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Genetic Basis of selection

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Genetic basis of selectionAlok kumarL-2012-A-80-M(Punjab Agricultural Univesity)[email protected]

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SELECTIONDifferential rate of reproductionComprises identification & isolation of plants having the desirable combination of charactersDetermine the success of breeding program

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Basis for SelectionEffective selection requires that traits be:HeritableRelatively easy to measureAssociated with economic valueGenetic estimates are accurateGenetic variation is available

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Self-pollinated CropsIn 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 selection5 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

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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

Johannsens results clarified the difference between phenotype and genotype and gave selection a firm scientific basis.

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The genetic basis of pure-line theoryThe 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.

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Percentage of homozygous and heterozygous individuals after self-fertilization of an individual heterozygous at single locus

GENERATIONGENOTYPE% HETEROZYGOTES% HOMOZYGOTESAAAaaaS00Aa0100NILS11/42/41/45050S23/82/83/82575S37/162/167/1612.587.5S101023/20482/20481023/20480.09899.902Sm2m-1 2m+112m2m-1 2m+1(1/2)m 100[1(1/2)m ] 100

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Percentage of completely homozygous individual for n segregating gene pairs after (m) generations of self-fertilization

GENERATIONFACTOR (gene) PAIRS1210nS00000S150250.10(1/2)n 100S27556.255.63(3/4)n 100S387.5076.5626.31(7/8)n 100Sm 2m 1 1 2m 100 2m 1 2 2m 100 2m 1 10 2m 100 2m 1 n 2m 100

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Sources of genetic variation in pure-lines 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 recombinationNew gene combination

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Application of pure-line breedingPure-line cultivar promotes mechanical farm operationCultivars 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

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GENETIC ISSUESPure-line breeding produces cultivars with a narrow genetic base

Depend primarily on production response and stability across environments

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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 population14

Bulk method

X

ParentsF1F2F3F4F5F6F7F8F9F10F11

GENETIC BASIS OF BULK SELECTIONGene frequencies in a population by the bulk method are determined by four variables associated with natural selection in a heterogeneous populationCompetitive ability of a genotypeInfluence of the environment on the genotype expressionSampling of genotypes to propagate the next generation Natural selection play important role in genetic shift in favour of good competitive genotype

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1 (1/2)

Recurrent parentDonor parentaaAAAaF1aaAaBC1F1BC2F1aaAaBC3F1BC2F1aaAAaaAaAaaaBC4F1

RemovedRemovedRemovedRemoved

Aa

Removed

Selfing(1)(2)(1) maintained(2) RemovedBackcross for a dominant alleleProgeny test

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Genetic basis of cross pollinated crops

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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 generationThe 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

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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 generationChanges in allele frequency can cause change in phenotype frequency; long-term change in allele frequency is evolutionary change

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Measure ofallele Frequencies in PopulationsPopulation 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

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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:

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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

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Mathematics ofthe Hardy-Weinberg Law

For a population, p + q = 1p = frequency of the dominant allele Aq = 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)

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Genotypic frequencies under the Hardy-Weinberg Law The Hardy-Weinberg Law indicates:At equilibrium, genotypic frequencies depend on the frequencies of the allelesThe maximum frequency for heterozygotes is 0.5If 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 driftIn 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

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MigrationIs 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 (q is large) when q is intermediate but is very ineffective when gene frequency is extreme (q is close to 0 or 1)27

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InbreedingInbreeding 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(1F)+FP] : [2PQ(1F)] : [Q2(1F)+FQ] If F=0, then it is reduced to P2+2PQ+Q2

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Results of inbreedingProlonged 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%.

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applicationInbreeds 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.

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Gene actionEffect of gene on trait Two types additive non additiveAdditive gene action: each additional gene enhances the expression of the trait by equal increments. Non additive gene action: it is devi