what already is present.
- Genes Within Populations Chapter 21
- Gene Variation is Raw Material
- Natural selection and evolutionary change
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- Some individuals in a population possess certain inherited
characteristics that play a role in producing more surviving
offspring than individuals without those characteristics.
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- The population gradually includes more individuals with
advantageous characteristics.
- Gene Variation In Nature
- Measuring levels of genetic variation
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- blood groups 30 blood grp genes
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- A locus with more variation than can be explained by mutation
is termed polymorphic .
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- Natural populations tend to have more polymorphic loci than can
be accounted for by mutation.
- Hardy-Weinberg Principle
- Population genetics - study of properties of genes in
populations
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- blending inheritance phenotypically intermediate (phenotypic
inheritance) was widely accepted
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- new genetic variants would quickly be diluted
- Hardy-Weinberg Principle
- Hardy-Weinberg - original proportions of genotypes in a
population will remain constant from generation to generation
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- Sexual reproduction (meiosis and fertilization) alone will not
change allelic (genotypic) proportions.
- Hardy-Weinberg Equilibrium Population of cats n=100 16 white
and 84 black bb = white B_ = black Can we figure out the allelic
frequencies of individuals BB and Bb?
- Hardy-Weinberg Principle
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- Allelic frequencies would remain constant if
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- population size is very large
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- no gene input from external sources
- Hardy-Weinberg Principle
- Calculate genotype frequencies with a binomial expansion
- (p+q) 2 = p 2 + 2pq + q 2
- p2 = individuals homozygous for first allele
- 2pq = individuals heterozygous for alleles
- q2 = individuals homozygous for second allele
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- p+q = 1 (always two alleles)
- 16 cats white = 16bb then ( q 2 = 0.16 )
- This we know we can see and count!!!!!
- If p + q = 1 then we can calculate p from q 2
- Q = square root of q 2 = q .16 q=0.4
- p + q = 1 then p = .6 (.6 +.4 = 1)
- All we need now are those that are heterozygous (2pq) (2 x .6 x
.4)= 0.48
Hardy-Weinberg Principle
- Hardy-Weinberg Equilibrium
- Five Agents of Evolutionary Change
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- Mutation rates are generally so low they have little effect on
Hardy-Weinberg proportions of common alleles.
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- ultimate source of genetic variation
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- movement of alleles from one population to another
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- tend to homogenize allele frequencies
- Five Agents of Evolutionary Change
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- assortative mating - phenotypically similar individuals
mate
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- Causes frequencies of particular genotypes to differ from those
predicted by Hardy-Weinberg.
- Five Agents of Evolutionary Change
- Genetic drift statistical accidents.
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- Frequencies of particular alleles may change by chance
alone.
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- important in small populations
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- founder effect - few individuals found new population (small
allelic pool)
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- bottleneck effect - drastic reduction in population, and gene
pool size
- Genetic Drift - Bottleneck Effect
- Five Agents of Evolutionary Change
- Selection Only agent that produces adaptive
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- artificial - breeders exert selection
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- natural - nature exerts selection
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- variation must exist among individuals
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- variation must result in differences in numbers of viable
offspring produced
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- variation must be genetically inherited
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- natural selection is a process, and evolution is an
outcome
- Five Agents of Evolutionary Change
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- matching climatic condition
- Measuring Fitness
- Fitness is defined by evolutionary biologists as the number of
surviving offspring left in the next generation.
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- Selection favors phenotypes with the greatest fitness.
- Interactions Among Evolutionary Forces
- Levels of variation retained in a population may be determined
by the relative strength of different evolutionary processes.
- Gene flow versus natural selection
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- Gene flow can be either a constructive or a constraining
force.
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- Allelic frequencies reflect a balance between gene flow and
natural selection.
- Natural Selection Can Maintain Variation
- Frequency-dependent selection
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- Phenotype fitness depends on its frequency within the
population.
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- Negative frequency-dependent selection favors rare
phenotypes.
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- Positive frequency-dependent selection eliminates
variation.
