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Constant Allele Frequencies AKA
Hardy Weinberg
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Population = An interbreeding group of the same species in a given geographical area
Gene pool = The collection of all alleles in the members of the population
Population genetics = The study of the genetics of a population and how the alleles vary with time
Gene Flow = Movement of alleles between populations when people migrate and mate
Terminology
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Hardy-Weinberg EquationDeveloped independently by an English
mathematician and a German physicianUsed algebra to explain how allele
frequencies predict genotypic and phenotypic frequencies in a population of diploid, sexually-reproducing species
Disproved the assumption that dominant traits would become more common, while recessive traits would become rarer
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Allele FrequenciesAllele frequency = # of particular allele
Total # of alleles in the population
Count both chromosomes of each individualFrequencies are often expressed as decimals
- The frequency of the two homozygotes and the heterozygote in the population= Hardy Weinberg equation
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Frequency of a trait varies in different populations
Table 14.1
Phenotype Frequencies
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6Figure 14.3
Source of the Hardy-Weinberg Equation
Figure 14.3
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Gamete (allele) Frequencies:
Freq(A) = pFreq(a) = q
Þ p + q = 1
Genotype Frequencies of 3 Possible Zygotes:
AA Aa aa
Freq (AA) = pA x pA = pA2
Freq (Aa) = (pA x qa) + (qa x pA) = 2pAqa
Freq (aa) = qa x qa = qa2
Þ p2 + 2pq + q2 = 1
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Solving a Problem
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The allele and genotypic frequencies do not change from one generation to the next
Thus, this gene is in Hardy-Weinberg equilibrium
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10Figure 14.3
Applying the Hardy-Weinberg Equation
Used to determine carrier probability
For autosomal recessive diseases, the homozygous recessive class is used to determine the frequency of alleles in a population - Its phenotype indicates its genotype
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11Table 14.3
Calculating the Carrier Frequency of an Autosomal Recessive
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12Figure 14.3
Calculating the Carrier Frequency of an Autosomal Recessive
Figure 14.5
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13Figure 14.3
Calculating the Carrier Frequency of an Autosomal Recessive disease ( CF)
What is the probability that two unrelated Caucasians will have an affected child?
Mendelian solution involves probability or punnett squares
Probability that both are carriers =1/23 x 1/23 = 1/529
Probability that their child has CF = 1/4 Therefore, probability = 1/529 x 1/4 = 1/2,116
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14Figure 14.3
Calculating the Risk withX-linked Traits
For females, the standard Hardy-Weinberg equation applies
p2 + 2pq + q2 = 1
However, in males the allele frequency is the phenotypic frequency
p + q = 1
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Calculating the Risk withX-linked Traits
Figure 14.6
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A few things to keep in mind as we take an excursion into population genetic theory:
“Make things as simple as possible, but no simpler.”---Einstein
“No theory should fit all the facts because some of the facts are wrong.”
---Bohr
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Mechanisms of Evolution: Mendelian Genetics in Populations
Genetic variation is the raw material of evolutionary change: how do we measure it?
What are the forces that cause genetic changes within populations? That is, what mechanisms cause evolutionary change?
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Evolution
1. Mutation = ultimate source of variation2. Natural selection = genotypes best
suited to survive and reproduce in a particular environment give rise to a disproportionate share of the offspring
3. Migration = the movement of organisms among subpopulations
4. Random genetic drift = the random, undirected changes in allele frequencies, especially in small populations
Changing Allele Frequencies
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MUTATION SELECTION
DRIFTMIGRATION
POPULATIONS
Phenotypic Evolution: Process
+
+/ —
—
—
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Darwin’s Observations and Inferences
Inference 1: Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation.
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Inference 2: Survival in the struggle for existence is not random, but depends in part on the inherited characteristics of individuals
Darwin’s Observations and Inferences
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Inference 3: The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection).
Darwin’s Observations and Inferences
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DARWINIAN EVOLUTION BY NATURAL SELECTION
Many more individuals are born than survive (COMPETITION).
Individuals within species are variable (VARIATION).
Some of these variations are passed on to offspring (HERITABILITY).
Survival and reproduction are not random. There must be a correlation between fitness and phenotype.
