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 27a

Chapter 19, Problem 27a

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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Understand the concept of multifactorial traits: These traits are influenced by both genetic and environmental factors. Examples include height, skin color, and susceptibility to certain diseases.

Define MZ (monozygotic) twins and DZ (dizygotic) twins: MZ twins are genetically identical because they originate from a single fertilized egg that splits into two embryos. DZ twins share about 50% of their genes, similar to regular siblings, as they originate from two separate fertilized eggs.

Consider the genetic influence on concordance rates: Concordance rate refers to the likelihood that both twins exhibit the same trait. If a trait is substantially influenced by genes, MZ twins, who share 100% of their genetic material, are more likely to have a higher concordance rate compared to DZ twins, who share only 50% of their genetic material.

Factor in environmental influences: While genes play a significant role, environmental factors can also impact concordance rates. However, for traits with strong genetic influence, the genetic similarity of MZ twins will result in a higher concordance rate compared to DZ twins.

Conclude the reasoning: Since MZ twins share all their genetic material, traits with substantial genetic influence will show higher concordance rates in MZ twins than in DZ twins. This is because the genetic similarity directly impacts the expression of such traits.

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Key Concepts

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

Concordance Rate

Concordance rate refers to the likelihood that both twins in a pair exhibit the same trait or condition. In genetic studies, this measure helps determine the influence of genetics versus environment on traits. A higher concordance rate in monozygotic (MZ) twins compared to dizygotic (DZ) twins suggests a stronger genetic component to the trait.

Monozygotic vs. Dizygotic Twins

Monozygotic (MZ) twins, or identical twins, originate from a single fertilized egg that splits into two embryos, sharing 100% of their genetic material. In contrast, dizygotic (DZ) twins, or fraternal twins, develop from two separate eggs fertilized by different sperm, sharing about 50% of their genes. This genetic difference is crucial for understanding the heritability of traits.

Multifactorial Traits

Multifactorial traits are influenced by multiple genes and environmental factors, making their inheritance complex. Examples include height, intelligence, and susceptibility to diseases. In the context of twins, examining concordance rates for multifactorial traits helps researchers discern the relative contributions of genetic and environmental influences.

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Textbook Question

In human gestational development, abnormalities of the closure of the lower part of the mid-face can result in cleft lip, if the lip alone is affected by the closure defect, or in cleft lip and palate (the roof of the mouth), if the closure defect is more extensive. Cleft lip and cleft lip with cleft palate are multifactorial disorders that are threshold traits. A family with a history of either condition has a significantly increased chance of a recurrence of mid-face cleft disorder in comparison with families without such a history. However, the recurrence risk of a mid-face cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone. Construct a similar explanation of why the recurrence risk of a cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone.

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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 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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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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