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- Selection favors different phenotypes at different times.
- Heterozygote Advantage
- Heterozygote advantage will favor heterozygotes, and maintain
both alleles instead of removing less successful alleles from a
population.
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- Homozygotes exhibit severe anemia, have abnormal blood cells,
and usually die before reproductive age.
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- Heterozygotes are less susceptible to malaria.
- Sickle Cell and Malaria
- Forms of Selection
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- Selection eliminates intermediate types.
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- Selection eliminates one extreme from a phenotypic array.
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- Selection acts to eliminate both extremes from an array of
phenotypes.
- Kinds of Selection
- Selection on Color in Guppies
- Guppies are found in small northeastern streams in South
America and in nearby mountainous streams in Trinidad.
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- Due to dispersal barriers, guppies can be found in pools below
waterfalls with high predation risk, or pools above waterfalls with
low predation risk.
- Evolution of Coloration in Guppies
- Selection on Color in Guppies
- High predation environment - Males exhibit drab coloration and
tend to be relatively small and reproduce at a younger age.
- Low predation environment - Males display bright coloration, a
larger number of spots, and tend to be more successful at defending
territories.
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- In the absence of predators, larger, more colorful fish may
produce more offspring.
- Evolutionary Change in Spot Number
- Limits to Selection
- Genes have multiple effects
- Evolution requires genetic variation
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- Intense selection may remove variation from a population at a
rate greater than mutation can replenish.
- Gene interactions affect allelic fitness
- Variations in genotype arise by random fusion of gametes,
mutation, and ______.
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- The total genetic information in a population is called the
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- Saint Bernards and Chihuahuas cant mate normally owing to great
differences in size. What type of reproductive isolationg mechanism
is operating here?
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- If a population is in genetic equilibrium,
- Allelic frequencies are changing
- Allelic frequencies are remaining stable
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- Mutations affect genetic equilibrium by
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- Directional selection, disruptive selection, and stabilizing
selection are all examples of
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- The most common way for new species to form is through
- Geographic and reproductive isolation
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- Population genetics
- genetic structure of a population
group of individuals of the same species that can interbreed
Patterns of genetic variation in populations Changes in genetic
structure through time
- Describing genetic structure
rr = white Rr = pink RR = red
- 200 white 500 pink 300 red
200/1000 = 0.2 rr 500/1000 = 0.5 Rr 300/1000 = 0.3 RR total = 1000
flowers genotype frequencies: Describing genetic structure
- 200 rr 500 Rr 300 RR
900/2000 = 0.45 r 1100/2000 = 0.55 R total = 2000 alleles allele
frequencies: = 400 r = 500 r = 500 R = 600 R Describing genetic
structure
- for a population with genotypes: 100 GG 160 Gg 140 gg Genotype
frequencies Phenotype frequencies Allele frequencies
calculate:
- for a population with genotypes: 100 GG 160 Gg 140 gg Genotype
frequencies Phenotype frequencies Allele frequencies 100/400 = 0.25
GG 160/400 = 0.40 Gg 140/400 = 0.35 gg 260/400 = 0.65 green 140/400
= 0.35 brown 360/800 = 0.45 G 440/800 = 0.55 g calculate: 0.65
260
- another way to calculate allele frequencies: 100 GG 160 Gg 140
gg Genotype frequencies Allele frequencies 0.25 GG 0.40 Gg 0.35 gg
360/800 = 0.45 G 440/800 = 0.55 g OR [0.25 + (0.40)/2] = 0.45 [0.35
+ (0.40)/2] = 0.65 0.25 0.40/2 = 0.20 0.40/2 = 0.20 0.35 G g G
g
- Population genetics Outline What is population genetics?
Calculate Why is genetic variation important?
How does genetic structure change?
