Constant Allele Frequencies AKA

Hardy Weinberg

2

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

3

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

4

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

5

Frequency of a trait varies in different populations

Table 14.1

Phenotype Frequencies

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

8

Solving a Problem

9

The allele and genotypic frequencies do not change from one generation to the next

Thus, this gene is in Hardy-Weinberg equilibrium

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

11Table 14.3

Calculating the Carrier Frequency of an Autosomal Recessive

12Figure 14.3

Calculating the Carrier Frequency of an Autosomal Recessive

Figure 14.5

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

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

1717

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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?

1919

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.

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

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