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PLANT BREEDINGAGR 3204
GENETICS AND VARIABILITY IN CROP
PLANTS
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Genetics and variability of traits are grouped by:
Qualitative traits
Traits that show variability that can be classified into discrete (clear-cut) classes that are easily identifiable.
Eg. Flower colour, fruit shape, stem colour etc. Quantitative traits
Traits that show variability in continuous form, and could only be identified through measurements. They do not show any particular discrete form.
Eg. Sugar content, plant height, fruit size, number of fruits per plant etc.
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QUALITATIVE TRAITS
Controlled by few number of genes, 1-3 loci (major gene)
Mostly expressed in dominant/ recessive forms
Absence / very minimum influence of environment on their expression
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Example of Qualitative Trait
Red(RR) Red (Rr) White (rr)
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EFFECT OF SELECTION ON QUALITATIVE TRAITSA) Recessive traits
Only expressed in the homozygous form in composition of segregating generation (e.g. F2)
RR 2Rr rr
No. of Loci(n) Rate of Recessive Homozygous Individuals (1/4)n
123
1/41/161/64
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A) Recessive Traits(cont.) Can be recognized and selected in
one generation only – but required an appropriate minimum population size (big enough) to detect its presence.
Dominant gene can be eliminated in one generation of selection.
EFFECT OF SELECTION ON QUALITATIVE TRAITS
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B) Dominant traits Expressed in the homozygous (RR) and
heterozygous (Rr) forms.
RR 2Rr rr
EFFECT OF SELECTION ON QUALITATIVE TRAITS
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EFFECT OF SELECTION ON QUALITATIVE TRAITS
B) Dominant Trait (Cont.) More difficult to select for dominant traits
– need more than one generation of selection.
Example to select Red Petunia with red flowers Colour of petunia flower: Red (RR, Rr) and white
(rr). F2 – ¾ red (RR, Rr) and ¼ white.
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B) Dominant Traits (Cont.) Methods:-1. Select for plant with red flower- 1/3 RR & 2/3 Rr2. Selfed pollinate plants with red flowers and grow
seeds from them. Selfing of Rr – gives progenies that are ¾ red & ¼ white
flowers Selfing of RR – gives all progenies with red flowers (RR).
3. Select only the plants that give progenies with all red flowers. Discard the plants with progenies segregating for the flower colour.
4. To select dominant gene or to eliminate the recessive gene requires two generations.
First generation: selection Second generation: progeny testing
EFFECT OF SELECTION ON QUALITATIVE TRAITS
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QUANTITATIVE TRAIT
Most of the economically valuable characters.
Controlled by many genes - polygenes. Each gene has cumulative contribution
to the expression of the character. Expression of quantitative genes usually
influenced by environment effects.
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PHENOTYPIC VARIATION (VP) OF QUANTITATIVE TRAITS
Consist of: 1. Genetic Variance (VG)
2. Environmental Variance (VE)
3. Variance Due to Interaction between Genetic and Environment (VGE)
Therefore: VP = VG + VE + VGE
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HERITABILITY
DEFINITION: Contribution of genetic component to a certain character, compared to that of the environment
Heritability (%) = VG / VP X 100
VG
Heritability calculated based on all genetic factors over phenotypic variance is called BROAD-SENSE HERITABILITY
VG + VE + VGE
X 100
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MAJOR COMPONENT OF GENETIC EFFECT
Genetic effect are divided to 3 components: 1. VA – Additive variance: Indicates the
number of favorable alleles needed for a particular locus
2. VD – Dominance variance: Interaction between alleles within the same locus
3. VI – Epistasis: Interaction among genes of different loci
Therefore: VG = VA + VD + VI
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Ratio of additive variance over phenotypic variance is called NARROW-SENSE HERITABILITY
Narrow-sense Heritability = VA x 100
VP
Narrow-sense heritability is more meaningful because:
Additive effect are transmitted to the next generation
Dominance (interaction between alleles within the same locus) and epistasis (interaction between loci) varied between generations.
Epistasis effects are usually small and could be neglected.
MAJOR COMPONENTS OF GENETIC EFFECTS
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EXAMPLE
Consider plant height controlled by one locus A/a
A=45 cm and a= 15cm
Additive effect: AA = 90cm, Aa = 60 cm aa=30cm
Dominance effect: AA = Aa = 90cm aa=30cm
aa=30cm
Aa = 60cm
AA = 90cm
aa=30cm
M AA = Aa = 90cm
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EXAMPLE
GENE EFFECTS
FEMALE PARENT
MALE PARENT
AVERAGE HEIGHT OF PROGENIES
(F1)
No Dominance(Completely additive)
Complete Dominance
AA (90 cm)
AA (90 cm)
aa (30 cm)
aa (30 cm)
Aa (60 cm)
Aa (90 cm)
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METHODS TO DETERMINE GENETIC VARIANCE COMPONENTS AND HERITABILITY
Crosses between 2 homozygous parents
Parent P1 x Parent P2
F1
A1A2
F2
1(A1A1) 2(A1A2) 1(A2A2)
(A1A1) (A2A2
)
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METHODS TO DETERMINE GENETIC VARIANCE COMPONENTS AND HERITABILITY (Cont.)
