Measuring Genetic Variation in Natural Populations Historical Method: Examining protein variation...
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Transcript of Measuring Genetic Variation in Natural Populations Historical Method: Examining protein variation...
Measuring Genetic Variation in Natural Populations
Historical Method: Examining protein variation via electrophoresis
Modern Method: DNA sequencing and typing
More than one allele at 30-50% of all loci in a population.Such loci are called polymorphic.
LDH-B cline in Fundulus
Most populations harbor considerable genetic diversity
Heterozygosity0.10 0.20 0.30
Heterozygosity has a couple of interpretations:
1) Average percentage of loci that are heterozygous per individual.
or2) Average percentage of individuals that are heterozygous per loci.
DNA Sequencing : How is it done?
Heat : Single strands separate
PrimerAnneals
Only use one primerDNA polymerase
All possible DNAfragments are synthesized
ATGCTAACGCCATTCGA
TTA
TAC
TACG
TACGA
TACGAT
2 kinds of nucleotides
(1) Regular Nucleotides
(2) Fluorescent (ACGT) terminating nucleotides
Double Stranded DNAATGCTAACGCCATTCGA
TACGATTGCGGTAAGCT
Laser
TACGAT
TACGA
TACG
TAC
TAT
(-) Cathode
(+) Anode
Gel Electrophoresis
SynthesizedDNA
TAC
ComputerInterprets
FluorescentCode
TTCTTCAGGGGAGGGGGTGGAANATAAAAACAAAAACCCTACAATGTATATTCATCGCCCATAATCGGCTACTTAGACA
Electropherogram
DNA Sequence DataFrom Automated Sequencer
Another Way to Isolate Genes : Polymerase Chain Reaction (PCR)
Heat Single Strands SeparateHeat
Double Stranded DNA
Heat Resistant DNAPolymerase makesnew strands
PrimersAnneal
PrimersAnneal
Two DNA fragmentsare produced
PCR
1 2 3
Cycles
4
Copies
2 4 8 16
etc
etc
DNA is Amplified
Anode (+)
Cathode (-)An Agarose Gel
DNA DNA DNA DNA DNA
BasePairs
1000
600
300
(75 x 2) + (24) / (102 x 2) = 85.3
75/102 + 1/2 (24/102) = 85.3
Counting alleles
or
Genotypic frequencies
Sequencing Studies Have Revealed Enormous Genetic Diversity
CFTR Locus
Population Genetics
Evolution depends upon mutation
to create new alleles.
Evolution occurs as a result of population
level changes in allele frequencies.
What evolutionary forces alter
allele frequencies?
How do allele frequencies changein a population from generationto generation?
Hardy-Weinberg Principle
(1) Allele frequencies in a population will not change, generation after generation.
(2) If allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq, and q2.
When none of the evolutionary forces (selection, mutation, drift, migration, non-random mating) are operative:
Allele frequencies in the gene pool:
A: 12 / 20 = 0.6a: 8 / 20 = 0.4
Alleles Combine to Yield Genotypic Frequencies
Our mice grow-up and generate gametesfor next generations gene pool
Allele frequency across generations: A General Single Locus, 2 Allele Model
Freq A1 = pFreq A2 = q
One locus, 2 Allele Model
Genotype A1A1 A1A2 A2A2
Frequency of allele A1 = pFrequency of allele A2 = 1 - p = q
In a diploid organism, there are two alleles for each locus.Therefore there are three possible genotypes:
Given:
Then:Genotype A1A1 A1A2 A2A2
Frequency p2 2pq q2
A population that maintains such frequencies is said to be at Hardy-Weinberg Equilibrium
Hardy-Weinberg Principle
(1) Allele frequencies in a population will not change, generation after generation.
(2) If allele frequencies are given by p and q, the genotype frequencies will be given by p2, 2pq, and q2
When none of the evolutionary forces (selection, mutation, drift, migration, non-random mating) are operative:
Hardy-Weinberg Principle Depends Upon the Following Assumptions
1. There is no selection
2. There is no mutation
3. There is no migration
4. There are no chance events
5. Individuals choose their mates at random