1. THEORY AND APPLICATION Department of Agronomy
PHENOTYPIC SELECTION
THEORY AND APPLICATION
Kendall R. Lamkey
USDA-ARS
Department of Agronomy
Iowa State University
Ames, IA
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2. OUTLINE Mendelian vs. Quantitative Traits Genotypic Value
Average Effect
Breeding Value
Heritability
Genetic Gain
Empirical Results
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3. REFERENCES
Falconer, D. S Introduction to quantitative genetics. 2nd ed. Longman, New York.
Lush, J. L The genetics of populations. Iowa Agriculture and Home Economics Experiment Station, College of Agriculture, Iowa State University, Ames.
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4. QUANTITATIVE GENETICS
Continuous
Variation
Mendelism
Fisher, R. A The correlation between relatives on the supposition of Mendelian Inheritance. Trans. Roy. Soc. Edinburgh 52:
Haldane, J. B. S The causes of evolution. Longmans, Green, London
Wright, S Systems of Mating. Genetics 6:
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5. QUANTITATIVE GENETICS
Temperature
Precipitation
Etc.
Genes
Environment
Genotype
Phenotype
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6. QUANTITATIVE GENETICS
SELECTION
Selection Must Be on Phenotype or Some Function of Phenotype
Breeders Only Rarely Know Genotypes for More Than a Few of the Loci an Individual Possesses
Individuals Chosen As Parents Transmit Only a Sample of Half of the Genes It Has
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7. QUANTITATIVE GENETICS
SELECTION
Show Us How to Choose Individuals With the Best Merit (Breeding Value)
Predict the Outcome of Selection to Compare Different Breeding Schemes
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8. QUANTITATIVE GENETICS
P = G + E
PHENOTYPIC VALUE (P) = Value observed when the character is measured on an individual
GENOTYPIC VALUE (G) = Value attributable to the particular assemblage of genes possessed by an individual
ENVIRONMENTAL DEVIATION (E) = Value attributable to all nongenetic circumstances that influence phenotype
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9. QUANTITATIVE GENETICS
Assumptions
Random Mating Equilibrium
No Linkage
No Epistasis
Diploid Inheritance
f(A) = p
f(a) = q
p + q = 1
p2AA + 2pqAa + q2aa
Random Mating Population
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10. GENOTYPIC VALUE aa Aa AA -a -4 d 2 a 4 aa Aa AA 6 12 14 Yieldd 2 a 4
aa Aa AA
Yield
MP = (14 + 6)/2 = 10 a = = 4
d = = 2 d/a = 2/4 = 0.5
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11. GENOTYPIC VALUE Genotype Frequency Value Freq x Value AA p2 a p2a
Aa 2pq d 2pqd
aa q2 -a -q2a
Mean = a(p-q) + 2pqd
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12. GENOTYPIC VALUE
With selection we are concerned with the transmission of value from parent to offspring.
This cannot be determined based on genotypic value alone.
Parents pass on their genes and not their genotypes to the next generation.
Genotypes are created anew in each generation.
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13. GENOTYPIC VALUE Individuals transmit genes (alleles) to their progeny.
One result of this is that some aspects of the value of a particular genotype are unpredictable.
Selection theory can work only with the predictable aspects of the union of two gametes.
Therefore, we need to introduce the average effect of a gene.
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14. P = ai + aj + dij + E ai = Average (or additive) effect of allele i
AVERAGE EFFECT
P = ai + aj + dij + E
ai = Average (or additive) effect of allele i
dij = Dominance deviation
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15. AVERAGE EFFECT
Average effect of a gene - the mean deviation from the population mean, of individuals that received the gene from one parent, the gene received from the other parent having come at random from the population.
Average effect of a gene - let a number of gametes carrying A unite at random with gametes from the population; then the mean of the genotypes so produced deviates from the population mean by an amount that is the average effect of the A gene.
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16. AVERAGE EFFECT Mean = a(p-q) + 2pqd Mean = pa + qd p2AA + 2pqAa + q2aa
pA + qa A
pA + qa
pAA + qaa
Mean = pa + qd
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17. AVERAGE EFFECT aA= q[a + d(q - p)] aa= -p[a + d(q - p)] a = aA - aa
Average effect of a gene substitution - The mean change in value produced by changing (a) genes at random into (A) genes.
