BIO 370 Lecture Notes - Lecture 13: Allele Frequency, Selection Coefficient, Allele
In this example, selection is against the light-colored phenotype, which is the product of the
genotype A2A2. Using the selection coefficient s, how do we represent fitness?
But we are really interested in what happens to allele frequencies (p and q)
Natural Selection – Dominant Allele
Let’s start with 360 A1A1, 480 A1A2, and 160 A2A2 individuals (notice that the initial allele
frequencies are p = 0.6 and q = 0.4), and that A2A2 is only 90% as successful as the other two
genotypes.
Let’s say that the A1A1 and A1A2 individuals replace themselves with equal numbers, and A2A2
individuals are reduced in number by 10%. In the next generation, the genotypes will be: 360
A1A1, 480 A1A2, and 144 A2A2.
find more resources at oneclass.com
find more resources at oneclass.com
360 A1A1, 480 A1A2, and 144 A2A2.
The population now has a total of 1968 alleles. Of these, 1200 are A1. So the new frequency of
A1 is 1200/1968 = 0.609756 (or an increase of about 0.01.)
So selection against the less fit homozygous A2A2 reduces the frequency of A2 (q) over every
generation, and so the frequency of A1 (p) increases.
Natural Selection – Dominance vs. Codominance vs. Recessive
The heterozygote provides a refuge for the allele which is selected against.
Natural Selection – Different Dominance Effects
What is overdominance?
First, note that this terminology is unfortunate
as it runs counter to the idea that the fitness of allele has nothing to do with whether it is
dominant or recessive.
When selection favors the heterozygote
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
In this example, selection is against the light-colored phenotype, which is the product of the genotype a2a2. But we are really interested in what happens to allele frequencies (p and q) Let"s start with 360 a1a1, 480 a1a2, and 160 a2a2 individuals (notice that the initial allele frequencies are p = 0. 6 and q = 0. 4), and that a2a2 is only 90% as successful as the other two genotypes. Let"s say that the a1a1 and a1a2 individuals replace themselves with equal numbers, and a2a2 individuals are reduced in number by 10%. In the next generation, the genotypes will be: 360. The population now has a total of 1968 alleles. A1 is 1200/1968 = 0. 609756 (or an increase of about 0. 01. ) So selection against the less fit homozygous a2a2 reduces the frequency of a2 (q) over every generation, and so the frequency of a1 (p) increases. Natural selection dominance vs. codominance vs. recessive.
