Quantitative Genetics

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Chapter 5 Quantitative Genetics
Problems 1-10, 12, 14
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discontinuous variation
continuous variation
quality
quantity
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Multiple-gene hypothesis (polygenic inheritance)
Many genes contribute to the phenotype in a
cumulative or quantitative way.
bell curve
Fig. 5.1
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Additional evidence for the multiple-gene
hypothesis (Edward M. East)
corolla
Fig. 5.2
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The Multifactor Hypothesis and Additive Alleles
1. Continuous variation can be quantified. 2.
Two or more genes interact in an additive way to
influence a phenotype. 3. These genes have
additive alleles and nonadditive alleles. 4.
Additive alleles at different loci are about
equal. 5. Many genes many alleles lots of
variation. 6. Large populations are needed to
study these characteristics.
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Example illustrating Additive Alleles
1. Phenotype grain color 2. 2 gene pairs 3.
A and B additive alleles a and b
nonadditive alleles 4. A B 1 a b
0 5. 5 different phenotypic categories 6. Many
plants were looked at.
Fig. 5.3
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Skin pigmentation in humans
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How many genes are involved?
n number of different genes
frequency of the extreme phenotype individuals

of different phenotypic classes
2n 1
Fig. 5.4
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Statistical Analysis

• Mathematical description of your data.
• Infer information about large populations from
  small samples.
• Compare two sets of data.

Data set 9, 10, 11, 12
? Xi 9 10 11 12 42
n 4
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Sample variance (V)- Degree to which values
within a data set diverge from the mean.
OR
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Standard Deviation (s)- The probability of a
measured value falling within a certain range.
s
x
s2 1.67 s 1.29
1 standard deviation 1.29
1 st. dev. (68.3)
2 st. dev. (95.5)
3 st. dev. (99.7)
This means that there is a 68.3 chance that a
randomly picked value from the population will be
between 10.5 /- 1.29 (11.79 and 9.21).
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Standard Error of the Mean (Sx)- A measure of how
accurate our sample mean is.
s standard deviation n sample size
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The environment may also play a role in
determining the phenotype.
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Heritability- Measure of how much of the
phenotypic variability (VP) is due to the
environment (VE) and how much is due to genetics
(VG).
Estimate of VG Grow two inbred strains (A and
B) in a constant environment and measure
variation.
VP VG (VE 0)
Estimate of VE Grow the same inbred strain
(strain A) under two different environmental
conditions and measure variation.
VP VE (VG 0)
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Broad-Sense Heritability (H2)- A measure of the
importance of genetic variation, relative to
environmental variation, in causing the
phenotypic variation of the trait of interest.
H2 close to 1 mostly genetic effect H2 close to
0 mostly environmental effect
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Sample calculation for H2
The Mexican cave-dwelling fish Astyanax fasciatus
has reduced eyes and reduced pigmentation
compared to its surface-dwelling relatives. How
much of this variation is due to genetic factors?
Cross cave-dwelling fish with surface-dwelling
fish and measure variance (eye diameter).
F1 variance 0.057 (VP VE) F2 variance
0.563 (VP VG VE)
VG
0.506
VG VP - VE VG 0.563 - 0.057 VG 0.506
H2

VP
0.563
H2 0.9
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Narrow-Sense Heritability (h2)- Measures how much
we can control the inheritance of a trait by
controlling which genes are passed on. Used
extensively in plant and animal breeding programs
(artificial selection).
M parental population M1 select group of the
parental population that expresses the desired
phenotype M2 offspring from breeding the M1
individuals
h2 close to 0 low heritability h2 close to 1
high heritability
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Sample calculation for h2
M 70 mm M1 81 mm M2 77 mm
M1
M2 - M M1 - M
h2
M2
77 - 70 81 - 70
h2
h2 0.636
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Heritability in Humans (Twin Studies)
monozygotic twins vs. dizygotic twins
familial vs. heritable
Table 5.6 A Comparison of Concordance of Various
Traits Between Monozygotic (MZ) and Dizygotic
(DZ) Twins
Table 5.6