do now: 5/14 (week 36) objectives : 1. define gene pool, phenotype frequency, and genotype...
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Do Now: 5/14 (Week 36)Objectives:
1. Define gene pool, phenotype frequency, and genotype frequency.
2. State the Hardy-Weinberg Principle.
3. Describe the conditions required for a population to be in H-W Equilibrium, and define genetic drift and bottlenecking.
4. Identify and explain the equations for f(A), f(a), f(AA), f(Aa), and f(aa).
TASK:
1. Pass forward labs & week 35 Do Nows.
2. (Don’t copy) In Cuban tree snails, brown shells (B) are dominant to yellow shells (b). Draw a Punnett square
representing a cross between a snail that is homozygous recessive and one that is heterozygous. What % of their
offspring will be yellow?
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Variation within a Population
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Some Terminology:• Gene Pool: all of the
genetic information (“genes”) in a population
• Phenotype frequency: How often a particular phenotype is observed in a population (0.00 – 1.00)
• Allele frequency: What percentage of the total # of alleles in a gene pool are a certain type. (0.00 – 1.00)
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Hardy-Weinberg Equilibrium
• the frequency of alleles and genotypes in a population will remain constant from generation to generation if the population is stable and in genetic equilibrium.
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HW Equilibrium: 5 Requirements
1. Large population.Small populations may experience genetic drift
(random changes) or bottlenecking.
The “bottleneck” effect
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HW Equilibrium: 5 Requirements
2. Random mating: Every phenotype is equally likely to mate with every other phenotype
3. No net mutation.
4. No immigration or emigration
5. No natural selection: there is no survival advantage to certain phenotypes.
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What to Know
• Definitions (gene pool, allele frequency)
• 5 conditions for HW equilibrium
• Equilibrium is theoretical, not practical.
• Disrupting equilibrium = evolution
• The rate of allele frequency change measures the rate of evolution.
• Microevolution: evolution within a species
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Hardy-Weinberg Genetic Equilibrium
• P = frequency of dominant allele (A)
• Q = frequency of recessive allele (a)
The Hardy-Weinberg Equation describes the relationship between allele frequencies in a gene pool and phenotype frequencies in a population
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P + Q = 1
• Consider the following Punnet square, showing a cross between 2 heterozygous individuals for allele A:
A (p) a (q)
A (p) AA Aa
a (q) Aa aa
In the gene pool,
f(A) = Pf(a) = Q
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Defining P2 and Q2
• In the population as a whole, the chance of a new individual receiving an “A” allele is equal to that allele’s frequency in the population, represented as p.
• Thus, the chance of receiving 2 “A” alleles is p x p, or p2, and the chance of receiving two “a” alleles is q2.
A (p) a (q)
A (p) AA Aa
a (q) Aa aa
In other words,
f(AA) = P2
f(aa) = Q2
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Heterozygotes• The probability for producing a
heterozygote = p x q, or pq.• Since there are 2 possible ways to
produce a heterozygote, the total probability is 2pq
A (p) a (q)
A (p) AA Aa
a (q) Aa aa
In other words,
f(Aa) = 2PQ
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The Hardy-Weinberg Equation
• In any population in equilibrium,
• p + q = 1
• Therefore, (p + q)2 = 1
• Expanded… p2 + 2pq + q2 = 1
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Relationships between allele frequency and phenotype
frequency
• In a population in equilibrium,
• f(AA) = p2
• f(Aa) = 2pq
• f(aa) = q2
• Remember: p = f(A) and q = f(a)
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How it works…
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Simple Application
• Like any formula with 2 variables, if one is known, the other can be determined.
• For this type of problem, use the simple form P + Q = 1
• Example: The allele for tongue rolling has a frequency of .95. What is the frequency of the allele for non-tongue rolling?