Download - Genes within Populations
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Genes within Populations
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What is a population?
How are populations characterized?
What does it mean to be diploid, haploid, polyploid?
How can we characterize populations based on their genes?
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What is a population?
• A group of individuals of the same species that have a high potential of interbreeding
• Share a common gene pool
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Phenotype & Genotype
• The phenotype is the expression of the genetic material (the genes) of the chromosomes.
• The genotype relates to the alleles found at loci on the chromosomes
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How many alleles can an individual have at a locus?
How many alleles can there be in a population at a specific locus?
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Determining the genotype for an enzyme in a fish.
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IDHP (isocitrate dehydrogenase) from a grasshopper
Individuals
1 2 3 . . .
AA
aa
Aa
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GenotypeThere are 3 Genotypes
AA and aa (Homozygous),
and
Aa (Heterozygous)
there are 2 alleles A and a
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Genotype frequency• If there are 100 individuals
60 are AA• 30 are Aa• 10 are aa
Genotype frequency AA= 60/100= 0.6 Aa = 30/100= 0.3 aa= 10/100 = 0.1
Total = 1.0
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Allele Frequency• in 100 individuals there are 200 alleles
60 indiv. AA = 120 A30 indiv. Aa = 30 A and 30 a10 indiv. aa = 20 aTherefore A = 150 A=150/200 = 0.75
a = 50 a=50/200 = 0.25
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Phenotype• there are 2 phenotypes
trait “X” (AA and Aa)
and
trait “Y” (aa)
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Phenotype frequency• trait X = 90 indiv. (AA and Aa)
trait Y = 10 indiv. (aa)
Freq. X = 90/100 = 0.9
Freq. Y =10/100 = 0.1
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Hardy-Wienberg Equilibrium• If p = proportion of allele A and
if q = proportion of allele a• Then p+q = 1
• Hardy-Wienberg Equilibrium gives the expected frequency of the three genotypes as:
(p+q)2 = p2 +2pq + q2 = 1
AA = p2, aa = q2 and Aa = 2pq
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Hardy-Wienberg EquilibriumAssumptions
• Population size is very large• Random mating is occurring• No mutation is taking place• No immigration (geneflow)• No selection is occurring
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If the proportion of genotype aa in a population = 1% or 0.01
• aa = 0.01• q2 = aa
• Therefore q = square root of 0.01 =0.1• p= 1 - 0.1 = 0.9• AA = p2 = 0.81• Aa = 2pq =0.18• aa = q2 = 0.010
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How the Hardy-Wienberg Equilibrium can be used!
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Extension to Hardy-Wienberg
• Three alleles
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p12 + 2p1p2 + 2p1p3 + p2
2 + 2p2p3 + p32 = 1
Genotype frequency
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• 3 alleles therefore 6 genotypes• genotype q2 =.01, z2 = 0.04
genotypes = q2+z2+p2+2qz+2zp+2pqalleles p+q+z=1Genotypes are:q= 0.1, z= 0.2, p= 0.7p2= 0.49q2=0.01z2=0.042pz=0.282pq=0.142zq=0.04
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Populations share a common gene pool!
What does this mean?
• At each gene locus a population will be characterized by a particular allele frequency. The combination of allele frequencies is what characterizes a population and potentially makes populations unique.
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What will cause deviation from Hardy-Wienberg Equilibrium?
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Genetic Drift
• For small populations random chance may result in the loss of an allele!
• Results in Fixation
• Loss of heterozygosity
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Genetic drift is a random process
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What is inbreeding and why is it bad?
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Inbreeding
alleles are common by descent
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• Genes common by descent
• Loss of heterozygosity
• No loss of alleles
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GENE FLOW – result of dispersal (an individual leaving one population and entering another population). Gene flowcounters genetic drift.
FOUNDER EFFECT – refers to the chance genecombination in newly founded population (the variation in a new pop. Generally less thanvariation in the source population).
GENETIC BOTTLENECK – when a population is reduced tonumbers but then recovers some genetic variation isgenerally lost.
Other Processes
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What generalities and patterns are there?
Selection is a change in allele frequencywhich is directional, NOT random.