genetics of complex traits: quantitative genetics
DESCRIPTION
Genetics of Complex Traits: Quantitative Genetics. Extra Credit Question Given in Email. Continuous Variation (height). Genetic Variation. Discrete Variation (presence/absence of tail). Height Weight Athletic ability Risk of heart disease Risk of diabetes Risk of cancer . Polygenic - PowerPoint PPT PresentationTRANSCRIPT
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Genetics of Complex Traits:Quantitative Genetics
Extra Credit Question Given in Email
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Genetic Variation
Continuous Variation (height)
Discrete Variation (presence/absence of tail)
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PolygenicEnvironmental influences
Have continuous (not discrete) distributions
HeightWeight
Athletic abilityRisk of heart disease
Risk of diabetesRisk of cancer
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PolygenicEnvironmental influences
Have continuous (not discrete) distributions
Can be measured on a quantitative scale
Quantitative Traits
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1
2
1
0
1
2
Genotype
Frequency
RR Rr rr
Discrete distribution
Partial dominanceIntermediate dominance= “additive” gene action
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F2: AB Ab aB abAB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
Red
Lt Red
Pink Lt Pink
Lt Red
PinkLt Red
Pink
Pink
Lt Pink
Lt Red
Pink
Pink
Lt Pink
Lt Pink
white
P: AABB red
x aabb white
F1: AaBb x AaBb All Pink
1 Red: 4 Lt Red: 6 Pink: 4 Lt Pink: 1 white
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Two additive genes: discrete phenotypic distribution
1
4
6
4
1
0
1
2
3
4
5
6
Red Lt Red Pink Light Pink White
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Color of wheat kernels: three additive genes
aabbcc AABBCC
AaBbCC
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Several additive genes
Plus environmental “noise”
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Additive Genes
# Additive genes # Phenotypes # Genotypes(n) (2n+1) (3n)
1 3 32 5 9 3 7 275 11 24310 21 59,049
20 41 3,486,784,401
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Frequency Distribution of Height of the Band
mean=68 inches
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Mean = = 68 inches
€
x =x i
i=1
n
∑n
Variance = = 9.5 in2
€
σ 2 =Var =x i − x( )
2
i=1
n
∑n −1( )
Properties of distributions
n=160
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Types of Variance
Phenotypic variance: total variance of the population, includes variation from
genes and from the environment
Genetic variance: the variance that is due to variation among individuals in the
alleles that they have, excludes environmentally-caused variation
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Phenotypic Variance
Var = 9.5 in2
Phenotypic variance = Genetic variance + Environ. variance VP = VG + VE
Mean = 68 in
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Genetic variance = Additive variance + Dominance Variance VG = VA + VD
VP = VA + VD + VE
Phenotypic variance = Genetic variance + Environ. variance VP = VG + VE
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Dominance effects Additive effectsAA Aa aa BB Bb bb+2 +2 +0 +2 +1 +0
P: AABB x aabb20cm 16 cm
F1: Aa Bb 19 cm
F2 Genotypes: AABB AABb AAbb AaBB AaBb Aabb aaBB aaBb aabbGenotypic
Effects +4 +3 +2 +4 +3 +2 +2 +1 +0
Phenotype(cm)
20 19 18 20 19 18 18 17 16
F2 proportions: 1/16 2/16 1/16 2/16 4/16 2/16 1/16 2/16 1/16
Additive and Dominance Effects (No Environmental Effects)
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Dominance effects Additive effectsAA Aa aa BB Bb bb+2 +2 +0 +2 +1 +0
F1: Aa Bb x Aa Bb19 cm 19 cm
F2 Genotypes: AABB AABb AAbb AaBB AaBb Aabb aaBB aaBb aabbGenotypic
Effects +4 +3 +2 +4 +3 +2 +2 +1 +0
Phenotype(cm)
20 19 18 20 19 18 18 17 16
F2 proportions: 1/16 2/16 1/16 2/16 4/16 2/16 1/16 2/16 1/16
Mean = 18.5 cm Var = 1.333 cm2
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Dominance effects Additive effectsAA Aa aa BB Bb bb+2 +2 +0 +2 +1 +0
F1: Aa Bb x Aa Bb19 cm 19 cm
F2 Genotypes: AABB AABb AAbb AaBB AaBb Aabb aaBB aaBb aabbGenotypic
Effects +4 +3 +2 +4 +3 +2 +2 +1 +0
Phenotype(cm)
20 19 18 20 19 18 18 17 16
F2 proportions: 1/16 2/16 1/16 2/16 4/16 2/16 1/16 2/16 1/16
VP = VA + VD + VE
Var = 1.333 cm2
1.333 = 1.0 + 0.333 + 0
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Heritability
Broad-sense heritability H2 = VG/VP
Narrow-sense heritability h2 = VA/VP
VP = VA + VD + VE
1.333 = 1.0 + 0.333 + 0
= 1.0
= 0.75
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Uses of heritability
• The degree to which offspring resemble their parents is determined by the narrow-sense heritability h2
• The efficacy of natural and artificial selection is also determined by h2
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h2 = 1
h2 = 0
VA/VP = 1
VA/VP = 0
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Efficacy of artificial selection: size of Labradors
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Breeder’s Question
Q: A horse breeder wants to win the Kentucky Derby. If she breeds her mare to a really fast stallion, how likely is it that the colt will be faster than all the other three-year-olds when it runs in the Derby?
A: It depends on the heritability of running speed!
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Breeder’s Equation
• R = h2 S• S = Selection differential difference between selected parents and the
population as a whole (within a generation)• R = response to selection difference between selected offspring and the
unselected population (across generations)
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Breeder’s EquationR = h2 SA dog breeder chooses his largest dogs to
breed together. The average height of the breed is 60 cm (at the shoulder), and the dogs he chooses to breed average 70 cm tall.
He knows from previous work that the heritability of height is 0.5.
How big can he expect the offspring to be?
R = h2 S = 0.5 * 10cm = 5cm
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Breeder’s Equation
R = h2 S = 0.5 * 10 cm = 5 cm
If the response to selection is 5 cm, he can expect his puppies to grow to be
60 cm + 5 cm = 65 cm tall
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Exactly the same equation can be used to understand
natural selection!
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Efficacy of natural selection:Darwin’s finches
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If large bills are favored in drought years, what effect will
an El Nino year have on
the population?
h2 = 0.8
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R = h2 S
Birds that survive the drought have bills that are 2 mm deeper (on average)
than the population mean.
Q: What will happen to the average bill depth in the next generation?