hardy weinberg equilibrium
DESCRIPTION
Hardy Weinberg Equilibrium. p 2 + 2pq + q 2 = 1. Two scientists independently derived the basic principle of population genetics called the Hardy – Weinberg Principle . This principle states that:. - PowerPoint PPT PresentationTRANSCRIPT
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Hardy Weinberg Equilibrium
p2 + 2pq + q2 = 1
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Two scientists independently derived the basic principle of population genetics called the Hardy – Weinberg Principle. This principle states that:
If all factors remain constant, the gene pool in a population will have exactly the same
composition generation after generation. This condition is called genetic equilibrium.
If the genetic equilibrium of a population is upset, the population is said to be evolving.
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Evolution
"the sum total of the genetically inherited changes in the individuals who are the members of a population's gene pool."
Evolution is simply a change in frequencies of alleles in the gene pool of a population.
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Population
A group of the same species living in the same place at the same time
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Gene pool
all of the genes / alleles that occur in a population.
Ex) human gene pool for blood type are IA, IB, and i.
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Allele frequency
– % or proportion of that allele in the population
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Conditions
Evolution will NOT occur and Hardy-Weinberg equilibrium will be met if the following conditions are met:
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Conditions
1. No Mutation
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Conditions
2. The population is infinitely large
- laws of probability must apply
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Conditions
3. All members of the population breed
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Conditions
4. All mating is totally random
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Conditions
5. Everyone produces the same number of offspring
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Conditions
6. There is no migration in or out of the population
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Equation
Equation used to find genotype frequencies:
p² + 2pq + q² = 1
And
p + q = 1
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p is the frequency of the dominant allele
q is the frequency of the recessive allele
p2 is the frequency of the homozygous dominant genotypes
q2 is the frequency of the homozygous recessive genotypes
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Equation
2pq is the frequency of the heterozygotes
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Example
Albinism is only expressed in the phenotype of homozygous recessive individuals (aa).
The average human frequency of albinism in North America is only about 1 in 20,000.
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Question
Calculate the frequencies of the alleles and all three genotypes in this population.
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Solution
Synthetic Theory of Evolution: Sample Hardy-Weinberg Problem
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Examples:
1. In a population, 21% of the individuals are homozygous dominant, 49% are heterozygous and 30% are homozygous recessive. What percentage of the next generation are predicted to be homozygous recessive?
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2. 16% of a population is observed to have a continuous hairline (recessive). What percentage of the population possesses the dominant allele? If there are 500 members in the population, how many would be heterozygous?
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3. A recessive genetic disorder occurs in 9% of the population. What percentage of the population will be carriers for the disorder? What percentage will be homozygous dominant?
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Quiz – Theoretical Ideas
http://w3.dwm.ks.edu.tw/bio/activelearner/18/ch18summary.html
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Disturbances to Equilibrium
There are some situations that may make H-W equilibrium of alleles more likely to change:
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1) Mutations
Whether a mutation is good or bad, often depends on the environment. A harmful mutation can turn out to have a selective advantage if the environment changes over time.
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2) Non-random Mating
Individuals are often attracted to one another because they value specific traits. Ex. In humans, wolves, elk
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-will reduce genetic diversity, thus decrease frequency of some alleles
3) Inbreeding
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4) Genetic Drift
- a reduction in the gene pool variation caused purely by chance. Usually in small populations. If a specific allele doesn’t reproduce (by chance) it may be lost entirely.
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Genetic Drift Example
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5 ) Gene Flow
Migration – is the movement of genes into (immigration) / out of (emigration) the population. Some genes may migrate more readily than others.
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6 ) Bottleneck Effect
occurs when a part of the population is eliminated by chance.
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7) Founder Effect
- occurs when the founders of a new population have a specific genotype. Ex. polydactyl hands in Amish in Pennsylvania.
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8) Natural Selection
Selective Advantage: the most important reason for changes to H-W equilibrium
New mutations may arise that give the organism an advantage over others of the same species
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These alleles become more common with time
Means that some alleles are helping individuals to survive and reproduce
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I. Stabilizing Selection:
atypical phenotypes are eliminated, and an average is favored. Ex. birth weight or color.
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II. Directional Selection
– an atypical phenotype is selected for because of a progression of change in the environment. Ex. horse evolution, peppered moth.
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III. Disruptive Selection
two or more phenotypes are selected due to different characteristics within a habitat. Ex. fish that feed on bottom vs fish that feed on top.
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Speciation
Divergence producing new species, two types:1.Allopatric speciation: physical
separation of species drives the splitting of one species into two (or more)
Eg. Grand Canyon SquirrelsDarwin’s Finches
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May not be immediately obvious Eg. Anole lizards
in Cuba – not physically separated now, but were 5 million years ago
Allopatry Animation
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2. Sympatry
Division of one species into two or more in absence of physical barriers
Disputed by some
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H-W Equilibrium - Summary
Does not change unless a force is acting upon it
This force is often natural selection – leads to evolution