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
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
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