This document provides an introduction to population genetics concepts including: - What is a population and how it relates to evolution - Key terms like gene pool, allele frequency, and genotype frequency - Mendelian populations and how they interact - Hardy-Weinberg equilibrium and how evolutionary forces can affect equilibrium - Factors like mutation, migration, non-random mating, natural selection, and genetic drift that influence population genetics

1. Welcome
P.TEJASREE
BAM-20-27
Dept Of GPBR

3. Learning Objectives
– what is population
– What is population genetics
– What is Mendelian population
– What is gene pool
– What is gene frequency
– What is genotypic frequency
– What is Hardy-Weinberg Equilibrium
– What are evolutionary forces in a population
– What are the significance of Hardy-Wienberg equilibrium
– Factors affecting H-W principle

4. What is a Population
– Definition: A group of individuals of a particular species occupying a
definite space, in which the individuals interact, interbreed and
exchange genetic materials.
– Species is a group of living organism comprising of similar individuals
capable of exchanging genes through interbreeding
– Individuals of same species of a particular region is called population
– A part from its ecological significance, population has a important role in
the process of evolution

5. What is Population genetics
 Study of distributions and changes of allele frequency and interaction of
allele in a population
 Population is prone to the four main evolutionary forces:
1. Natural selection
2. Genetic drift
3. Mutation
4. Migration
 Study of population genetics is essential for species adaptation and
evolution

6. – For better understanding of population genetics we must study
1.Mendalian population
2.Gene pool
3.Gene frequency
4.Genotypic frequency
5.Hardy Weinberg Law
6.Evolutionary forces in population

7. Mendelian Population
 A group of sexually interbreeding organism also called as genetic
population or random mating population
 Definition: A community of similar individuals living within a
circumscribed area at a given time and capable of interbreeding
to produce fertile offspring's.
 Mendelian population is characterized by individuals having
somewhat similar genetic constitution

8. Gene pool
– Sum total of genes present in the Reproductive individuals of a
population called Gene pool
– A gene pool of population describes:
1. Gene present in the population
2. Properties different kinds of gene
3. Pattern of distribution of gene in the individuals of population

9. Allelic frequency
 Suppose there are 100 individuals in a population, in which
 40 are homozygous dominant (AA)
 40 are heterozygous dominant(Aa)
 20 are homozygous recessive(aa)
 The frequency of ‘A’ allele will be (p)
(119+1/2 *76)/208=0.75
 The frequency of ‘a’ allele will be (q)
(13+1/2*76)/208=0.25
 Gene frequency calculated by dividing the no.of a particular allele
with total no.of allele present on that locus in the population

10. GENOTYPE FREQUENCY
– Definition: The Proportions of different genotypes for a gene in a
population are known as genotypic frequency
– Genotype frequency is the proportion of a genotype in a sample
will be the ratio of the number of individuals having that genotype
to the total number of individuals in the sample.
– Genotypic frequencies for MN blood group
– L^M L^M = 0.57 (119/208) p2=0.56
– L^M L^N = 0.36 (76/208) 2pq = 0.38
– L^N L^N = 0.06 (13/208) q2=0.06

11. Gene frequency
– Definition : Proportion of different alleles of a gene present in a Mendelian Population
are known as Gene frequency
– If a gene has two alleles for eg; L^M And L^N, their frequencies may be represented by p
and q
– p = 0.75 , q = 0.25
– p+q = (0.75+0.25) p+q = 1
– p = 1-q ,q = 1-p
– For gene with multiple alleles
– The A-B-O blood types are determined by three alleles IA,IB,and i
– Frequencies of these are p,q and r respectively
– The frequencies of 6 different genotypes in the A-B-O blood typing system are
obtained by expanding the trinomial (p+q+r)2=p2+q2+r2+2pq+2qr+2pr