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18. AVERAGE EFFECT Dependent on genotypic value.
Dependent on gene frequencies.
Property of the population as well as the genes concerned.
Average effect of a gene cannot be measured.
So, we introduce the concept of breeding value.
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19. BREEDING VALUE
Breeding value is the value of an individual judged by the mean value of its progeny.
The Breeding Value of an individual is equal to the sum of the average effects of the genes it carries. The summation is over pairs of alleles at a locus and over all loci.
Genotype Breeding Value
AA 2aA
Aa aA + aa
aa 2aa
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20. BREEDING VALUE 2qa +a d (q - p)a a -2pa -a 1 2 aa Aa AA q2 2pq p2
(q - p)a a
-2pa -a 1 2 aa Aa AA q2
2pq p2
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21. Cp Cs Z c P
PHENOTYPIC SELECTION
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22. PHENOTYPIC SELECTION R = bopS R = Response S = Selection Differential
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23. PHENOTYPIC SELECTION Base Population C1 C2 . Cn Develop Progenies
Intermate
Selections
Evaluate
Progenies
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24. OBJECTIVES OF SELECTION
PHENOTYPIC SELECTION
OBJECTIVES OF SELECTION
Increase the Frequency of Favorable Alleles, Which Is to Increase the Mean of the Population in the Favorable Direction
Maintain Genetic Variability for Continued Selection by Intermating Superior Progenies for Each Cycle of Selection.
Therefore, Enhancing the Probability of Obtaining Superior Genotypes (Lines or Hybrids) From the Population.
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25. PHENOTYPIC SELECTION Original Population Improved Population
Best Hybrid From
Original Population
Best Hybrid From
Improved Population
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26. PHENOTYPIC SELECTION
Plant Breeders Determine Merit Based on Phenotypes.
The Goal of Plant Breeding Is to Separate the Environment From the Breeding Value.
From a Quantitative Genetic Point of View This Is Equivalent to Maximizing the Heritability.
From a Statistics/Regression Point of View This Is Equivalent to Maximizing the Correlation Between Phenotype and Breeding Value.
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27. V(P) = V(ai) + V(aj) + V(dij) + V(E)
HERITABILITY
V(P) = V(G) + V(E)
V(P) = V(ai) + V(aj) + V(dij) + V(E)
V(P) = V(A) + V(D) + V(E)
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28. HERITABILITY V(P) = Variance among phenotypes Phenotypic variance
V(A) = Variance among breeding values
Additive variance
V(D) = Variance among dominance deviations
Dominance Variance
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29. HERITABILITY h2 = V(A)/V(P)
Heritability = The extent to which phenotypes are determined by genes transmitted from the parents.
h2 = V(A)/V(P)
The goal of an artificial selection program is to maximize heritability
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30. GENETIC GAIN
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31. GENETIC GAIN p Z
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32. GENETIC GAIN where, i = Standardized selection differential
c = parental control
y = years per cycle
r = number of replications per environment
e = number of environments
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33. INCREASING GENETIC GAIN
Increase Selection Intensity
Increase Genetic Variance
Decrease Years per Cycle
Decrease Phenotypic Variance
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34. INCREASING GENETIC GAIN
Increase Selection Intensity
Proportion Selected I
30 of
20 of
10 of
5 of
1 of
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35. INCREASING GENETIC GAIN
Increase Genetic Variance
