The document discusses various concepts in plant breeding and genetics including selection methods, genetic variation and inheritance, gene action, and estimation of genetic parameters. It provides information on selection in self-pollinated and cross-pollinated crops, the pure line theory, bulk and pure line selection methods, genetic variation and inbreeding, gene action and effects, estimation of genetic variance and heritability, and recurrent selection approaches. Examples and equations are given to explain key concepts in plant breeding and population genetics.

1. Alok kumar
L-2012-A-80-M
(Punjab Agricultural Univesity)
kumaralokgupta1987@gmail.com

2. SELECTION
 Differential rate of reproduction
 Comprises identification & isolation of plants
having the desirable combination of
characters
 Determine the success of breeding program

3. Basis for Selection
Effective selection requires that traits be:
 Heritable
 Relatively easy to measure
 Associated with economic value
 Genetic estimates are accurate
 Genetic variation is available

4. Self-pollinated Crops
In self-pollinated species:
 Homozygous loci will remain homozygous following
self-pollination
 Heterozygous loci will segregate producing half
homozygous progeny and half heterozygous progeny
 Plants selected from mixed populations after 5-8 self
generations will normally have reached a practical
level of homozygosity

5.  In general, a mixed population of self-pollinated plants is
composed of plants with different homozygous
genotypes
 If single plants are selected from this population and seed
increased, each plant will produce a ‘pure’ population,
but each population will be different, based on the
parental selection
5
Self-pollinated Crops

6. The Pure Line Theory
His first conclusion was that selection for
seed weight was effective.
His second conclusion was that the original
landrace consisted of a mixture of homozygous
plants

7. Thus, his third conclusion was that the within-line
phenotypic variation was environmental in nature and
further selection within a pure line will not result in
further genetic change
Johannsen’s results clarified the difference between
phenotype and genotype and gave selection a firm
scientific basis.

8. The genetic basis of pure-line theory
 The variation for seed size in the original commercial seed lot of
beans was due to joint effects of heredity and environment.
 The variation within a particular pure-line was due to differences in
the micro-environment of each individual plant of the line.
 Few generations of selfing are required to reduce
heterozygosity(Aa)
 Reduction of heterozygosity at each locus occurs irrespective of the
number of other heterozygous loci.
 The percentage of homozygosity at a given locus is not affected by
the number of gene pairs.
 All the heterozygous loci approach homozygosity at the same rate.
 The proportion of completely homozygous individuals increases at
slower rates as the number of gene pairs increases whereas increase
in rate of homozygosity is independent of number of genes.

9. Percentage of homozygous and heterozygous
individuals after self-fertilization of an individual
heterozygous at single locus
GENERATI
ON
GENOTYPE %
HETEROZYGOT
ES
%
HOMOZYGOTES
AA Aa aa
S0 0 Aa 0 100 NIL
S1 1/4 2/4 1/4 50 50
S2 3/8 2/8 3/8 25 75
S3 7/16 2/16 7/16 12.5 87.5
S10 1023/2048 2/20
48
1023/
2048
0.098 99.902
Sm 2m-1
2m+1
1
2m
2m-1
2m+1
(1/2)m × 100 [1−(1/2)m ] ×100

10. Percentage of completely homozygous individual for ‘n’
segregating gene pairs after (m) generations of self-
fertilization
GENERATI
ON
FACTOR (gene) PAIRS
1 2 10 n
S0 0 0 0 0
S1 50 25 0.10 (1/2)n × 100
S2 75 56.25 5.63 (3/4)n × 100
S3 87.50 76.56 26.31 (7/8)n × 100
Sm 2m −1 1
2m × 100
2m −1 2
2m × 100
2m −1 10
2m ×100
2m −1 n
2m ×100

11. Sources of genetic variation in pure-
lines
1. Gene mutation
 creates variability within the pure line. The
rate of mutation is different for different loci.
 Alleles of same locus mutate at a variable rate
2. Natural crossing and recombination
 New gene combination

12. Application of pure-line breeding
 Pure-line cultivar promotes mechanical farm
operation
 Cultivars developed for a discriminating
market that puts a premium on eye-appeal (e.g.
uniform shape, size).
 Improving newly domesticated crops that have
some variability.
 Integral part of other breeding method

