Transcript Document

Phenotypic Variation
• Variation of a trait can be separated into genetic and
environmental components
• Genotypic variance sg2 = variation in phenotype caused
by differences in genotype
• Environmental variance se2 = variation in phenotype
caused by environment
• Total variance sp2 = combined effects of genotypic and
environmental variance
s p 2 = s g 2 + s e2
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Fig. 15.9
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Phenotypic Variation
• Genotype and environment can interact or they can
be associated
• Genotype-environment (G-E) interaction =
environmental effects on phenotype differ
according to genotype
• Genotype-by-sex interaction: same genotype
produces different phenotype in males and females
(distribution of height among women and men)
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Fig. 15.10
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Genetic Variation
• Genotype-environment (G-E) association = certain
genotypes are preferentially associated with
certain environments
• There is no genotypic variance in a genetically
homogeneous population sg2 = 0
• When the number of genes affecting a quantitative
trait is not too large, the number, n, of genes
contributing to the trait is
n = D2/8sg2
D = difference between parental strains
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Broad-Sense Heritability
• Broad-sense heritability (H2) includes all genetic
effects combined
H2 = sg2 / sp2 = sg2 / sg2 + se2
• It measures the importance of genetic variation,
relative to environmental variation, in causing
variation in the phenotype of a trait of interest.
• The higher H2 is the higher genetic
contribution to phenotype.
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Discussion Question 2:
 In humans, the estimated values of broad-sense
heritability for some traits are:
 Maximum heart rate H2 = 0.84
 Systolic blood pressure H2 = 0.57
 Serum lipid level H2 = 0.44
 For which of these traits is the phenotypic variance
most affected by genetic differences between
individuals?
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Discussion Question 2 Answer:
 In humans, the estimated values of broad-sense heritability
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for some traits are:
Maximum heart rate H2 = 0.84
Systolic blood pressure H2 = 0.57
Serum lipid level H2 = 0.44
For which of these traits is the phenotypic variance most
affected by genetic differences between individuals?
 Answer: Maximum heart rate is most affected
by genetic differences between individuals.
The higher H2 is the higher genetic
contribution to phenotype.
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Discussion Question 3A:
 Some estimates of broad-sense heritabilities of human
traits are:
 0.85 for adult height,
 0.62 for body weight,
 0.57 for systolic blood pressure,
 0.55 for diastolic blood pressure,
 0.5 for twinning, and
 0.1 to 0.2 for overall fertility.
 Which of these family characteristics is most likely to
“run in families”?
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Discussion Question 3A Answer:
 Some estimates of broad-sense heritabilities of human traits are:
 0.85 for adult height,
 0.62 for body weight,
 0.57 for systolic blood pressure,
 0.55 for diastolic blood pressure,
 0.5 for twinning, and
 0.1 to 0.2 for overall fertility.
 Which of these family characteristics is most likely to “run in
families”?
 Answer: The traits with the higher broad-sense
heritabilities show higher correlations among family
members. Therefore, adult height and weight are
more likely to “run in families” than is overall fertility.
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Discussion Question 3B:
 Some estimates of broad-sense heritabilities of human
traits are:
 0.85 for adult height,
 0.62 for body weight,
 0.57 for systolic blood pressure,
 0.55 for diastolic blood pressure,
 0.5 for twinning, and
 0.1 to 0.2 for overall fertility.
 If one of your parents and one of your grandparents
has high blood pressure, should you be concerned
about the likelihood of you having the same problems?
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Discussion Question 3B Answer:
 Some estimates of broad-sense heritabilities of human traits are:
 0.57 for systolic blood pressure,
 0.55 for diastolic blood pressure,
 If one of your parents and one of your grandparents has high
blood pressure, should you be concerned about the likelihood of
you having the same problems?
 Answer: The broad-sense heritabilities of systolic and
diastolic blood pressure readings suggest a genetic
component to variation in this trait, and hence
individuals with affected relatives (in this example, a
parent and a grand parent) are at somewhat higher risk
than those with a negative family history.
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Discussion Question 4:
 Two varieties of corn, A and B, are field-tested in
Indiana and North Carolina. Strain A is more
productive in Indiana, but strain B is more productive
in North Carolina. What phenomenon in quantitative
genetics does this example illustrate?
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Discussion Question 4 Answer:
 Two varieties of corn, A and B, are field-tested in
Indiana and North Carolina. Strain A is more
productive in Indiana, but strain B is more productive
in North Carolina. What phenomenon in quantitative
genetics does this example illustrate?
 Answer: Genotype-environment interactions.
