POPULATION
GENETICS
Dr. Attya Bhatti
General Genetics
Population genetics
 The branch of genetics that studies the genetic makeup of
groups of individuals and how a group’s genetic composition
changes with time.
 Population geneticists usually focus their attention on a
Mendelian population, which is a group of interbreeding, sexually
reproducing individuals that have a common set of genes, the
gene pool.
 A population evolves through changes in its gene pool; so
population genetics is therefore also the study of evolution.
Population genetics
 Study the variation in alleles within and between groups
and the evolutionary forces responsible for shaping the
patterns of genetic variation found in nature
Population Genetics
The genetical study of the process of evolution
Change of allele frequencies
Genotype frequencies
Phenotype frequencies
Terminology used in population Genetics
• A population is a community of sexually inbreeding individuals.
Mating b/w individuals can be random or assortative.
– Random mating is the condition in which an individual in a
population has equal chance of mating with other individual in
that population.
– Assortative mating is non random.
Terminology used in population Genetics
• The Gene pool of a population is the total of all the genes in the
reproductive gametes of the population, Each gene in the gene
pool have different alleles.
• The allele frequency is the frequency of a specific allele of a
gene in a population.
• The genotypic frequency is the frequency of individuals with
specific genotype in the populations.
Population genetics
• The phenotypic frequency is the frequency of individuals with
specific phenotype in the populations.
Factors causing genotype frequency changes
• Selection
• Mutation
• Random Drift
• Migration
• Recombination
• Non-random Mating
• The primary goal of population genetics is to understand the
processes that shape a population’s gene pool.
• First, must ask
– what effects reproduction and Mendelian principles have on
the genotypic and allelic frequencies:
– How do the segregation of alleles in gamete formation and
the combining of alleles in fertilization influence the gene
pool?
Assumptions
•
For an autosomal locus with two alleles, the Hardy- Weinberg law
can be stated as follows:
•
Assumptions—If a population is large, randomly mating, and not
affected by mutation, migration, or natural selection, then:
•
Prediction 1—the allelic frequencies of a population
do not change; and
•
Prediction 2—the genotypic frequencies stabilize (will not change)
after one generation in the proportions p2 (the frequency of AA),
2pq (the frequency of Aa), and q2 (the frequency of aa), where p
equals the frequency
allele a.
of allele A and q equals the frequency of
Hardy-Weinberg Equilibrium
Godfrey H. Hardy and Wilhelm Weinberg in 1908
.
States that “ Under certain conditions , if the population is large
and randomly mating, the genotypic frequencies of the population
will remain stable from generation to generation.”
Hardy-Weinberg conditions:
• No mutations
• No selection against one of the genotypes.
• No migration or immigration.
•
No consanguineous mating
•
Equations: Allele freq. at locus can be expressed as
•
•
•
p + q = 1
Genotypic frequencies are expressed as
p2 + 2pq + q2 = 1
•
p2-= freq. of indivi. with genotype AA
•
q2= freq. of indivi. with genotype aa
•
2pq = freq. of indivi. with genotype Aa
Genotype Frequency
The proportion of individuals in a group with a particular genotype.
(Genotype can refer to one locus, two loci, or the whole genome)
40 AA, 47 Aa,
13 aa =
pAA = 40/100 = 0.4
pAa = 47/100 = 0.47
paa = 13/100 = 0.13
100 Total individuals
Calculation of Genotypic Frequencies
•
A frequency is simply a proportion or a percentage, usually expressed as
a decimal fraction.
•
For example, if 20% of the alleles at a particular locus in a population
are A, we would say that the frequency of the A allele in the population
is 20.
•
For large populations, where it is not practical to determine the genes
of all individuals, a sample of individuals from the population is usually
taken and the genotypic and allelic frequencies are calculated for this
sample 22 for a discussion of samples).
•
The genotypic and allelic frequencies of the sample are then used to
represent the gene pool of the population.
Calculation of Genotypic
Frequencies
• To calculate a genotypic frequency, we simply add up
the number of individuals possessing the genotype and
divide by the total number of individuals in the sample (N).
• For a locus with three genotypes AA, Aa, and aa, the
frequency (f ) of each genotype is:
The sum of all the genotypic frequencies always equals 1.
Hardy-Weinberg Equilibrium
How to predict genotype frequencies from
allele frequencies?
Assumptions
(1) Organism is diploid
(2) Reproduction is sexual
(3) Generations are non-overlapping
(4) Mating occurs at random
(5) Population size is very large
(6) Migration is zero
(7) Mutation is zero
(8) Natural selection does not affect the gene in
question
1-locus, 2-alleles
Assume that all of the H-W conditions are met.
pAA + pAa + paa = 1
Frequency of A allele?
These are the Hardy-Weinberg frequencies.
A (frequency p)
a
(frequency q=1—p)
A
AA
(pAA=p2)
a
Aa
(pq)
Aa
(pq)
aa
(q2)
Review of Hardy-Weinberg
Allele Frequency
The proportion of all alleles in all individuals in the group in question
which are of a particular type. (often referred to as "gene frequency")
e.g.
40 individuals which are AA
47 individuals which are Aa
13 individuals which are aa
Genotype
AA
Aa
aa
Total
# of
individuals
40
47
13
100
# of A
alleles
80
47
0
127
# of a alleles
0
47
26
73
Total # of
alleles
200
Allele frequency of A = 127/200 = 0.635
pA=0.635
pa = 73/200 = 0.365 = 1- pA