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36. INCREASING GENETIC GAIN
Decrease Years per Cycle
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37. INCREASING GENETIC GAIN
Decrease Phenotypic Variance
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38. APPLICATION EMPIRICAL RESULTS
PHENOTYPIC SELECTION
APPLICATION
EMPIRICAL RESULTS
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39. RECIPROCAL RECURRENT SELECTION
BSSSC0
BSCB1C0
HS Families
BSSS(R)C1
BSCB1(R)C1
BSSS(R)C9
BSCB1(R)C9
BSSS(R)C12
BSCB1(R)C12
BSSS(R)C1 X BSCB1(R)C1
FS Families
BSSS(R)C12 X BSCB1(R)C12 S1
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40. RECIPROCAL RECURRENT SELECTION
BSSS - Iowa Stiff Stalk Synthetic
- 16 Inbred Line Synthetic
- Synthesized in Early 1930s
BSCB1 - Iowa Corn Borer Synthetic #1
- 12 Inbred Line Synthetic
- Synthesized in 1940s
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41. IOWA STIFF STALK SYNTHETIC
Os WD Ind Ill CI CI Ill Oh Ind TR A3G- CI Le
I159 I E HY 3167B AH F1B
Single
Crosses
Double
Crosses
Double-Double Crosses
Bulk Equal Quantities Seed
BSSSC0
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42. IOWA STIFF STALK SYNTHETIC
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43. IOWA STIFF STALK SYNTHETIC
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44. IOWA STIFF STALK SYNTHETIC
BSSS(R)C0 X
BSCB1(R)C0
BSSS(R)C5 X
BSCB1(R)C5
BSSS(R)C11 X
BSCB1(R)C11
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Grain Yield Mg ha-1

45. IOWA STIFF STALK SYNTHETIC
BSSS(R)
BSCB1(R) C4 C4 C0 C0 C7 C9 C7 C9
C11
C11
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46. IOWA STIFF STALK SYNTHETIC
Heterozygosity
Population Observed Expected
Progenitors C C
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47. IOWA CORN BORER SYNTHETIC #1
Heterozygosity
Population Observed Expected
Progenitors C C
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48. NEI’S GENETIC DISTANCE
BSSS(R)
BSCB1(R) P
C12
0.33
0.26
0.07
0.66
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49. Gene diversity GENE DIVERSITY BSSS(R) and BSCB1(R) 0.7 Total 0.6 0.5P
0.5
Gene diversity C0
Mean
0.4
within
0.3
C12
0.2
BSSS(R) and BSCB1(R)
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50. BS11 SELECTION METHODS STUDY
Last Cycle
Method Evaluated
Full-sib (FS) 5
Half-sib with Inbred Test. (HI) 4
Modified Ear-to-row (MER) 5
Mass selection (MASS)
Reciprocal Full-sib (FR) 5
S1 Progeny (S1) 5
S2 Progeny (S2) 4
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51. BS11 SELECTION METHODS STUDY
Cycle # Progeny Selection
Method Tester Time Evaluated * Intermated Intensity
FS BS
MER BS
HI B
S2 BS
S1 BS
FR BS
MASS BS
*Evaluations based on 2 Reps at 3 Locations
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52. BS11 SELECTION METHODS STUDY Grain Yield - Populations Per Se
5.8
Response
R2 = 0.83
Per Cycle % S2
MER
5.6
S2 0.21** 4.5
MER 0.17** 3.6
FR 0.12** 2.6
S1 0.09** 1.9
HI 0.08** 1.6
FS 0.07** 1.4
MASS 0.03** 0.6
5.4 FR
Mg ha-1
5.2 S1 HI FS
5.0
4.8
MASS
4.6 1 2 3 4 5
Cycle
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53. BS11 SELECTION METHODS STUDY Stalk Lodging - Populations Per Se
24.0
MASS
22.0 FR
20.0
18.0
Response
S **
MER -2.2**
FS -2.2**
HI -2.1**
S **
FR 0.0
MASS 0.3**
Per Cycle %
16.0
14.0 HI
12.0 S1
10.0 S2 FS
R2 = 0.85
MER
8.0 1 2 3 4 5
Cycle
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54. ACKNOWLEDGEMENTS Dr. Joanne Labate Roger Weyhrich Jode Edwards
Peter Guzman
Chris Mack
Kebede Mulatu
John Golden
Jim Sears
Dr. Arnel R. Hallauer
Dr. Michael Lee
Dr. Howie Smith
Dr. Paul Scott
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55. U.S. CORN YIELD - 1866 to 1996 R2 = 0.96 Single Crosses b = 2.06
Double Crosses
b = 1.10
Open pollinated
b = -0.08
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