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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. - PowerPoint PPT Presentation
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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
• 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.
Terminology used in population Genetics
• The phenotypic frequency is the frequency of individuals
with specific phenotype in the populations.
Population genetics
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 of allele A and
q equals the frequency of allele a.
Hardy-Weinberg EquilibriumGodfrey 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 FrequencyThe 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 = 100 Total individuals
pAA = 40/100 = 0.4
pAa = 47/100 = 0.47
paa = 13/100 = 0.13
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.
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 question1-locus, 2-allelesAssume that all of the H-W conditions are met.pAA + pAa + paa = 1Frequency of A allele?
These are the Hardy-Weinberg frequencies.
A (frequency p) a (frequency q=1—p)
A AA
(pAA=p2)
Aa(pq)
a Aa (pq)
aa(q2)
Review of Hardy-WeinbergAllele FrequencyThe 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 Aa13 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