Population Genetics is the field of genetics which studies allele distributions and genetic variation in populations. Population geneticists study the processes of mutation, migration, natural selection and genetic drift on populations, and in doing so are studying evolution as it occurs.
Editor's note
Chapter or book approach: The approach of this chapter / book is to work through a series of models. The first model will be the Hardy-Weinberg Model, and then progressively, the models will move away from the premises of Hardy-Weinberg.
There are three major properties that a population must maintain with replication:
They are composed of reproducing individuals
They are distributed over space and time
They host a population of genes
The first property indicates that individuals of the population must reproduce to keep the population stable. This is necessary because individuals breakdown over time do to the introduction of entropy and inability for an individual to continuously remove the entropy added by the environment. Thus to maintain the population, individuals must pass down their DNA, or organizational encoding, to the next generation. Through continuous reproduction, a population can be maintained over a much longer time than the individuals that comprise it. In addition, the continuous reproduction of over time enables for the population to have properties and components of its own.
The second property is that a population is distributed over a space. Populations can exists as:
small isolated groups
a collection of groups with a varied amount of genetic exchange
a large interbreeding population that exists over a vast space
In general though the population can be divided into a primary group that can be considered as interbreeding and a secondary group that mates occasionally with the primary group. It is this primary group that population geneticists generally study, as it is generally stable. They define the group as a group of interbreeding individuals that share a common system of mating. The secondary group is generally ignored, and treated as noise in the system, unless it is having a major effect on the primary group.
The third property that must be maintained with reproduction is the population's gene pool. The gene pool is the collection of all the genes, organizational templates, in the population that can be used to create new individuals. By studying this gene pool, geneticists can determine the frequency of alleles, and or groups of alleles in the population and how they are changing over time. From the patterns of result that are obtained, geneticists then can start to understand what forces are acting on the population.
Change is a requirement of evolution and one method of introducing change is through modification of the templates used sustain the population. In the case of living life, these templates are genes.
Sources of Mutation:
Insertions
Deletions
Single Nucleotide Substitutions (sometimes changing the protein sequences and sometimes not)
Transpositions
Duplications
An allele is an alternate form of an template. In the case of biological systems, an allele is a form of a gene. Zooming further out, a version of a region of templates is called a haplotype. Biological systems would call this a sequence of nucleotides, while a in a computer system, this would be a sequence of linked objects.
If a population has no forces of evolution acting upon it is in Hardy Weinberg Equilibrium.
Quantitatively it says that if the allele proportions of two alleles A and a are denoted p and q then the genotype proportions will be such that the homozygote AA will be of proportion p2, the heterozygote Aa will have proportion 2pq and the homozygote aa will be of proportion q2.
Extending the H.W. model to two autosomal locus model with two alleles.
For the purpose of this discussion, the first locus will have alleles A and a and the second locus will have B and b. From this we can get the following gamete types and their frequencies through recombination:
Gamate
Frequency
AB
FreqAB
Ab
FreqAb
aB
FreqaB
ab
Freqab
Sum
1
A population producing the above four gamates can produce the following genotypes:
Gamates
AB
Ab
aB
ab
AB
FreqAB . FreqAB
FreqAB . FreqAb
FreqAB . FreqaB
FreqAB . Freqab
Ab
FreqAb . FreqAB
FreqAb . FreqAb
FreqAb . FreqaB
FreqAb . Freqab
aB
FreqaB . FreqAB
FreqaB . FreqAb
FreqaB . FreqaB
FreqaB . Freqab
ab
Freqab . FreqAB
Freqab . FreqAb
Freqab . FreqaB
Freqab . Freqab
If you analyze the table above, it can be noticed that there are only ten unique combinations. Four combinations correspond to homozygous zygotes and the remaining six are the heterozygous zygotes.
Notice that the sum of FreqAB, FreqAb, FreqaB, and Freqab is one. This follows from the earlier model where the sum of p and q equaled one.
Formally, if we define pairwise LD, we consider indicator variables on alleles at two loci, say . We define the LD parameter as:
Here denote the marginal allele frequencies at the two loci and denotes the haplotype frequency in the joint distribution of both alleles.
Various derivatives of this parameter have been developed. In the genetic literature the wording "two alleles are in LD" usually means to imply . Contrariwise, linkage equilibrium, denotes the case .
Editor's note
This section is in a total flux right now and is being outlined.
First experiments were done in the early 30's. German was a major scientific language before the second world war.
Genetic drift is a function of population size N.
The effects of genetic drift is inversely proportional to population size. This means as the population increases, the deviation from expected allelic frequencies will decrease. (Templeton, p. 84).
Genetic drift is non-directional.
There is no attraction to return ancestral allele frequencies.
Genetic drift is a cumulative function. Changes in allele frequencies from the previous generations are added to the changes that occur in the current generation.
Genetic drift is only occurs when there is variability.
To study genetic drift this section will create a simplified model and expand upon it. The model will just have two states, a) genetic drift is occurring and b) genetic drift is not occurring.
In each population, 2N=32, po=qo=0.5 initially and then did 19 generations of data. (Buri, P. 1956 "Gene frequency in small populations of mutant Drosophila. Evolution 10: 367-402
(See Figure 6.3 from Hedrick, P.W. 2005, Genetics of populations, 3rd edition. Jones and Bartlett, Sudbury, MA)
Note that in this figure, variance is increasing, but mean allele frequency over populations is staying relatively the same.
Genetic Drift can be simulated using a Monte Carlo Simulation.
(See Figure 6.2 in Hedrick)
The proportion of populations expected to go to fixation for a given allele is equal to the initial frequency of that allele
Only the allele frequencies are changing and the distribution of the allele frequency. The mean allele frequency over multiple replicate populations does not change due to genetic drift.
Heterzygositity or the variance of the allele frequency over the replicate population can be used to understand genetic drift.
We can calculate the number of generations necessary to reduce the reduce the heterozygostity
t=ln(x)*-2*N, where x is how much heterozgostity is left, and N is the population size.
Example 4 - Sex Linked (1 locus, 2 allele)[edit | edit source]
It is possible to solve for the amount of time it will take till an initial frequency is less than or equal to a particular threshold by using the following formula:
Example 5 - Inbreeding - Self Pollination[edit | edit source]
Genotype
Count
Homodom
20
Hetro
50
Homorec
30
Calculate F Value
Chi-squared then can be calculated by:
Example 6 - Inbreeding (1 Locus / 2 alleles)[edit | edit source]
Given the frequency of the dominant allele and an F value:
Allele
Frequency
p
0.6
q
0.4
What sample size would be necessary to detect an effect of F=0.01 at the S% significant level?
Look up the X2 value for S% significant level (usually S = 5%)