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But, Darwin’s theory was not complete
Because Darwin knew nothing about mutation, he had no idea how variation was generated in populations
Because Darwin knew nothing about genetics or genes, he had no idea how variation was passed on to offspring (Mendel)
Darwin did not know about nonadaptive evolutionary forces, such as Genetic Drift
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Rediscovery of Mendel’s laws of inheritance
In 1900, Mendel’s laws of inheritance were “rediscovered”
Worked out laws of inheritance independently
Discovered Mendel’s work as they were publishing their own
Formed the beginning of the foundation of Genetics: Mendel is considered the “Father of Genetics”
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MUTATIONISM AND THE IMPACT OF MENDEL
Gregor Mendel’s research was published in 1866, but was not noticed until 1900.
NOTE: Darwin knew nothing about the mechanism of inheritance when he conceived of natural selection.
MUTATIONIST THEORIES (based on Mendel’s work):
Emphasized the importance of VARIATION T. H. Morgan -- the founder of Drosophila
genetics.
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BUT… Mendel and Darwin’s ideas seemed Incompatible
Mendel: dealt with particulate traits Darwin: observed continuous traits
Q: How would continuous traits get passed on?
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Controversy between Mutationists vs Darwinists
Mutationists (+ Mendelianism) They thought that evolution
required only mutations and passing on of discrete traits
Darwinists They thought that evolution
required only Natural Selection on continuous variation
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Problems to Resolve:
At the heart was the question of whether Mendelian genetics and Mutation could be reconciled with mechanisms of Natural Selection.
A second issue was whether the broad-scale changes (macroevolution) seen by palaeontologists could be explained by changes seen in local populations (microevolution).
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The Modern Synthesis1930s ~ 1940s
Also called the “Synthesis of Evolution and Genetics”The synthesis of population genetics(role of mutation, selection, genetic drift),paleontology, systematics
Darwin and Mendel Reconciled
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J. B. S. Haldane1892-1964
The Causes of Evolution1932
Developed the mathematical theory of gene frequency change under selection (and many other interesting applications).
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Sir R. A. Fisher1890-1962
The Genetical Theory of Natural Selection
1930
Fisher united Mendelian population genetics with the inheritance of continuous traits.
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3434
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DNA ProfilingDeveloped in the 1980s by British geneticist
Sir Alec Jeffreys
Also called DNA fingerprinting
Identifies individuals
Used in forensics, agriculture, paternity testing, and historical investigations
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DNA RepeatsShort repeated segments are distributed all
over the genomeThe repeat numbers can be considered
alleles and used to classify individuals Two types of repeats are important:
- Variable number of tandem repeats (VNTRs)- Short tandem repeats (STRs)
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DNA Repeats
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DNA ProfilingA technique that detects differences in
repeat copy numberCalculates the probability that certain
combinations can occur in two sources of DNA by chance
DNA evidence is more often valuable in excluding a suspect- Should be considered along with other types of evidence
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1) A blood sample is collected from suspect
2) White blood cells release DNA
3) Restriction enzymes cut DNA
4) Electrophoresis aligns fragments by size
5) Pattern of DNA fragments transferred to a nylon sheet
DNA Profiling Technique
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6) Exposed to radioactive probes
7) Probes bind to DNA
8) Sheet placed against X ray film
9) Pattern of bands constitutes DNA profile
10) Identify individuals
DNA Profiling Technique
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41Figure 2.3
DNA Fingerprinting Animation
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DNA can be obtained from any cell with a nucleus
STRs are used when DNA is scarce
If DNA is extremely damaged, mitochondrial DNA (mtDNA) is often used
For forensics, the FBI developed the Combined DNA Index System (CODIS)
- Uses 13 STRs
DNA Sources
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The probability that any two individuals have same thirteen markers is 1 in 250 trillion
CODIS- Combined DNA Index System
Figure 14.10
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The power of DNA profiling is greatly expanded by tracking repeats in different chromosomes
The number of copies of a repeat are assigned probabilities based on their observed frequency in a population
The product rule is then used to calculate probability of a certain repeat combination
Population Statistics Is Used to Interpret DNA Profiles
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To Solve A Crime
Table 14.6Figure 14.11
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Recent examples of large-scale disasters
- World Trade Center attack (2001)
- Indian Ocean Tsunami (2004)
- Hurricane Katrina (2005)
Using DNA Profiling to Identify Victims
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Challenges to DNA Profiling
Figure 14.12
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Today’s population genetics presents a powerful way to identify individuals
Our genomes can vary in more ways than there are people in the world
DNA profiling introduces privacy issues
Genetic Privacy