Population genetics is the study of the distribution and change in frequency of alleles within populations. It examines how processes like natural selection, genetic drift, gene flow, and mutation cause evolution in a population over time. The key concepts are: 1) Hardy-Weinberg law states that allele and genotype frequencies remain constant in a population with no evolutionary influences. 2) Variation within populations arises through migration, recombination, mutation, and gene flow. 3) Gene pools are sets of genes found in related species that can be used in crop breeding. They are divided into primary, secondary, and tertiary pools based on ease of gene transfer.

1. POPULATION GENETICS
PREPARED BY: ARUSHI ARORA
A-2016-30-050
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2. CONTENTS
TOPICS
• Population genetics
• Variation in population
• Hardy Weinberg law
• Gene pool concept
• Allele frequency
• Rare and common alleles
• References
SLIDE NUMBER
• 3-8
• 9-19
• 20-26
• 27-37
• 38-42
• 43
• 44
2

3. POPULATION
• A population is a summation of all the organisms of
the same group or species, which live in a particular
geographical area, and have the capability of
interbreeding.
3

4. POPULATION GENETICS
• Defined as study of the distribution and change
in frequency of alleles within populations and is
related to evolution.
• Population genetics is the study of the frequency and
interaction of alleles and genes in populations.
• In population genetics population is a set of
organisms in which any pair of members
can breed together.
• This means that they can regularly exchange gametes
to produce normally-fertile offspring, and such a
breeding group is also known therefore as
a gamodeme.
4

5. PROCESSES CAUSING
EVOLUTION IN A
POPULATION
NATURAL
SELECTION
GENE FLOW
MUTATION
GENETIC
DRIFT
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6.  Population genetics is a vital ingredient in the
emergence of the modern evolutionary synthesis.
 Its primary founders were
• Sewall Wright(1945)
• J. B. S. Haldane(1924)
• Ronald Fisher,(1930) who also laid the foundations
for the related discipline of quantitative genetics.
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7. • Population genetics began as a reconciliation
of Mendelian inheritance and biostatistics models
• In a series of papers starting in 1918 and culminating
in 1930 the book The Genetical Theory of Natural
Selection by Fisher.
• He showed that the continuous variation measured by
the biometricians could be produced by the combined
action of many discrete genes, and that natural
selection could change allele frequencies in a
population, resulting in evolution.
7

8. • In a series of papers beginning in 1924, another
British geneticist, J.B.S. Haldane worked out the
mathematics of allele frequency change at a single
gene locus under a broad range of conditions.
• Haldane also applied statistical analysis to real-world
examples of natural selection, such as the Peppered
moth evolution and industrial melanism
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9. VARIATION IN POPULATION
• If all members of a species have the same set of
genes, how can there be genetic variation?
NO VARIATION = NO NEW VARIETY= NO PLANT
BREEDING
• In a population, for any given gene there can be one
to many different alleles;
• however, because most organisms carry only one or
two chromosome sets per cell, any individual
organism can carry only one or two alleles per gene.
• The alleles of one gene will always be found in one
chromosomal position. Allelic variation is the basis
for hereditary variation. 9

10. TYPES OF VARIATION IN
POPULATION
CONTINOUS
VARIATION
DISCONTINOUS
VARIATION
10

11. CONTINOUS VARIATION DISCONTINOUS VARIATION
1. They cannot be classified into distinct
classes.
They can be put into various classes and
categories.
2.Caused by number of genes Caused by a single genes or few genes
3. Tends to be quantative Tends to be qualitative
4. They refer to small, indistinct differences
from the normal condition.
large, conspicuous differences from the
parents.
5. . They are also known as fluctuations. They are also known as mutations or spurts
6. due to chance segregation of genes during
gamete formation, crossing over or chance
combination during fertilization.
They are produced by change in genes or
genome.
Eg.height, weight,leaf length Eg.finger prints, blood groups
11

12. Continous variation
12

13. Discontinous variation
13

14. HOW VARIATION ARISE?
MIGRATION
RECOMBINATION
MUTATION
GENE FLOW
14

15. MIGRATION
• It is the movement of organisms from one location to
another.
• If the migrating individuals mate or cross with the
destination individuals, they can provide a sudden
influx of alleles
• This way migrating individuals contribute gametes
carrying alleles that can alter the existing proportion
of alleles in the destination population.
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16. MUTATION
• They are the changes in DNA or nucleotide
sequences which happen a sudden.
• A single mutation has a large efeect in creation of
variation in a population
• It creates an entirely new character in a population
which may be capable of selection
16

