Post on 21-Dec-2015
BIOE 109Summer 2009
Lecture 7- Part IISelection on quantitative characters
Selection on quantitative characters
What is a quantitative (continuous) character?
Selection on quantitative characters
What is a quantitative character?
• quantitative characters exhibit continuous variation among individuals.
Selection on quantitative characters
What is a quantitative character?
• quantitative characters exhibit continuous variation among individuals.
• unlike discrete characters, it is not possible to assign phenotypes to discrete groups.
Examples of discrete characters
Example of a continuous character
Two characteristics of quantitative traits:
1. Controlled by many genetic loci
Two characteristics of quantitative traits:
1. Controlled by many genetic loci
2. Exhibit variation due to both genetic and environmental effects
Two characteristics of quantitative traits:
1. Controlled by many genetic loci
2. Exhibit variation due to both genetic and environmental effects
• the genes that influence quantitative traits are now called quantitative trait loci or QTLs.
What are QTLs?
What are QTLs?
• QTLs possess multiple alleles, exhibit varying degrees of dominance, and experience selection and drift.
What are QTLs?
• QTLs possess multiple alleles, exhibit varying degrees of dominance, and experience selection and drift.
• some QTLs exhibit stronger effects than others – these are called major effect and minor effect genes, respectively.
What are QTLs?
• QTLs possess multiple alleles, exhibit varying degrees of dominance, and experience selection and drift.
• some QTLs exhibit stronger effects than others – these are called major effect and minor effect genes, respectively.
• the number and relative contributions of major effect and minor effect genes underlies the genetic architecture of the trait.
Mapping QTLs is expensive, labor intensive, and fraught with statistical problems!
Mapping QTLs is expensive, labor intensive, and fraught with statistical problems!
QTL mapping can reveal:1. Number of loci that influence a QT2. Magnitude of their effects on phenotype3. Their location on genome
Mapping QTLs is expensive, labor intensive, and fraught with statistical problems!
QTL mapping can reveal:1. Number of loci that influence a QT2. Magnitude of their effects on phenotype3. Their location on genome
QTL mapping CANNOT reveal:1. Identity of loci2. Proteins they encode
What is heritability?
What is heritability?
• heritability is the proportion of the total phenotypic variation controlled by genetic rather than environmental factors.
What is heritability?
• heritability is the proportion of the total phenotypic variation controlled by genetic rather than environmental factors.
The total phenotypic variance may be decomposed:
VP = total phenotypic variance
The total phenotypic variance may be decomposed:
VP = total phenotypic varianceVG = total genetic variance
The total phenotypic variance may be decomposed:
VP = total phenotypic varianceVG = total genetic varianceVE = environmental variance
The total phenotypic variance may be decomposed:
VP = total phenotypic varianceVG = total genetic varianceVE = environmental variance
VP = VG + VE
The total phenotypic variance may be decomposed:
VP = total phenotypic varianceVG = total genetic varianceVE = environmental variance
heritability = VG/VP (broad-sense)
The total genetic variance (VG) may be decomposed:
The total genetic variance (VG) may be decomposed:
VA = additive genetic variance
The total genetic variance (VG) may be decomposed:
VA = additive genetic varianceVD = dominance genetic variance
The total genetic variance (VG) may be decomposed:
VA = additive genetic varianceVD = dominance genetic varianceVI = epistatic (interactive) genetic variance
The total genetic variance (VG) may be decomposed:
VA = additive genetic varianceVD = dominance genetic varianceVI = epistatic (interactive) genetic variance
VG = VA + VD + VI
The total genetic variance (VG) may be decomposed:
VA = additive genetic varianceVD = dominance genetic varianceVI = epistatic (interactive) genetic variance
heritability = h2 = VA/VP (narrow sense)
Estimating heritability
Estimating heritability
