Title: Genetics of Complex Traits: Quantitative Genetics
1Genetics of Complex TraitsQuantitative Genetics
2Genetic Variation
Discrete Variation (presence/absence of tail)
- Continuous Variation (height)
3Quantitative Genetics
- Polygenic
- Environmental influences
- Have continuous (not discrete) distributions
- Can be measured on a quantitative scale
Height Weight Athletic ability Risk of heart
disease Risk of diabetes Risk of cancer
4Intermediate dominance additive gene action
Partial dominance
Discrete distribution
5Red
Pink
Lt Red
Lt Red
Lt Red
Pink
Pink
Lt Pink
Pink
Lt Pink
Pink
Lt Red
Lt Pink
white
Lt Pink
Pink
1 Red 4 Lt Red 6 Pink 4 Lt Pink 1 white
6Two additive genes discrete phenotypic
distribution
7Color of wheat kernels three additive genes
aabbcc AABBCC
AaBbCC
8Frequency Distribution of Height of the Band
mean68 inches
9Properties of distributions
n 160
Mean 68 inches
Variance
9.5 in2
10(No Transcript)
11Types 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
12Phenotypic Variance
Mean 68 in
Var 9.5 in2
Phenotypic variance Genetic variance Environ.
variance VP
VG VE
13Phenotypic variance Genetic variance Environ.
variance VP
VG VE
Genetic variance Additive variance Dominance
Variance VG VA
VD
VP VA VD VE
14Additive and Dominance Effects (No Environmental
Effects)
15Mean 18.5 cm
Var 1.333 cm2
VP VA VD VE
1.333
1.0
0.333
0
16VP 1.333 cm2 VA
1.0 VD 0.333 VE
0
17Heritability
VP VA VD VE
1.0
Broad-sense heritability H2 VG/VP
0.75
Narrow-sense heritability h2 VA/VP
18Uses 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
19(No Transcript)
20h2 1
VA/VP 1
h2 0
VA/VP 0
21Efficacy of artificial selection size of
Labradors
22Breeders 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
23Breeders 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)
24Breeders Equation
- R h2 S
- A 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?
0.5 10cm 5cm
R h2 S
25Breeders 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
26Exactly the same equation can be used to
understand natural selection!
27Efficacy of natural selectionDarwins finches
28If large bills are favored in drought years, what
effect will an El Nino year have on the
population?
h2 0.8
29R 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?