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Flashcards in Population Genetics Deck (39):
1

Study of allele frequencies in a population

Population Genetics

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All the alleles in a population

Gene Pool

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A focus of population genetics:

To understand the forces that change the gene pool

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Why measure genetic variation?

-microevolution
-genetic engineering
-infer diversity in preparation for selective pressure
-inheritance patterns (ex. founder mutation)
-genetic counselling/ disease probability
-adaptation
-history of interbreeding

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The proportion of a specific genotype within a population.
-ranges from 0 to 1.

Genotypic frequency

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Genotypic frequency example: Scarlet tiger moth wing pattern gene (B)
Frequency of BB genotype?
Genotype: BB; # individuals: 452; total # in population: 497

F(BB)= #of individuals/total # in population
Frequency=0.909

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Portion of an allele within a population.

Allelic frequency

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Allelic frequency example: Scarlet tiger moth
What is the frequency of the B allele?

F(B)= #of a specific allele/total # of all alleles
=(2*BB)+(1*Bb)/2(total # individuals)

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Frequency of dominant allele (A)

p

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Frequency of recessive allele (a)

q

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Frequency of heterozygous allele (Aa)

2pq

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Frequency of Homozygous dominant (AA)

p^2

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Frequency of Homozygous recessive (aa)

q^2

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p+q=1

Allelic frequency shortcut

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Allows us to calculate allelic and genotypic frequencies in the absence of evolutionary forces.

Hardy-Weinberg Law

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Assumptions about the population for Hardy-Weinberg to work:

1) No migration
2) Large population
3) Random mating
4) No mutation
5) No natural selection

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-Genetic equilibrium; the frequency of alleles do not change over time.
-Gives us an idea about genetic variation

Hardy-Weinberg Law

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p^2 + 2pq + q^2

Hardy-Weinberg Equation

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-Like crossing population with itself
-Each individual passes both alleles that it possesses with equal frequency.
(p+q)(p+q)=p^2+2pq+q^2

Derivation of Hardy-Weinberg Law

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If Hardy-Weinberg assumes no evolutionary influences, then what is its purpose?

-Standard of comparison
-Predictor
-Control

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Example: Study geographic-dependent allelic variation

-Allele frequencies vary for populations separated by space across geographic transect.
-Cline

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Gradient for allele frequencies that changes in a systematic way according to the physical attributes of the environment.
-Blue mussles and aminopeptidase: LAP
North to South=increasing frequency for LAP94
*LAP94 allows for survival in lower salinity.

Cline

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How is genetic variation measured in a lab?

-Measure gene product

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How is genetic variation measured at the DNA level?

-Examine nucleotide sequence directly (BLAST) after PCR
-Examine RNA products (sequence & size differences)
-Look for polymorphism
(For Repeats)
*visualize by microscopy
*PCR & look for size differences on gel
(For SNPs)
*RFLP analysis

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Sequence changes between individuals.

Polymorphisms

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-Look for single nt. changes in DNA
-a SNP has changed a restriction enzyme site.

RFLP analysis

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Forces that change allele frequencies:

-Mutation
-Natural Selection
-Genetic Drift
-Migration

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Converts one allele to another
-New allele may be neutral, can be detrimental or advantageous, thus subject to natural selection.

Mutation:
Frequency of alleles in a population is determined by interaction of mutation rates and natural selection

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Individuals with alleles that confer an advantage in the environment, produce more offspring on average than other in the population.
-Frequency of alleles that confer survival and reproduction advantages increase population over time.

Natural Selection

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The relative reproductive ability of a genotype.

Fitness

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Selective mating type of the same phenotype.

Positive

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Selective mating type of the opposite phenotype.

Negative

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Changes in allele frequencies due to random sampling.
-reduces genetic diversity

Genetic Drift

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Contributes to genetic drift; when population is drastically reduced in #.

Bottleneck

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Contributes to genetic drift; when a population is established from a small # of breeding individuals.

Founder Effect

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Types of migration

Immigration
Emigration

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Type of migration that introduces new alleles

Immigration

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Type of migration that takes away alleles

Emigration

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When an individual migrates and contributes their alleles to the gene pool of the receptive population.

Gene Flow