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 26b

Chapter 19, Problem 26b

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 DZ twins in this study? Explain why this answer is different from the answer to part (a).

Verified step by step guidance

Step 1: Begin by understanding the genetic inheritance of blood types. Blood type is determined by the ABO gene, which has three alleles: Iᴬ, Iᴮ, and i. The Iᴬ and Iᴮ alleles are codominant, meaning both are expressed when present together, while the i allele is recessive.

Step 2: Analyze the parental genotypes. One parent has blood type O (genotype ii), meaning they can only pass on the i allele. The other parent has blood type AB (genotype IᴬIᴮ), meaning they can pass on either the Iᴬ or Iᴮ allele.

Step 3: Determine the possible genotypes of the offspring. Using a Punnett square, combine the alleles from each parent. The offspring can inherit either Iᴬi or Iᴮi, resulting in blood types A or B, respectively. There is no possibility of blood type AB or O in this case.

Step 4: Understand the concept of concordance rate in DZ (dizygotic) twins. DZ twins share approximately 50% of their genetic material, as they are formed from two separate fertilized eggs. The concordance rate for blood type depends on whether both twins inherit the same genotype (Iᴬi or Iᴮi) or different genotypes.

Step 5: Compare this result to part (a). The concordance rate for DZ twins is lower than for monozygotic (MZ) twins because MZ twins are genetically identical, while DZ twins are not. This difference arises due to the independent assortment of alleles during gamete formation and fertilization.

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Key Concepts

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

Blood Type Inheritance

Blood type is determined by the ABO gene, which has three alleles: A (Iᴬ), B (Iᴮ), and O (i). Individuals with blood type O have the genotype ii, while those with blood type AB have the genotype IᴬIᴮ. The offspring of these parents can inherit one allele from each parent, leading to potential blood types A (Iᴬi) or B (Iᴮi), but not AB or O.

Dizygotic (DZ) Twins

Dizygotic twins, or fraternal twins, arise from two separate eggs fertilized by two different sperm cells. They share about 50% of their genetic material, similar to regular siblings. The genetic diversity in DZ twins can lead to different blood types among them, depending on the alleles inherited from their parents.

Concordance Rate

Concordance rate refers to the likelihood that both twins in a pair exhibit the same trait or characteristic, such as blood type. In the case of DZ twins from the given parental genotypes, the expected concordance rate for blood type would be lower than that of monozygotic twins, as DZ twins share only half of their genetic information, leading to a greater chance of differing blood types.

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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. Suppose a friend of yours who has not taken genetics asks you to explain these observations. Construct a genetic explanation for the increased recurrence risk of mid-face clefting in families that have a history of cleft disorders versus families without a history of such disorders.

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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.

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Textbook Question

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

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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