1 Phenotypic Variation Variation of a trait can be separated into genetic and environmental...

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Phenotypic Variation• Variation of a trait can be separated into genetic and

environmental components

• Genotypic variance g2 = variation in phenotype caused

by differences in genotype

• Environmental variance e2 = variation in phenotype

caused by environment

• Total variance p2 = combined effects of genotypic and

environmental variance

p2 = g

2 + e2

2Fig. 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)

4Fig. 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 g

2 = 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 = D2/8g2

D = difference between parental strains

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Broad-Sense Heritability• Broad-sense heritability (H2) includes all genetic effects

combined

H2 = g2 / p

2 = g2 / g

2 + e2

• 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.

Discussion Question 2:In humans, the estimated values of broad-

sense heritability for some traits are:

Maximum heart rate H2 = 0.84Systolic blood pressure H2 = 0.57Serum 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 for some traits are: Maximum heart rate H2 = 0.84Systolic blood pressure H2 = 0.57Serum lipid level H2 = 0.44For 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: 2e/2

t = 0.13 is given. Therefore 1-2

e/2t) = 1 – 0.13 = 0.87.

But 1-2e/2

t) = (2t - 2

e)/2t = 2

g/2t = 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 = D2/8g2

D = difference between parental strains

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 = D2/8g2

n = number of genesD = difference between parental strains

g2 = 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 e

2 , and between fraternal twins - g

2/2 + e2

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Heritability: Twin StudiesPotential 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

Any questions?Any questions?

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