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Chapter 15 Complex Inheritance
15.1 quantitative traits
15.2 gene/environment interactions
15.3 artificial selection
© 2006 Jones and Bartlett Publishers
Up until now…
traits have been discrete
either round or wrinkled,
either yellow or green,
red eyes or white eyes,…
a single gene has different alleles
having different phenotypes
very easy to study and understand
But many traits are the result of
interactions between multiple genes as
well as being affected by the environment
The traits are called:
multifactorial traits
quantitative traits
multifactorial traits
quantitative traits
influenced by:
alternative genotypes of one or more genes
environmental factors
inbred lines
Fig. 15.1. A completely inbred line is homozygous for every gene
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multifactorial traits
quantitative traits
influenced by:
alternative genotypes of one or more genes
environmental factors
example
height
continuous traits
height, blood pressure, weight
crop yield, milk production
categorical traits
ears of corn/stalk
eggs from hen
ridges in fingerprints
threshold traits
few phenotypes
multiple genes/environement
“predisposition to express”
Quantification (how do we describe the results)
“discrete” traits
like seed color
75% yellow, 25% green
continuous traits
like height
distributions
mean, variance (std. deviation)
mean
=
average
sum of all heights
divided by # of people measured
62” 65” 63” 70”
260 /4 = 65” = mean
54*5+
56*33+
58*254+…
divided by 4995
=63.1 in. = mean height
Table 15.1. Distribution
of height among British
women
x

 fi xi
N
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mean=average
mean=
sum of all heights
divided by
number of people
Fig. 15.2. Graph of distribution of height among 4995 British women
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Fig. 15.3. A living histogram of human height
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mean
variance?
standard deviation?
mean

variance
standard
deviation
 fi xi
x  
N
 f i x i  x 
N 1
2
 s2 =

σ=
s
2
mean = 63.1 inches
variance = 7.24 inches2
std dev = 2.69 inches
Fig. 15.2. Graph of distribution of height among 4995 British women
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67%
annotated bib.
95%
99.7%
36.3 = mean
2.4 = stdev
bell curve
Fig. 15.5. Features of a normal distribution
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Fig. 15.4. Variance of a distribution measures the spread of the distribution
around the mean
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Variation in a trait
genetic
environmental
•genotypic variation
•environmental variation
•variation due to genotypeby-environment interaction
•variation due to genotypeby-environment association
Variation in a trait
genotypic variation
due to differences in genotype
the distribution of phenotypes, by
itself, provides no information about
how many genes influence a trait
3 genes affect trait
A or a, B or b, C or c
each dominant
contributes some
to phenotype
Fig. 15.6. Segregation of independent genes affecting a quantitative trait
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3 vs 30 genes?
distribution is
the same
Fig. 15.7. Distribution of phenotypes determined by the
segregation of 3 and 30 independent genes
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Variation in a trait
genotypic variation
due to differences in genotype
the distribution of phenotypes, by
itself, provides no information about
how many genes influence a trait
Variation in a trait
environmental variation
due to differences in environment
inbred
beans
normal
bell curve
Fig. 15.8. Distribution of seed weight in a homozygous line of edible beans
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Fig. 15.8. Distribution of seed weight in a homozygous line of edible beans
© 2006 Jones and Bartlett Publishers
Variation in a trait
environmental variation
due to differences in environment
the distribution provides no
information about the relative
importance of genotype or
environment. Could be either/or
or both
Variation in a trait
genetic and environmental variation
when both affect phenotype
independently, the total variance is the
sum of the individual variances
Fig. 15.9. Combined
effects of genotypic
and environmental
variance
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Variation in a trait
genetic and environmental variation
when both affect phenotype
independently, the total variance is the
sum of the individual variances
total
=
variance
genotypic
environmental
+
variance
variance
  
2
p
2
g
2
e
(eq. 15.3)
Variation in a trait
genetic and environmental variation
REVIEW:
•genotypic (G) variation
•environmental (E) variation
•variation due to G-E interaction
•variation due to G-E association
variation due to G-E interaction
(genotype-by-environment)
corn
poor environment
strain A does better than B
good environment
strain B does better than A
e.g.,
special varieties of
plants developed to
suit different
growing areas
Fig. 15.10. Genotype-by-environment interaction in maize. [Data from
W. A. Russell. 1974. Annual Corn & Sorghum Research Conference 29: 81]
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variation due to G-E (?) interaction
(genotype-by-sex)
sex
different phenotype depending on
gender of organism
living histogram
and height
Fig. 15.3. A living histogram of human height
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variation due to G-E association
(genotype-by-environment)
cow example ?
A homogeneous population…
…will have no genotypic variance.
  
 0
2
p
Therefore:
2
g
2
g
2
e
 
2
p
2
e
cave dwelling fish
inbred
inbred
cross
F1
homogeneous
population
cross
F2
heterogeneous
population
see fig 15.6
Fig. 15.6. Segregation of independent genes affecting a quantitative trait
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inbred
inbred
cross
F1
measure
eye size
variation
homogeneous
population
cross
F2
heterogeneous
population
inbred
inbred
cross
F1
measure
eye size
variation
  0.057
2
cross
F2
  0.563
2
F1
  0.057  
F2
  0.563      
2
2
2
e
2
p
2
g
0.563  0.057      
2
g
0.506  
2
g
2
e
2
e
2
e
Which is more important
genotype
or
environment?
broad-sense heritability H2
shows the importance of genetic variation,
relative to environmental variation, in causing
variation in phenotype
ratio of genotypic variance to total
phenotypic variance


0.506
H 
 2

 0.90
2

 g   e 0.563
2
2
g
2
p
2
g
90% of eye variation in fish is genetic
Fig. 15.13. Selection for increased length of corolla tube in tobacco
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Fig. 15.13. Selection for increased length of corolla tube in tobacco
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M
=
mean of parental generation
M*
=
mean of selected parents
M’
=
mean of progeny of selected parents
M  M h M
'
2
*
 M
M M
h  *
M M
'
narrow-sense heritability

2
narrow-sense heritability
M M
h  *
M M
'
2
ratio of additive genetic variance to
 total phenotypic variance
the
broad-sense heritability H2
proportion of phenotypic variance
due to genetic differences
narrow-sense heritability h2
proportion of phenotypic variance
due to differences in additive alleles
that’s all from Chapter 15
for now folks
© 2006 Jones and Bartlett Publishers