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Discussion Question 5a:
 For the difference between the domestic tomato,
Solanum esculentum, and its wild South American
relative, Solanum chmielewskii, the environmental
variance accounts for 13 percent of the total
phenotypic variance of fruit weight, for 9 percent of
the total variance of soluble-solid content and for 11
percent of the total variance in acidity.
 What are the broad-sense heritability of fruit weight?
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Discussion Question 5a Answer:
 Environmental variance for the following:
 Fruit weight – 13%
 What are the broad-sense heritability of fruit weight?
 Answer: For fruit weight: s2e/s2t = 0.13 is given.
Therefore 1- (s2e/s2t) = 1 – 0.13 = 0.87.
 But 1- (s2e/s2t) = (s2t - s2e)/s2t = s2g/s2t = H2
 Hence for fruit weight H2 = 0.87. Therefore, 87% is
the broad-sense heritability of fruit weight.
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Discussion Question 5b:
 For the difference between the domestic tomato,
Solanum esculentum, and its wild South American
relative, Solanum chmielewskii, the environmental
variance accounts for 13 percent of the total
phenotypic variance of fruit weight, for 9 percent of
the total variance of soluble-solid content and for 11
percent of the total variance in acidity.
 What are the broad-sense heritability of soluble-solid
content and acidity?
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Discussion Question 5b Answer:
 Environmental variance for the following:
 Soluble-solid content – 9%
 Acidity – 11%
 What are the broad-sense heritabilities of these traits?
 Answer: For soluble-solid content, H2 = 1 – 0.09 =
0.91. For acidity, H2 = 1 – 0.11 = 0.89. The values of
H2 for soluble-solid content and acidity are 91%
and 89% respectively.
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Question 6a:
 You are studying fruit weigh, seed number and acid content
in tomatoes. From 100 greenhouse plants, you sample a
single fruit and measure it for three traits obtaining the
following data:
A. Can you infer which of these traits likely has the
greatest genetic variance? Explain your reasoning.
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Question 6a Answer:
 A. Can you infer which of these traits likely has the
greatest genetic variance? Explain your reasoning.
 Answer: It is impossible to tell from these data.
Phenotypic variance is composed of both
environmental and genetic components. By growing
plants in a green-house environment, we can
eliminate some but not all of the environmental
components of variant. Although one may suspect
that acid content (with the highest phenotypic
variance) also has the largest genetic variance, there is
no way to be sure without actually putting the genetics
into action by making crosses or examining
correlations among related plants.
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Question 6b:
 You are studying fruit weigh, seed number and acid content
in tomatoes. From 100 greenhouse plants, you sample a
single fruit and measure it for three traits obtaining the
following data:
B. Can you infer which of these traits has the greatest
heritability? Explain your reasoning.
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Question 6b Answer:
 B. Can you infer which of these traits has the greatest
heritability? Explain your reasoning.
 Answer: Just as with the previous answer, heritability
cannot be inferred directly from phenotypic
observations of a single sample. We must observe
something about the transmission of genetic
information, from parent to offspring, or response to
selection from generation to generation.
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Discussion Question 7:
 Estimate the minimum number of genes affecting fruit
weight in a population of the domestic tomato
produced by crossing two inbred strains. Measured as
the logarithm of fruit weight in grams, the inbred lines
have average fruit weights of 0.137 and 1.689. The F1
generation has a variance of 0.0144 and the F2
generation has a variance of 0.0570.
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Genetic Variation
• When the number of genes affecting a quantitative
trait is not too large, the number, n, of genes
contributing to the trait is
n = D2/8sg2
D = difference between parental strains
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Discussion Question 7 Answer:
The inbred lines have average fruit weights of 0.137 and
1.689. The F1 generation has a variance of 0.0144 and
the F2 generation has a variance of 0.0570.
Answer:
n = D2/8sg2
n = number of genes
D = difference between parental strains
 sg2 = 0.570-0.0144 = 0.0426 and D = 1.689 – (0.137) = 1.826. The minimum number of genes
is estimated to be n = (1.826)2/8X0.0426 = 9.8
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Heritability: Twin Studies
• Twin studies are often used to assess genetic
effects on variation in a trait
• Identical twins arise from the splitting of a single
fertilized egg = genetically identical
• Fraternal twins arise from two fertilized eggs =
only half of the genes are identical
• Theoretically, the variance between identical twins
would be equivalent to se2 , and between fraternal
twins - sg2/2 + se2
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Heritability: Twin Studies
Potential sources of error in twin studies of heritability:
– Genotype-environment interaction increases the
variance in fraternal twins but not identical twins
– Frequent sharing of embryonic membranes by identical
twins creates similar intrauterine environment
– Greater similarity in treatment of identical twins results in
decreased environmental variance
– Different sexes can occur in fraternal but not identical
twins
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Any questions?
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