• one common approach is to compare phenotypic scores of parents and their offspring:
Estimating heritability
• one common approach is to compare phenotypic scores of parents and their offspring:
Junco tarsus length (cm)
Cross Midparent value Offspring value
Estimating heritability
• one common approach is to compare phenotypic scores of parents and their offspring:
Junco tarsus length (cm)
Cross Midparent value Offspring value
F1 x M1 4.34 4.73
Estimating heritability
• one common approach is to compare phenotypic scores of parents and their offspring:
Junco tarsus length (cm)
Cross Midparent value Offspring value
F1 x M1 4.34 4.73
F2 x M2 5.56 5.31
Estimating heritability
• one common approach is to compare phenotypic scores of parents and their offspring:
Junco tarsus length (cm)
Cross Midparent value Offspring value
F1 x M1 4.34 4.73
F2 x M2 5.56 5.31
F3 x M3 3.88 4.02
Slope = h2
Regress offspring value on midparent value
Heritability estimates from other regression analyses
Comparison Slope
Heritability estimates from other regression analyses
Comparison Slope
Midparent-offspring h2
Heritability estimates from other regression analyses
Comparison Slope
Midparent-offspring h2
Parent-offspring 1/2h2
Heritability estimates from other regression analyses
Comparison Slope
Midparent-offspring h2
Parent-offspring 1/2h2
Half-sibs 1/4h2
Heritability estimates from other regression analyses
Comparison Slope
Midparent-offspring h2
Parent-offspring 1/2h2
Half-sibs 1/4h2
First cousins 1/8h2
Heritability estimates from other regression analyses
Comparison Slope
Midparent-offspring h2
Parent-offspring 1/2h2
Half-sibs 1/4h2
First cousins 1/8h2
• as the groups become less related, the precision of the h2 estimate is reduced.
Heritabilities vary between 0 and 1
Cross-fostering is a common approach
Heritability of beak size in song sparrows
Q: Why is knowing heritability important?
Q: Why is knowing heritability important?
A: Because it allows us to predict a trait’s response to selection
Q: Why is knowing heritability important?
A: Because it allows us to predict a trait’s response to selection
Let S = selection differential
Q: Why is knowing heritability important?
A: Because it allows us to predict a trait’s response to selection
Let S = selection differential
Let h2 = heritability
Q: Why is knowing heritability important?
A: Because it allows us to predict a trait’s response to selection
Let S = selection differential
Let h2 = heritability
Let R = response to selection
Q: Why is knowing heritability important?
A: Because it allows us to predict a trait’s response to selection
Let S = selection differential
Let h2 = heritability
Let R = response to selection
R = h2S
Predicting the response to selection
Example: the large ground finch, Geospiza magnirostris
Predicting the response to selection
Example: the large ground finch, Geospiza magnirostris
Mean beak depth of survivors = 10.11 mm
Predicting the response to selection
Example: the large ground finch, Geospiza magnirostris
Mean beak depth of survivors = 10.11 mm
Mean beak depth of initial pop = 8.82 mm
Predicting the response to selection
Example: the large ground finch, Geospiza magnirostris
Mean beak depth of survivors = 10.11 mm
Mean beak depth of initial pop = 8.82 mm
S = 10.11 – 8.82 = 1.29
Predicting the response to selection
Example: the large ground finch, Geospiza magnirostris
Mean beak depth of survivors = 10.11 mm
Mean beak depth of initial pop = 8.82 mm
S = 10.11 – 8.82 = 1.29
h2 = 0.72
Predicting the response to selection
Example: the large ground finch, Geospiza magnirostris
Mean beak depth of survivors = 10.11 mm
Mean beak depth of initial pop = 8.82 mm
S = 10.11 – 8.82 = 1.29
h2 = 0.72
R = h2S = (1.29)(0.72) = 0.93
Predicting the response to selection
Example: the large ground finch, Geospiza magnirostris
Mean beak depth of survivors = 10.11 mm
Mean beak depth of initial pop = 8.82 mm
S = 10.11 – 8.82 = 1.29
h2 = 0.72
R = h2S = (1.29)(0.72) = 0.93
Beak depth next generation = 10.11 + 0.93 = 11.04 mm
Modes of selection on quantitative traits
Directional selection on oil content in corn
Modes of selection on quantitative traits
Modes of selection on quantitative traits