13. GENETIC ISSUES
 Pure-line breeding produces cultivars with a
narrow genetic base
 Depend primarily on production response and
stability across environments

14. PURE-LINE SELECTION
 A pure line consists of progeny descended solely by
self-pollination from a single homozygous plant
 Pure line selection is therefore a procedure for
isolating pure line(s) from a mixed population
14

15. Bulk method
XParents
F1
F2
F3
F4
F5
F6
F7
F8
F9
F10
F11

16. GENETIC BASIS OF BULK SELECTION
 Gene frequencies in a population by the bulk method
are determined by four variables associated with natural
selection in a heterogeneous population
1) Competitive ability of a genotype
2) Influence of the environment on the genotype
expression
3) Sampling of genotypes to propagate the next
generation
Natural selection play important role in genetic shift in
favour of good competitive genotype

17. 1− (1/2)
Recurrent
parent
Donor parent
aa AA
Aa F1
aaAa BC1F1
BC2F1
aaAa
BC3F1
BC2F1
aa
AA
aa
Aa
Aa aa
BC4F1
Removed
Removed
Removed
Removed
Aa
Removed
Selfing
(1) (2)
(1) maintained
(2) Removed
Backcross for a dominant allele
Progeny test

18. Genetic basis of cross pollinated crops

19.  Compared to self-pollinated species, cross-pollinated
species differ in their gene pool structure, and in the
extent of genetic recombination
 Unselected populations typically consist of a
heterogeneous mixture of heterozygotes; as a result of
outcrossing, genes are re-shuffled in every generation
 The breeder focuses more on populations, rather than
individual plants, and on quantitative analysis, rather
than qualitative traits
 Progeny do not breed true, since the parent plant is
pollinated by another plant with a different complement
of alleles

20. Allele Frequency
• Allele frequency
 The frequency with which alleles of a particular
gene are present in a population
 The frequency of alleles in a population may
change from generation to generation
 Changes in allele frequency can cause change in
phenotype frequency; long-term change in allele
frequency is evolutionary change

21. Measure of
allele Frequencies in Populations
 Population genetics studies allele frequencies
in populations, not offspring of single mating
 In some cases allele frequency in a population
can be measured directly
 In other cases, the Hardy-Weinberg Law is
used to estimate allele frequencies within
populations

22.  all mating is totally random
 there is no migration
 there is no mutation
 there is no selection
 the population is infinitely large
If these conditions are violated, a change in
frequencies will occur.
Allele and genotype frequencies will
remain stable if:

23. The Hardy-Weinberg Equation
p2 + 2pq + q2 = 1
 1 = 100% of genotypes in the new generation
 p2 and q2 are the frequencies of homozygous dominant
and recessive genotypes
 2pq is the frequency of the heterozygous genotype in
the population

24. Mathematics of
the Hardy-Weinberg Law
 For a population, p + q = 1
 p = frequency of the dominant allele A
 q = frequency of the recessive allele a
 The chance of a fertilized egg carrying the same
alleles is p2 (AA) or q2 (aa)
 The chance of a fertilized egg carrying different
alleles is pq (Aa)

25. Genotypic frequencies under the
Hardy-Weinberg Law
• The Hardy-Weinberg Law
indicates:
 At equilibrium, genotypic
frequencies depend on the
frequencies of the alleles
 The maximum frequency
for heterozygotes is 0.5
 If allelic frequencies are
between 0.33 and 0.66, the
heterozygote is the most
common genotype

26.  Mutation: a change in the sequence of a gene. May
produce new alleles.
 On short term, the effect of mutation is negligible
because mutation rate is very low.
Random drift
 In small finite populations, gene frequencies are not
stable. They are subject to random fluctuations
arising from the sampling of gametes
 Random fluctuations (changes) of gene frequencies
from one generation to the next in small populations
is called random genetic drift.
26

27.  Migration
 Is the movement of pollen from one population to
another.
 The effect of migration in changing gene frequency
depends on migration rate (m) the difference in allele
frequency between migrants and natives.
 Selection:
 Selection increases the frequency of favorable alleles
and decreases the frequency of unfavorable alleles.
 Selection is most effective (Δq is large) when q is
intermediate but is very ineffective when gene
frequency is extreme (q is close to 0 or 1)
27

