Genetic Basis of selection

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1. Alok kumar L-2012-A-80-M (Punjab Agricultural Univesity) 2. SELECTION  Differential rate of reproduction  Comprises identification…
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  • 1. Alok kumar L-2012-A-80-M (Punjab Agricultural Univesity)
  • 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 YOU
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