17. RECOMBINATION
• It is the production of offspring with the
combinations of traits that differ from those found in
parents.
• Genetic recombination occurs during meiosis
• It is a major contributor in variation in a population
17

18. GENE FLOW
• Gene flow is the transfer of alleles or genes from one
population to another.
• These alleles bring about variation in population and
this is related to migration as brings new alleles to
the population.
• Generally horizonal gene transfers that is through
hybridization which is used by breeder.
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19. GENE POOL CENTRES
• Gene pool centres refers to :
areas on the earth where important crop plants and
domestic animals originated.
They have an extraordinary range of the wild
counterparts of cultivated plant species and useful
tropical plants.
Gene pool centres also contain different sub tropical
and temperate region species
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20. HARDY- WEINBERG LAW
HARDY (1908) IN ENGLAND
WEINBERG (1909) IN GERMANY
20

21. Hardy- Weinberg law states that gene and
genotype frequencies in a Mendelian
population remain constant generation after
generation if there is no selection, mutation,
migration or random drift.
21

22. ACCORDING TO HARDY
WEINBERG LAW
• The frequencies of 3 genotypes for a locus
with two alleles,
• Say A and a, will remain constant at:
• p² (AA), 2pq (Aa) and q² (aa). Where p and q
are frequencies of alleles A and a, respectively
• Since, p+q=1. So,
p² + 2pq +q²=1
22

23. ASSUMPTIONS OF HARDY –
WEINBERG LAW
Large population
Random mating
No mutation
No migration
Natural selection
23

24. ASSUMPTIONS
 LARGE POPULATION: large size of population limits
errors and also Allele frequencies in a small population
are more likely to get affected by random drift as
compared to allele frequencies in a large population.
 RANDOM MATING. It refers to matings in a population
that occur in proportion to their genotypic frequencies.
If deviation from the expected value occurs, then no
random mating. So, the assumption will be eliminated.
 NO MUTATIONS. Mutations create and maintain
variations within a population and these introduce new
genes and alleles in a gene pool which violate the hardy
weinberg law.
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25.  NO MIGRATION. Immigration results in addition
of new alleles into the existing gene pool which
results in large changes in allele frequencies.
Migration in population may cause unbalance in
the law.
 NO NATURAL SELECTION: Due to natural
selection, certain phenotypes are favored over
other. So, the allele frequencies will change and
the population will evolve which will disturb the
law.
25

26. Hardy–Weinberg genotype frequencies for two alleles: the
horizontal axis shows the two allele frequencies p and q and the
vertical axis shows the genotype frequencies. Each curve shows
one of the three possible genotypes.
26

27. GENE POOL CONCEPT IN CROP
BREEDING
• Harlan and de Wet (1971) proposed classifying each
crop and its related species by gene pools rather
than by formal taxonomy.
27

28. GENE POOL
Set of all genes, or
genetic information,
in a single
population, usually of
a particular species
28

29. GENE POOL
Sum of a population’s
genetic material at a
given time.
29

30. 30

31. TYPES OF GENE POOL
TERTIARY
GENE POOL
PRIMARY
GENE POOL
SECONDARY
GENE POOL
31

32. PRIMARY GENE POOL (GP-1):
• Members of this gene pool are probably in the same
"species" and can intermate freely.
• Among forms of this gene pool, crossing is easy;
hybrids are generally fertile with good chromosome
pairing;
• gene segregation is approximately normal and gene
transfer is generally easy.
32

33. Example of primary gene pool
• The primary gene pool of both cultivated and wild
varieties of sunflower(Helianthus annus) .
• A winter’s sunflower (H.winterii) a perennial grass in
south Siberia whose genes are easy to be bought in
cultivated ones.
• So both constitute members of gene pool 1.
33

34. SECONDARY GENE POOL (GP-2):
• Members of this pool are normally classified as
different species than the crop species under
consideration (the primary gene pool).
• These species are closely related and can cross and
produce at least some fertile hybrids. BUT…
 hybrids may be weak
 hybrids may be partially sterile
 chromosomes may pair poorly or not at all
 recovery of desired phenotypes may be difficult in
subsequent generations
 The gene pool is available to be utilized, if the plant breeder
or geneticist is willing to put out the effort required.
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35. Example of secondary gene pool
• Aegilops tauschii AND Aegilopes speltoides,
two wild relatives in secondary gene pool of
bread wheat (Triticum aestivum)are diploid.
That means that they have paired
chromosomes whereas bread wheat is
hexaploid(six copies).
• So, some crosses if result from this would be
partially sterile or weak.
• THESE CONSTITUTE SECONDARY GENE POOL
35

