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Population Genetics
Refresher…
• What is a population?
• What is a species?
• What is an allele?
• What is a gene pool?
Genetic Diversity
• Genetic diversity is generally high within
populations
– High degree of heterozygosity
– Genetic diversity may not be easily detectable
• Two methods to detect genetic diversity:
– Artificial selection
– Nucleotide sequencing
Adh gene in Drosophila
14 18
11
Of the 14 exon mutations, 13 are silent mutations
Only one changes an amino acid
Human CFTR gene
• >1900 different mutations identified in this
gene
– Missense
– Amino acid deletions
– Nonsense mutations
– Frameshifts
– Splice defects
• Cystic fibrosis transmembrane
conductance regulator
– Most common genetic disease in Caucasians
Clicker Question: True or False:
High amounts of genetic diversity
are always accompanied by high
phenotypic variation in a population
• A) True
• B) False
Microevolution
• Changes in allele frequencies in a
population that do NOT lead to speciation.
– No reproductive isolation
• Key elements of population genetics:
– Allele frequencies in a population
– Genotype frequencies in a population
– How these frequencies change from one
generation to the next
Hardy-Weinberg Law • Examines allele and genotype frequencies
in an “ideal” population:
– Infinitely large
– Randomly mating
– Not subject to evolutionary forces
• No migration
• No mutation
• No selection (artificial or natural)
Hardy-Weinberg
• Refresher of Mendelian inheritance and
the multiplication rule
• Consider an autosomal gene with two
alleles: A and a
• A has a frequency of 0.7 (=70%)
• a has a frequency of 0.3 (=30%)
• A + a = 1.0 (=100%) – there are no other
alleles to consider
Hardy-Weinberg
Figure 22-3
0.49 + 0.21 + 0.21 + 0.09 = 1.00
Hardy-Weinberg
• What is the frequency of
alleles in the offspring
generation?
• AA: 0.49
• aa: 0.09
• Aa: 0.42
• A = 0.49 + (½)*(0.42) = ?
• a = 0.09 + (½)*(0.42) = ?
Figure 22-3
Hardy-Weinberg: Generic
• First allele is p; second allele is q
• Homozygous: pp = p2 or qq = q2
• Heterozygous: pq
• In typical matings, heterozygotes occur
twice as often as either homozygote type
• So… p2 + 2pq + q2 = 1.0
Hardy-Weinberg: Predictions
• Allele frequencies do not change from
generation to generation
– No speciation!
• Genotype frequencies in offspring can be
predicted from allele frequencies
Hardy-Weinberg: Applications
• When real-life allele frequencies do
change, scientists can determine what
factors may be responsible
• Neutral genes can be identified in
populations
Hardy-Weinberg: Implications
• Dominant traits do not increase from
generation to generation
• Genetic variability remains in populations
once established
• Knowing frequency of one genotype
allows calculation of frequency of other
genotypes.
– Can calculate frequency of disease carriers
when we only know the frequency of affected
individuals
In a group of 125 students, 88 can taste
PTC; 37 cannot (homozygous recessive).
Calculate frequency of T and t alleles in this
population, and frequency of the genotypes.
Hardy-Weinberg in Human
Population • Example: CCR5 gene product allows HIV-
1 to enter cells (allows infection)
• CCR5-1 is normal allele
• CCR5-D32 is “mutant” allele, resistant to
HIV infection
• Heterozygotes can be infected, but
disease progresses more slowly than
homozygous “normal” patients
CCR5 gene data
Hardy-Weinberg Equilibrium • Do the offspring allele and genotype
frequencies match those of the parental
generation?
– Generate prediction for offspring based on
parents (calculate “expected”)
– Measure offspring frequencies (“observed”)
– Use c2 analysis to determine whether observed
frequencies match expected frequencies
• Population is in H-W equilibrium if there are
no changes in frequencies from generation
to generation
Hardy-Weinberg Equilibrium • Reasons a population is NOT in H-W
equilibrium:
– Selection
– Genetic drift
– Mutation
– Migration
– Nonrandom mating