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SELECTION Differential rate of reproduction Comprises identification & isolation of plants
having the desirable combination of characters
Determine the success of breeding program
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
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
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
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Self-pollinated Crops
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
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.
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.
Percentage of homozygous and heterozygous individuals after self-fertilization of an individual
heterozygous at single locus
GENERATION
GENOTYPE % HETEROZYGOTES
% HOMOZYGOTES
AA Aa aaS0 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 2/2048
1023/2048
0.098 99.902
Sm 2m-1
2m+112m
2m-1
2m+1(1/2)m × 100 [1−(1/2)m ] ×100
Percentage of completely homozygous individual for ‘n’ segregating gene pairs after (m) generations of self-
fertilizationGENERATION
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
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
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
GENETIC ISSUES
Pure-line breeding produces cultivars with a narrow genetic base
Depend primarily on production response and stability across environments
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
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Bulk method
XParents
F1
F2
F3
F4
F5
F6
F7
F8
F9
F10
F11
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 genotype2) Influence of the environment on the genotype
expression3) Sampling of genotypes to propagate the next generation Natural selection play important role in genetic shift in
favour of good competitive genotype
1− (1/2)
Recurrent parent
Donor parent
aa AA
Aa F1
aaAa BC1F1
BC2F1aaAa
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
Genetic basis of cross pollinated crops
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
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
Measure ofallele 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
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:
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
Mathematics ofthe 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)
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
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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.
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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)
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
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%.
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.
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.
Gene action and plant breedingSelf 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
Genetic variance
Heritable portion of total variance Three type Additive Dominance Epistatic Estimating of component of genetic variane
Estimating of component of genetic varianeP1
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
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
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
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 =
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 / VpIt is more useful then the broad sence
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
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
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.
Application of recurrent selection
Establish broad genetic base, add new germplasmIt break linkage blocks
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