hardy weinberg equilibrium
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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
Hardy Weinberg Equilibrium
p2 + 2pq + q2 = 1
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.
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.
Population
A group of the same species living in the same place at the same time
Gene pool
all of the genes / alleles that occur in a population.
Ex) human gene pool for blood type are IA, IB, and i.
Allele frequency
– % or proportion of that allele in the population
Conditions
Evolution will NOT occur and Hardy-Weinberg equilibrium will be met if the following conditions are met:
Conditions
1. No Mutation
Conditions
2. The population is infinitely large
- laws of probability must apply
Conditions
3. All members of the population breed
Conditions
4. All mating is totally random
Conditions
5. Everyone produces the same number of offspring
Conditions
6. There is no migration in or out of the population
Equation
Equation used to find genotype frequencies:
p² + 2pq + q² = 1
And
p + q = 1
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
Equation
2pq is the frequency of the heterozygotes
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.
Question
Calculate the frequencies of the alleles and all three genotypes in this population.
Solution
Synthetic Theory of Evolution: Sample Hardy-Weinberg Problem
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?
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?
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?
Quiz – Theoretical Ideas
http://w3.dwm.ks.edu.tw/bio/activelearner/18/ch18summary.html
Disturbances to Equilibrium
There are some situations that may make H-W equilibrium of alleles more likely to change:
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.
2) Non-random Mating
Individuals are often attracted to one another because they value specific traits. Ex. In humans, wolves, elk
-will reduce genetic diversity, thus decrease frequency of some alleles
3) Inbreeding
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.
Genetic Drift Example
5 ) Gene Flow
Migration – is the movement of genes into (immigration) / out of (emigration) the population. Some genes may migrate more readily than others.
6 ) Bottleneck Effect
occurs when a part of the population is eliminated by chance.
7) Founder Effect
- occurs when the founders of a new population have a specific genotype. Ex. polydactyl hands in Amish in Pennsylvania.
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
These alleles become more common with time
Means that some alleles are helping individuals to survive and reproduce
I. Stabilizing Selection:
atypical phenotypes are eliminated, and an average is favored. Ex. birth weight or color.
II. Directional Selection
– an atypical phenotype is selected for because of a progression of change in the environment. Ex. horse evolution, peppered moth.
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.
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
May not be immediately obvious Eg. Anole lizards
in Cuba – not physically separated now, but were 5 million years ago
Allopatry Animation
2. Sympatry
Division of one species into two or more in absence of physical barriers
Disputed by some
H-W Equilibrium - Summary
Does not change unless a force is acting upon it
This force is often natural selection – leads to evolution