- Genetic variation in space and time Frequency of Mdh-1 alleles
in snail colonies in two city blocks
- Changes in frequency of allele F at the Lap locus in prairie
vole populations over 20 generations Genetic variation in space and
time
- Why is genetic variation important? potential for change in
genetic structure
- adaptation to environmental change
Genetic variation in space and time
- divergence of populations
- Why is genetic variation important? variation no variation
EXTINCTION!! global warming survival
- Why is genetic variation important? variation no variation
north south north south
- Why is genetic variation important? variation no variation
divergence NO DIVERGENCE!! north south north south
- Natural selection Resistance to antibacterial soap Generation
1: 1.00 not resistant 0.00 resistant
- Natural selection Generation 1: 1.00 not resistant 0.00
resistant Resistance to antibacterial soap
- Natural selection Resistance to antibacterial soap mutation!
Generation 1: 1.00 not resistant 0.00 resistant Generation 2: 0.96
not resistant 0.04 resistant
- Natural selection Resistance to antibacterial soap Generation
1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not
resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24
resistant
- Natural selection Resistance to antibacterial soap Generation
1: 1.00 not resistant 0.00 resistant Generation 2: 0.96 not
resistant 0.04 resistant Generation 3: 0.76 not resistant 0.24
resistant Generation 4: 0.12 not resistant 0.88 resistant
- Natural selection can cause populations to diverge divergence
north south
- Selection on sickle-cell allele aa abnormal hemoglobin
sickle-cell anemia very low fitness intermed. fitness high fitness
Selection favors heterozygotes ( Aa ). Both alleles maintained in
population ( a at low level). Aa both hemoglobins resistant to
malaria AA normal hemoglobin vulnerable to malaria
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genetic change by chance alone
How does genetic structure change?
- Genetic drift 8 RR 8 rr Before: After: 2 RR 6 rr 0.50 R 0.50 r
0.25 R 0.75 r
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cause changes in allele frequencies How does genetic structure
change?
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mating combines alleles into genotypes How does genetic structure
change?
- AA x AA AA aa x aa aa genotype frequencies: AA = 0.8 x 0.8 =
0.64 Aa = 2(0.8 x0.2) = 0.32 aa = 0.2 x 0.2 = 0.04 allele
frequencies: A = 0.8 A = 0.2 A A A A A A A A a a AA 0.8 x 0.8 Aa
0.8 x 0.2 aA 0.2 x 0.8 A 0.8 A 0.8 a 0.2 a 0.2 aa 0.2 x 0.2
- Example: Coat color
- Allele frequency
- No. B alleles = 2(100) + 1(50) = 250
- No. b alleles = 2(50) + 1(50) = 150
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- Previous example: counted alleles to compute
- frequencies. Can also compute allele frequency
- from genotypic frequency.
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- f(BB) = .50, f(Bb) = .25, f(bb) = .25.
- allele frequencies can be computed as:
- Mink color example:
- bb = platinum (blue-gray)
- Group of females (.5 BB, .4 Bb, .1 bb) bred to
- heterozygous males (0 BB, 1.0 Bb, 0 bb).
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- Allele frequencies among the females?
- Allele frequencies among the males?
- Expected genotypic, phenotypic and allele frequencies in the
offspring?
- Expected frequencies in offspring
- Allele frequencies in offspring
- Also note: allele freq. of offspring = average of sire and dam
- f(B) = 1/2 (.5 + .7) = .6
- f(b) = 1/2 (.5 + .3) = .4
- Hardy-Weinberg Theorem
- Population gene and genotypic frequencies dont change over
generations if is at or near equilibrium.
Population in equilibrium means that the populations isnt under
evolutionary forces (Assumptions for Equilibrium*)
- Assumptions for equilibrium
- large population (no random drift)
- no migration (closed population)
- Hardy-Weinberg Theorem
- Under these assumptions populations remains stable over
generations.
- It means: If frequency of allele A in a population is =.5, the
sires and cows will generate gametes with frequency =.5 and the
frequency of allele A on next generation will be =.5!!!!!
- Hardy-Weinberg Theorem
- It can be used to estimate frequencies when the genotypic
frequencies are unknown.
- Predict frequencies on the next generation.
- Hardy-Weinberg Theorem
- If predicted frequencies differ from observed frequencies
Population is not under Hardy-Weinberg Equilibrium.
