Ch. 19 - Genetic Analysis of Quantitative Traits

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 19 - Genetic Analysis of Quantitative Traits

Problem 27b

Chapter 19, Problem 27b

Answer the following in regard to multifactorial traits in human twins. If the trait is produced with little contribution from genetic variation, what would you expect to see if you compared the concordance rates of MZ twins versus DZ twins? Explain your reasoning.

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Understand the concept of multifactorial traits: Multifactorial traits are influenced by both genetic and environmental factors. Concordance rates measure the likelihood that both twins exhibit the same trait.

Define MZ (monozygotic) and DZ (dizygotic) twins: MZ twins share nearly 100% of their genetic material, while DZ twins share approximately 50% of their genetic material, similar to regular siblings.

Analyze the role of genetic variation: If the trait has little contribution from genetic variation, it means environmental factors play a dominant role in determining the trait.

Compare concordance rates: Since MZ twins share more genetic material than DZ twins, if genetic variation has minimal influence, the concordance rates for MZ twins and DZ twins would be similar. This is because the trait is primarily determined by environmental factors, which affect both types of twins equally.

Explain reasoning: The similarity in concordance rates arises because the genetic contribution is negligible, and environmental factors are shared to a similar extent by both MZ and DZ twins, especially if they are raised in the same environment.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Concordance Rates

Concordance rates refer to the likelihood that both twins in a pair exhibit the same trait. In studies of twins, these rates are compared between monozygotic (MZ) twins, who share nearly all their genes, and dizygotic (DZ) twins, who share about 50% of their genes. Higher concordance rates in MZ twins compared to DZ twins suggest a genetic influence on the trait, while similar rates indicate that environmental factors may play a more significant role.

Multifactorial Traits

Multifactorial traits are characteristics influenced by multiple genetic and environmental factors. Unlike single-gene traits, these traits do not follow simple Mendelian inheritance patterns. Examples include height, intelligence, and susceptibility to diseases, where both genetic predispositions and environmental conditions contribute to the final phenotype.

Genetic Variation

Genetic variation refers to the differences in DNA sequences among individuals, which can influence traits. In the context of the question, if a trait shows little contribution from genetic variation, it implies that environmental factors are more significant in determining the trait's expression. This would likely result in similar concordance rates for MZ and DZ twins, as both sets would be equally affected by shared environmental influences.

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Genomic Variation

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Textbook Question

The children of couples in which one partner has blood type O (genotype ii) and the other partner has blood type AB (genotype IᴬIᴮ) are studied. What is the expected concordance rate for blood type of MZ twins in this study? Explain your answer.

Answer the following in regard to multifactorial traits in human twins. If the trait is substantially influenced by genes, would you expect the concordance rate to be higher in MZ twins or higher in DZ twins? Explain your reasoning.

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Textbook Question

Suppose the mature height of a plant is a multifactorial trait under the control of five independently assorting genes, designated A, B, C, D, and E, and five environmental factors. There are two alleles of each gene (A₁, A₂, etc.). Each allele with a subscript 1 (i.e., A₁) contributes 5 cm to potential plant height, and each allele with a 2 subscript (i.e., A₂, etc.) contributes 10 cm to potential plant height. In other words, a genotype containing only 1 alleles (A₁A₁B₁B₁C₁C₁D₁D₁E₁E₁) would have a potential height of [(10)(5)]=50cm, and a genotype with only 2 alleles (A₂A₂B₂B₂C₂C₂D₂D₂E₂E₂) would have a potential height of [(10)(10)]=100cm.The five environmental factors are (1) amount of water, (2) amount of sunlight, (3) soil drainage, (4) nutrient content of soil, and (5) temperature. Each environmental factor can vary from optimal to poor. If all factors are optimal, assume that full potential height is attained. However, if one or more of the environmental factors is less than optimal, then height is reduced. The state of each environmental factor has an effect on growth. In this exercise, we'll assume that the growth is affected according to the following scale:Environmental Factor State Height LostOptimal (O) 0 cmGood (G) 4 cmFair (F) 8 cmMarginal (M) 12 cmPoor (P) 16 cmThus, for example, if one environmental factor is optimal, two are good, one is fair, and one is marginal, the loss of potential height is . If the loss of height potential is greater than the height potential of the plant, the plant does not survive.How many 1 and 2 alleles must be present to give a height potential of 80 cm?

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Textbook Question

Suppose the mature height of a plant is a multifactorial trait under the control of five independently assorting genes, designated A, B, C, D, and E, and five environmental factors. There are two alleles of each gene (A₁, A₂, etc.). Each allele with a subscript 1 (i.e., A₁) contributes 5 cm to potential plant height, and each allele with a 2 subscript (i.e., A₂, etc.) contributes 10 cm to potential plant height. In other words, a genotype containing only 1 alleles (A₁A₁B₁B₁C₁C₁D₁D₁E₁E₁) would have a potential height of [(10)(5)]=50cm, and a genotype with only 2 alleles (A₂A₂B₂B₂C₂C₂D₂D₂E₂E₂) would have a potential height of [(10)(10)]=100cm.The five environmental factors are (1) amount of water, (2) amount of sunlight, (3) soil drainage, (4) nutrient content of soil, and (5) temperature. Each environmental factor can vary from optimal to poor. If all factors are optimal, assume that full potential height is attained. However, if one or more of the environmental factors is less than optimal, then height is reduced. The state of each environmental factor has an effect on growth. In this exercise, we'll assume that the growth is affected according to the following scale:Environmental Factor State Height LostOptimal (O) 0 cmGood (G) 4 cmFair (F) 8 cmMarginal (M) 12 cmPoor (P) 16 cmThus, for example, if one environmental factor is optimal, two are good, one is fair, and one is marginal, the loss of potential height is . If the loss of height potential is greater than the height potential of the plant, the plant does not survive.List two genotypes that have a height potential of 80 cm.

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Textbook Question

Suppose the mature height of a plant is a multifactorial trait under the control of five independently assorting genes, designated A, B, C, D, and E, and five environmental factors. There are two alleles of each gene (A₁, A₂, etc.). Each allele with a subscript 1 (i.e., A₁) contributes 5 cm to potential plant height, and each allele with a subscript 2 (i.e., A₂, etc.) contributes 10 cm to potential plant height. In other words, a genotype containing only 1 allele (A₁A₁B₁B₁C₁C₁D₁D₁E₁E₁) would have a potential height of [(10)(5)]=50 cm, and a genotype with only 2 alleles (A₂A₂B₂B₂C₂C₂D₂D₂E₂E₂) would have a potential height of [(10)(10)]=100 cm. The five environmental factors are (1) amount of water, (2) amount of sunlight, (3) soil drainage, (4) nutrient content of soil, and (5) temperature. Each environmental factor can vary from optimal to poor. If all factors are optimal, assume that full potential height is attained. However, if one or more of the environmental factors is less than optimal, then height is reduced. The state of each environmental factor has an effect on growth. In this exercise, we'll assume that the growth is affected according to the following scale:

Thus, for example, if one environmental factor is optimal, two are good, one is fair, and one is marginal, the loss of potential height is 0 + 4 + 4 + 8 + 12 = 28 cm. If the loss of height potential is greater than the height potential of the plant, the plant does not survive. Calculate the potential height, based on inherited alleles, and the attained height, based on growth in the environmental circumstances given, for the three plants (a, b, and c) in the accompanying table.

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