BIOL 2040 Lecture Notes - Lecture 4: Mutation Rate, Zygosity, Mutation
27 Apr 2018
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Lecture 4 - Selection (end)
January 22, 2018
8:38 PM
Selection table: laying out a way to see what happens during selection - use it to work through selection
mathematically
Frequency
W
AA
0.36
1
Aa
0.48
0.75
aa
0.16
0.5
W = fitness
W-bar = mean/average fitness
(0.36 x 1) + (0.48 x 0.75) + (0.16 x 0.5) = 0.8
W-bar = 0.8
Fitness levels do not add up to 1. The average fitness is 0.8.
Divide by W-bar to get the new genotype alleles, to figure out frequency of alleles that are going to be
selected
Selection: Predictions
• x-axis must be stretched out hundreds of generation to be able to see a change in selection
o You could see a change faster in mutation
Selection experiment
• Alcohol Dehydrogenase in fruit flies
o An important metabolic enzyme
o Single gene
o Technique that draws alleles to one side of a gel (alleles travel at different rates) is used
• Does selection on ADH come from exposure to ethanol?
• Experimental treatment: food is spiked with ethanol, VS, Control: no ethanol
• Both groups:
o Same number of flies
o No migration
o Flies chosen at random (researchers did not know ADH genotype when transferring flies
between generations)
• When spiked with ethanol, experimental treatment group drosophila ADH allele frequency
increased
• Control group drosophila frequency stayed the same
• Believed to prove selection, and is repeatable
o But how strong is the selection?
Codominant (additive) vs Recessive alleles
• Generally, heterozygote is less fit than homozygote dominant, but it's not always the case
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o Fitness may be roughly the same
W (%)
W (decimals)
AA
100%
1
Aa
100%
1
aa
60%
0.6
S = selection value
Convert percentages into decimals (never do calculations in percentages). How much less fit is the
homozygote recessive compared to the heterozygote? S = 0.4
W
AA
100%
1
Aa
100%
1
aa
0%
0
S = 1, Waa = 0
In this case, homozygote recessive is lethal.
A lot of dominant lethal mutations die out quickly - that's why we don't see many of them.
Frequency
AA
0.8
Aa
0.18
aa
0.01
Most of the aa alleles are hiding out in the heterozygotes, or "hidden from selection". If the mutation
makes the organism suffer from a little loss of fitness, recessive alleles take longer to die out from a
population.
Equilibrium for Balancing Selection
• What happens when heterozygote alleles are most dominant?
• Start with a frequency of 50% for both homozygous alleles. Each allele has 40% mortality rate if
homozygous, and the fitness for AA is 0.8, fitness for Aa is 1, and fitness for aa is 0.6.
W
AA
0.8
Aa
1
aa
0.6
o 1 - t = WAA
• T is used because we need something to represent another number - same idea as (1 -
S) though
find more resources at oneclass.com
find more resources at oneclass.com