36. TERTIARY GENE POOL (GP-3):
• Members of this gene pool are more distantly related
to the members of the primary gene pool. The
primary and tertiary gene pools can be intermated,
but gene transfer between them is impossible
without the use of "rather extreme or radical
measures" such as:
• embryo rescue (or embryo culture, a form of plant
organ culture)
• induced polyploidy (chromosome doubling)
• bridging crosses (e.g., with members of the
secondary gene pool). 36

37. Example of tertiary gene pool
• Triticum turgidum (AABB, 2n=28) and
• Aegilops speltoides(BB,2n=14)
produce amphdipoid hybrid (INTERGENERIC CROSS).
This can be made fertile using various techniques like
colchicine treatment.
EXAMPLE OF A SUCESSFUL CROSS BEING Triticale
DEVELOPED BY RIMPAU.
37

38. ALLELE FREQUENCY
• The allele frequency for an allele is the fraction of the
genes in the pool that is composed of that allele (for
example, what fraction of moth coloration genes are
the black allele).
• Evolution occurs when there are changes in the
frequencies of alleles within a population; for
example, the allele for black color in a population of
moths becoming more common
38

39. ALLELE FREQUENCY
• Allele frequency, or gene frequency, is the relative
frequency of an allele (variant of a gene) at a
particular locus in a population, expressed as a
fraction or percentage.
• It is the fraction of all chromosomes in the
population that carry that allele.
• Microevolution is the change in allele frequencies
that occurs over time within a population.
39

40. CALCULATION OF ALLELE
FREQUENCY
Allele = number of specific type of allele
frequency total number of allele in population
MONOPLOIDS
• The frequency (p) of an allele A is the fraction of the
number of copies (i) of the A allele and the
population or sample size (N), so
• p = i/N.
40

41. DIPLOIDS
• If f(AA),f(AB) AND F(BB) are the frequencies of the
three genotypes at a locus with two alleles,
• Then,
o p=f(AA)+1/2f(AB)=frequency of A
o q=f(BB)+1/2f(AB)=frequency of B
o Their sum must be equal to 1 as p+q=1
41

42. EXAMPLE
• Consider a locus that carries two alleles, A and B. In a
diploid population there are three possible genotypes,
two homozygous genotypes (AA and BB), and one
heterozygous genotype (AB). If we sample 10
individuals from the population, and we observe the
genotype frequencies.
1.freq(AA) = 6
2.freq(AB) = 3
3.freq(BB) = 1
• Then there are 6 × 2 + 3 = 15 observed copies of the A
allele and 1 × 2 + 3 = 5 of the B allele, out of 20 total
chromosome copies. The frequency p of the A allele is
p = 15/20 = 0.75, and the frequency q of the B allele is
q = 5/20 = 0.25.
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43. RARE AND COMMON ALLELES
COMMON ALLELES
• The alleles in a population
with frequencies >1%
• These alleles are frequent
enough in a population
• They can be observed by
genotyping in standard
marker panels
RARE ALLELES
• They are polymorphic
alleles with <1% frequency
• They are seen in only
handful of individuals or are
private to a single individual
• They can be observed by
sequencing technologies.
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44. REFRENCES
https://en.m.wikipedia.org/wiki/Gene_pool
http://www.fao.org/docrep/006/y4011e/y4011e0e.htm
http://www.yourarticlelibrary.com/biology/5-factors-
affecting-genetic-equilibrium-hardy-weinberg-equilibrium-
theory/27259/
https://en.m.wikipedia.org/wiki/Allele_frequency
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2914559/
https://en.m.wikipedia.org/wiki/Hardy%E2%80%93Weinbe
rg_principle
http://www.cwrdiversity.org/about/what-is-a-genepool/
http://study.com/academy/lesson/what-is-a-gene-pool-
definition-example-quiz.html
https://en.m.wikipedia.org/wiki/Population_genetics
http://evolution.berkeley.edu/evolibrary/article/evo_17
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