28. Inbreeding
 Inbreeding is the mating of individuals that are closely
related by ancestry.
 A genetic consequence of inbreeding is the exposure of
cryptic genetic variability that was inaccessible to
selection and was being protected by heterozygosity.
 Inbreeding encourages non-random mating and it
effects Hardy-Weinberg equilibrium.
 It is measured by coefficients of inbreeding (F).
 Mathematically,
[P2(1−F)+FP] : [2PQ(1−F)] : [Q2(1−F)+FQ]
If F=0, then it is reduced to P2+2PQ+Q2

29. Results of inbreeding
 Prolonged selfing is an extreme form of
inbreeding with each selfing heterozygosity
decreases at a rate of 50%, whereas,
homozygosity increases at a rate of 50%.

30. application
 Inbreeds are used as parent for hybrid seed
production.
 Partial inbreds are used as parent in the
breeding of synthetic cultivar.
 It increases the diversity among individuals
among population, thereby, facilitating the
selection process in a breeding program.

31. Gene action
 Effect of gene on trait
Two types
1. additive
2. non additive
 Additive gene action: each additional gene
enhances the expression of the trait by equal
increments.
 Non additive gene action: it is deviation from
additivity that make the heterozygote resemble
more to one parent then other.

32. Gene action and plant breeding
Self pollinated crop
 Additive gene action: Pure line selection, mass
selection, progeny selection and hybridization.
 Non additive gene action: Heterosis
Cross pollinated spesies
 Additive gene action: Recurrent selection to
aceive general combining ability.
 Non genetic action: Heterosis

33. Genetic variance
 Heritable portion of total variance
 Three type
 Additive
 Dominance
 Epistatic
 Estimating of component of genetic variane

34. Estimating of component of genetic variane
P1
F1
F2
P2
BC1 BC2
Four genration F1, F2 ,BC1,BC2
AA
ha
−da+da
Aa aa
Relationship between two homozygote and heterozygote at a single locus in respect of
the phenotypic expression of polygenic trait

35.  More than one gene affecting a character,
phenotype of a homozygous line would be
 X=∑(+d)+∑(−d)+c
 ∑(+d)=additive effect of positive alleles at all
the loci
 ∑(−d)=additive effect of all the negative alleles
 C=effect of genotypic background and
environment

36.  Variance of different seggregating generation
with respect to single gene, Aa is
 Generation F2:
 genotype AA Aa aa
 phenotype da ha −da
 frequency ¼ ½ ¼
 VF2 (variance of F2)= ½ ∑d2 + ¼ ∑h2 +E
 VB1 (variance of B1)= ½ ∑d2 + ¼ ∑h2− ½ ∑dh +E
 VB2 (voriance of B2)= ½ ∑d2 + ¼ ∑h2 +1/2∑dh+E
VB1 +VB2 = 1/2D + 1/2H + 2E where
∑d2=D , ∑h2=H

37. HERITABILITY
 The heritability (H) of a trait is a measure of the degree of resemblance between
relatives.
genetic variance (VG)
phenotypic variance (VP)
H= VG / VP = VG / (VG + VE)
Heritability ranges from 0 to 1
(Traits with no genetic variation have a heritability of 0)
H =

38.  There are two estimate of heritability
1 Broad sence heritability:
heritability using the total genetic variance
H= VG / VP
2 Narrow sence heritability:
Ratio of additive variance to phenotypic variance
h2 =VA / Vp
It is more useful then the broad sence

39. Application of heritability
 Determine most effective selection strategy in
plant breeding
 Predicts gain from selection
 Determine whether a trait woud be
benefificial from breeding point of view

40.  A cyclical and systematic technique in which desirable
individuals are selected from a population and mated to
form new population , the cycle is then repeated.
 The purpose of a recurrent selection in a plant breeding
program is to improve the performance of a population.
 The improved population may be released as new
cultivar or used as breeding material in other breeding
programs.
 Population is improved without reduction in genetic
variability
Concept of Recurrent selection

41. Genetic basis of recurrent selection
 Recurrent for GCA is more effective when
additive gene effects are more important.
 Recurrent for SCA is more effective when
overdominance gene effects are more
important.
 Reciprocal recurrent selection is more
effective when both additive and
overdominance gene effects are important.

42. Application of recurrent selection
Establish broad genetic base,
add new germplasm
It break linkage blocks

43. THANK
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