Backcross 1 to parent P1 (BC1P1)
Parent P1 x Parent P2
F1
(A1A2)
BC1P1
1(A1A1) 1(A1A2)
(A1A1) (A2A2
)
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METHODS TO DETERMINE GENETIC VARIANCE COMPONENTS AND HERITABILITY (Cont.)
Backcross 1 to parent P2 (BC1P2)
Parent P1 x Parent P2
F1
A1A2
BC1P2
1(A2A2) 1(A1A2)
(A1A1) (A2A2
)
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METHODS TO DETERMINE GENETIC VARIANCE COMPONENTS AND HERITABILITY (Cont.) All populations are planted at the
same time in the same environmentPopulation Variance
Expected Variance Components
Genotype
VP1
VP2
VF1
VF2
VBC1P1
VBC1P2
VE
VE
VE
VA+VD+VE
½VA+VD+VE
½VA+VD+VE
A1A1
A1A1
A1A1
A1A2
2A1A2
A1A2
A1A2
A2A2
A2A2
A2A2
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METHODS TO DETERMINE GENETIC VARIANCE COMPONENTS AND HERITABILITY (Cont.)
1. Environmental Variance (VE)
VE = (VP1 + VP2 + VF1)/3
2. Phenotypic Variance (VP)
VP = VG + VE = VA + VD + VE = VF2
3. Genetic Variance (VG) VG = VP - VE
= VF2 –[(VP1 + VP2 + VF1)/3]
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METHODS TO DETERMINE GENETIC VARIANCE COMPONENTS AND HERITABILITY (Cont.)
4. Additive Variance (VA) 2VF2 = 2VA + 2VD + 2VE VBC1P1 + VBC1P2 = VA + 2VD + 2VE VA = 2VF2 - (VBC1P1 + VBC1P2 )
5. Dominance Variance (VD) VD = VG - VA
= {VF2 –[(VP1 + VP2 + VF1)/3]} - {2VF2 - (VBC1P1 + VBC1P2 )}
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METHODS TO CALCULATE HERITABILITY
1. Based on P1, P2, F1, dan F2 Population Variation Broad-sense Heritability (HB) = VG/VP
= VF2 –[(VP1 + VP2 + VF1)/3]
VF2
2. Based on F2 , BCP1 & BCP2 Population Variation Narrow-sense Heritability (HN) = VA/VP
= 2VF2 - (VBC1P1 + VBC1P2 )
VF2
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METHODS TO CALCULATE HERITABILITY (Cont.)3. Parent (X) to Offsprings (Y) Regression
Method
Y= a + bX
High Heritability value = character from the parent is highly inherited by the offsprings
Y
X
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METHODS TO CALCULATE HERITABILITY (Cont.)3. Parent (X) to Offspring (Y)
Regression Method Arrangement of parent and offspring
dataFemale Parent
Male Parent
Parent Average
Offspring
X1 X1 X1 Y1
X2 X2 X2 Y2
X3 X3 X3 Y3
. . . .
. . . .
Xn Xn Xn Y4
SX SX SX SY
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Parent- Offspring Regression (bxy)
= Sxy - {(SxSy)/n}Sx2 - {(Sx)2/n}
where: y = offspring value
x = parent value If X is the value of one of the parent (male or female):
Narrow-sense Heritability (HN) = 2b If X is the average value of the parents:
Narrow-sense Heritability(HN) = b
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METHODS TO CALCULATE HERITABILITY (Cont.)
4. Components in Analysis of Variance (ANOVA) Method
Source of variation
d.f. Mean squares
Expected Mean Squares
ReplicationGenotypeError
r-1g-1(r-1)(g-1)
M1M2
se2 + rsg
2
se2
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METHODS TO CALCULATE HERITABILITY (Cont.)
Computation of Variance Components:
VG = sg2 = (M1 – M2)/r
= (se2 + rsg
2 - se2)/r
= rsg2/r
= sg2
VE = se2 = M2
Broad-sense Heritability (HB) = VG /(VG + VE )
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Genetic Advance From Selection From heritability value, genetic advance
from selection can be estimated:
Original populati
on
Selected
parent
Progenies (offspring) of
selected parents
XO =10 t/ha
XS=14 t/ha
XE = ?
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Original population
Selected population
Progenies of Selected population
Genetic Advance From Selection (Cont.)
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Genetic Advance From Selection (Cont.)
Computation of Genetic Advance (GA) and population mean of progenies of selected population (XE):-Consider the Heritability (H) = 60% GA = (XS- XO)H
= (14 – 10)0.6=2.4 t/ha
XE = XO + (XS - XO)H= 10 + 2.4 t/ha= 12.4 t/ha