12. – N :- Total no.of individuals in the population
– D :- Total no.of homozygous dominant
– H :- Total no.of heterozygous dominant
– R :- Total no.of homozygous recessive
– Genotypic frequency of AA individuals D/N
– G.f of Aa individuals H/N
– G.f of aa individuals R/N

13. HARDY WIENBERG’S EQUILIBRIUM
– Hardy – Wienberg law is the fundamental law of population genetics
– This law was developed independently by Hardy (1908) in england and
Weinberg (1909) in germany
– The Hardy-Weinberg law States that gene and genotype frequencies in a
mendelian population remain constant generation after generation if there
is no selection , mutation , or random drift.
Hardy & Wienberg

15. – H-W law describes a theoretic situation in which a population is undergoing
NO evolutionary changes.
– It explains that:
 The evolutionary forces are absent
 The population is large
 Its individuals have random mating
 Each parent produce roughly equal number of gametes
– The gametes produce by the mates combine at random and the gene
frequency remain constant
– The genetic equilibrium of the gene is maintained and the variability
present in the population is preserved

16. Significance
– Gene and genotype frequencies of different allele of a gene in a population remain
equilibrium
– Mating is completely random phenomenon in a population
– Only large population follow H-W law of equilibrium
– Gene frequencies will be unpredictable in small population
– All the genotypes in a population reproduce equally successfully
– The variation in H-W equilibrium is produced by mutation , selection , and
genetic drift these are the evolutionary forces in a population
– If these evolutionary forces not there in the population, it follows H-W
equilibrium
– A population in H-W equilibrium do not show evolution

21. Evolutionary Factors Affecting
MUTATIONS: - p = v / u+v , q = u / u+v
* Mutation in only one direction can cause one allele slowly to replace another.
* Mutation in both directions results in an equilibrium with frequencies determined
by the mutation rates.
* Mutation may produce new allele or may change the frequency of existing allele
which is not in that population.
MIGRATION: Ne=4N*Nm / (Nf+Nm) , m = M / N+M
* Migration between population always causes the gene frequencies of the
receiving population to shift towards those of the immigrants
* This addition or removal of alleles when individuals enter or leave a population
from another locality is called gene flow
* Wright estimated small amount of migration m=0.0001

23. NON RANDOM MATING
INBREEDING : results in more homozygotes
ASSORTATIVE MATING : mating according to phenotype , mating between
phenotypically similar individuals being either more frequent or less frequent
NATURAL SELECTION: Fitness (W)=Ri / Rh
• If selection occurs in such a way that out of the 2 alleles only 1 allele is
preferred by nature . It leads to change in the equilibrium .
• The genotype having fitness of 1 are favoured by selection , the magnitude of
selection aganist a genotype is called selection coefficient or selection
differential .
RANDOM GENETIC DRIFT : Random change in gene frequency or allele
frequency due to sampling error . It occurs in small populations becoz of
greater sampling error .
• Sampling errors lead to – Fixation of certain alleles
Elimination of some other alleles

26. Gametic selection : when selection acts on gametes or on the haploid phase of life cycle
is called gametic selection . Ex :- fungi
• Thus in each generation , the frequency of a allele will decline by the proportion s till
it reaches zero , and A becomes fixed in the population .
Zygotic selection :- selection operates on the zygotes or the diploid phase is called
zygotic selection .
• Zygotic selection may be :-
• aganist the recessive phenotype
• Aganist the dominant phenotype
• In the favour of heterozygote
FOUNDER PRINCIPLE :- the genetic divergence created by the limited no.of founders
of new populations is called the founder principle .
Ex :- Evolution of Darwin finches on Galapagos Islands which were probably derived
from a few initial founders
• A new population of a species is often initiated by a relatively small no.of individuals
of a population ; these individuals are called founders of the new population .

34. Reference
– GENETICS – P.S VERMA
– GENETICS –B.D.SINGH

35. Thank You
Submitted to :-
Dr Lal Ahamad
Associate Professor
Dept of GPBR