BIOL 2040 Lecture Notes - Lecture 5: Genotype Frequency, Allele Frequency, Genetic Drift
27 Apr 2018
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Department
Course
Professor
Lecture 5 - Migration and Drift
January 23, 2018
9:46 PM
Example: there are two mice colours, dark and light fur. The fur color is determined by one gene, and 2
alleles. The dark fur allele is dominant over the light fur allele. Value of p = 0.4, and value of q = 0.6.
Predators invade, and having dark fur provides better camouflage against the dirt. The light-furred mice
end up suffering a 75% mortality rate. What is the mean fitness and the new allele frequencies?
S = 0.75 (mortality rate).
Frequency
W (fitness)
-->
Product
-->
New genotype
frequencies
AA
0.16
1
(0.16 x 1)
0.16
(0.16/0.73)
0.22
Aa
0.48
1
(0.48 x 1)
0.48
(0.48/0.73)
0.66
aa
0.36
0.25
(0.36 x 0.25)
0.09
(0.09/0.73)
0.12
Total
1
0.73
1
Fitness for the dark-furred mice is 1 because none of them suffered a mortality rate.
Step 1: get the frequency of the 3 genotypes.
P2 = AA
q2 = aa
2pq = Aa
Step 2: Get W (fitness). Dark-furred mice do not suffer mortality. Only 25% of light-furred mice survives,
so the fitness for aa is 0.25.
Step 3: Get W(bar) by adding up the values of the product. W(bar) = 0.73
Step 4: To find the new genotype frequency, divide the product by the W(bar).
Step 5: Figure out allele frequency. Take the new genotype frequency and calculate the allele frequency.
AA: 0.22 --> 0.22 + (0.66/2) = 0.55
Aa: 0.66
aa: 0.12 --> 0.12 + (0.66/2) = 0.45
Genetic Drift
• Changes in allele frequency due to:
o Random sampling variation when drawing alleles to form zygotes
o Chance variation in survival, unrelated to genotype
• Most pronounced in small populations
• Ex. in the first generation, there are 5 individuals with genotypes: 1 AA, 3 Aa, 1 aa, with a p of 0.5.
Randomly, alleles are chosen to form generation 2: 1 AA, 1 Aa, 3 aa, with a p of 0.3.
• Genetic drift changes aren't based on fitness, more offspring, etc. It's just random.
o Usually, changes that have no affect on fitness have no affect on phenotype.
o Mutations that affect DNA replication or protein production are generally silent
(synonymous/silent mutations)
• Nonsynonymous = mutation changes the amino acid production into a different one
find more resources at oneclass.com
find more resources at oneclass.com
The graph is a computer simulation of a population of 4, and over many generations. The A1 allele
frequency quickly reaches a frequency of 0.
The graph is a computer simulation of a population of 40, and over many generations. The A1 allele
frequency has many more fluctuations, at one point nearly reaching 1, but eventually reaches 0.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Example: there are two mice colours, dark and light fur. The fur color is determined by one gene, and 2 alleles. The dark fur allele is dominant over the light fur allele. Value of p = 0. 4, and value of q = 0. 6. Predators invade, and having dark fur provides better camouflage against the dirt. The light-furred mice end up suffering a 75% mortality rate. Fitness for the dark-furred mice is 1 because none of them suffered a mortality rate. Step 1: get the frequency of the 3 genotypes. Only 25% of light-furred mice survives, so the fitness for aa is 0. 25. Step 3: get w(bar) by adding up the values of the product. Step 4: to find the new genotype frequency, divide the product by the w(bar). Take the new genotype frequency and calculate the allele frequency. Aa: 0. 22 --> 0. 22 + (0. 66/2) = 0. 55. Aa: 0. 66 aa: 0. 12 --> 0. 12 + (0. 66/2) = 0. 45.
