Lecture 26
In this lecture we will consider how allele frequencies can change under the
influence of mutation and selection.
The first consider the conversion of a wild type gene to an altered allele by
mutation:
µ
A → a µ =mutation rate (probability of a mutation/generation)
Δ
q
mut = µ ƒ(A) = µ
p
≈ µ
Typical mutation rates vary from µ = 10-4 — 10-8
Thus, in the absence of any other effects, such as selection, for any given gene the
frequency of mutant alleles will increase a little each generation because of new
mutations
Consider the disease phenylketonuria (PKU), which is an autosomal recessive defect
in the enzyme phenylalanine hydroxylase. The absence of the enzyme prevents
phenylalanine from being metabolized causing unusually high levels of phenylalanine
in the body leading to severe mental retardation.
Say, that for PKU, µ = 10-4. The frequency of PKU will then slowly increase each
generation.
When the allele frequency gets high enough selection against homozygotes will
counterbalance new mutations and
q
will stay constant. In order to treat selection
quantitatively we need an additional concept.
S = selective disadvantage; and fitness = 1–S
If a genotype has S = 0.75 then fitness = 0.25, meaning that individuals with this
genotype will reproduce at a rate of only 25% relative to an average individual.
Fitness can be thought of as a combination of survival and fertility.
