POPULATION GENETICS

Dr. Attya BhattiGeneral 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

• 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.

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 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

Allele frequency of A = 127/200 = 0.635

pA=0.635

pa = 73/200 = 0.365 = 1- pA