Population
genetics deals with terms like gene pools, allelic
frequencies, genotypic frequencies, phenotypic
frequencies. Instead of studying the gene from the level
of the molecule, cell, or organism, it looks a gene from
the population point of view. A population is a local
group of organisms of the same species (interbreeding or
potentially so). |
Hardy-Weinberg Equilibrium: This
mathematical
expression predicts what proportions to expect in each of
the three genotypic classes (looking at only one gene and
assuming there are only two alleles).
- Genotypic Frequencies
Are Determined by Allelic Frequencies (Gene
Frequencies): The frequency of AA
individuals should be p2, Aa
2pq, and aa q2. (p is the
allelic frequency of A in the gene pool and q is the
allelic frequency of a in the gene pool). If we assume
HW equilibrium, we can estimate the gene and genotypic
frequencies from the homozygous recessive frequency (q2=[aa]).
HW equilibrium applies to autosomal genes and to X
linked genes in females.
- Assumptions
of Hardy-Weinberg Equilibrium: These
frequencies will be observed if we assume the
following.
- Large, Randomly
Mating Population
- No Selection
- No Mutation
- No Migration (no
immigration or emigration)
Changes in Gene Frequency: The
gene and genotypic frequencies will not remain at HW
equilibrium if the above condition are not met.
- Selection: With
selection,
all genotypes are not equally viable. Fitness values
(W) and selection coefficients (s) are assigned to
different genotypes
- Complete Selection:
With complete selection against aa,
it is assigned an s value of 1.
- qnew=qold/(1+qold)
- qn=q0/(1+nq0)
- Partial Selection: With
partial selection against aa, it is assigned an s
value less than 1 but greater than 0.
- qnew=(qold(1-sqold))/(1+sqold)
- Heterozygote
Advantage: With this type of
selection, both homozygotes are selected against
(have an s value greater than 0). Sickle-cell anemia
in a malaria environment is an example of this kind
of selection. The value of q will reach an
equilibrium independent of the initial genotypic
frequencies.
- q at equilibrium = s1/(s1+s3)
- Drift: If
the
mating population is small, dramatic fluctuations in
gene and genotypic frequencies may be seen due to
sampling error. Founder's Principle is a "one-time"
drift event due to a new population being founded from
a small sample of the original population.
- Inbreeding: Inbreeding
does not change the gene frequency but changes the
genotypic frequencies.
- Migration
Balanced
Polymorphisms: In nature,
population often exist with genotypic frequencies balanced
at some level. This may be due to neutral mutations,
heterozygous advantage, equilibrium between mutation and
selection, equilibrium between forward and backward
mutation, or density-dependent selection.
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