- Therefore the population is under selection, migration,
mutation or genetic drift.
- Or a particular locus is been affected by the forces mentioned
above.
- Hardy-Weinberg Theorem (2 alleles at 1 locus)
- Sum of all alleles = 100%
- f(AA) = p 2 Dominant homozygous
- f(aa) = q 2 Recessive homozygous
- Sum of all genotypes = 100%
- Hardy-Weinberg Theorem
- Sum of all alleles = 100%
- f(AA) = p 2 Dominant homozygous
- f(aa) = q 2 Recessive homozygous
- Sum of all genotypes = 100%
AA = p*p = p 2 Aa = pq + qp = 2pq Aa = q*q = q 2 Gametes A (p) a(q)
A(p) AA (pp) Aa (pq) a(q) aA (qp) aa (qq)
- Example use of H-W theorem
- 1000-head sheep flock. No selection for color. Closed to
outside breeding.
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- Start with known: f(black) = f(bb) = .09 =q 2
- Then, p = 1 q = .7 = f(B)
- In summary:
- f(bb) = q 2 = .09 (actual)
- Mink example using H-W
- Group of 2000 (1920 brown, 80 platinum) in equilibrium. We know
f(bb) = 80/2000 = .04 = q 2
- Forces that affect allele freq.
- 4. Random (genetic) drift
- Selection and migration most important for livestock
breeders.
- Mutation
- Change in base DNA sequence.
- Important over long time-frame.
- Migration
- Introduction of allele(s) into a population from an outside
source.
- Classic example: introduction of animals into an isolated
population.
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- under-the-counter addition to a breed.
- Change in allele freq. due to migration
- p mig = m(P m -P o ) where
- P m = allele freq. in migrants
- P o = allele freq. in original population
- m = proportion of migrants in mixed pop.
- Migration example
- 100 Red Angus (all bb, p 0 = 0)
- purchase 100 Bb (p m = .5)
- p = m(p m - p o ) = .5(.5 - 0) = .25
- new p = p 0 + p = 0 + .25 = .25 = f(B)
- new q = q 0 + q = 1 - .25 = .75 = f(b)
- Random (genetic) drift
- Changes in allele frequency due to random segregation.
- Important only in very small pop.
- Selection
- Some individuals leave more offspring than others.
- Primary tool to improve genetics of livestock.
- Does not create new alleles. Does alter freq.
- Primary effect change allele frequency of desirable
alleles.
- Example: horned/polled cattle
- 100-head herd (70 HH, 20 Hh, and 10 hh).
- f(HH) = .7, f(Hh) = .2, f(hh) = .1
- Suppose we cull all horned cows. Calculate
- allele and genotypic frequencies after culling?
- After culling
- f(H) = .778 + .5(.222) = .889
- f(h) = 0 + .5(.222) = .111
- 2nd example
- Cow herd with 20 HH, 20 Hh, and 60 hh
- Initial genotypic freq.: .2HH, .2Hh, .6hh
- Initial allele frequencies:
- Again, cull all horned cows.
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- freq (HH) = .2/.4 = .5 f(H) = .5 + 1/2(.5) = .75
- freq (Hh) = .2/.4 = .5 f(h) = 0 + 1/2(.5) = .25
- Note: more change can be made when the initial
- frequency of desirable gene is low.
- 3rd example
- initial genotypic freq. .2 HH, .2 Hh, .6 hh.
- Initial allele freq. f(H) = .3 and f(h) = .7
- Cull half of the horned cows.
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- f (HH) = .2/.7 = .2857 f(H) = .2857 + 1/2(.2857) = .429
- f (Hh) = .2/.7 = .2857 f(h) = .4286 + 1/2(.2857) = .571
- Note: the higher proportion that can be culled, the
- more you can change allele freq.
- Selection Against Recessive Allele
- Allele Freq. Genotypic Freq.
- Factors affecting response to selection
- (dominance slows progress)
- Initial allele frequency (for a one locus)
- Genetic Variability (Bell Curve)