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Ch. 9 - The Molecular Biology of Translation
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 9 - The Molecular Biology of TranslationProblem B.1b
Chapter 9, Problem B.1b

Answer the following questions for autosomal conditions such as PKU.
Parents who are each heterozygous carriers for a recessive mutant allele have a child who does not have the condition. What is the chance this child is a heterozygous carrier of the condition?

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Identify the genotype of the parents: Since both parents are heterozygous carriers for a recessive mutant allele, their genotypes can be represented as Aa, where A is the normal allele and a is the recessive mutant allele.
Determine the possible genotypes of their child using a Punnett square: The possible genotypes are AA (homozygous normal), Aa (heterozygous carrier), and aa (affected with the condition). The expected genotypic ratio is 1 AA : 2 Aa : 1 aa.
Since the child does not have the condition, exclude the aa genotype from consideration. This leaves the possible genotypes for the child as either AA or Aa.
Calculate the conditional probability that the child is a heterozygous carrier (Aa) given that the child is not affected. Use the formula: \text{Probability} = \frac{\text{Number of } Aa \text{ genotypes}}{\text{Number of } AA + Aa \text{ genotypes}}.
Substitute the values from the Punnett square into the formula to find the chance that the child is a heterozygous carrier given they do not have the condition.

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Key Concepts

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

Autosomal Recessive Inheritance

Autosomal recessive inheritance means a condition manifests only when an individual inherits two copies of a mutant allele, one from each parent. Carriers have one normal and one mutant allele but typically do not show symptoms. Understanding this pattern helps predict the likelihood of offspring being affected or carriers.
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Autosomal Pedigrees

Genotype Probabilities in Carrier Parents

When both parents are heterozygous carriers, each child has a 25% chance of being affected (homozygous recessive), 50% chance of being a carrier (heterozygous), and 25% chance of being unaffected and not a carrier (homozygous dominant). These probabilities are derived from a Punnett square analysis.
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Conditional Probability Given Phenotype

If a child does not have the condition (is unaffected), the probability that the child is a carrier changes because the affected genotype is excluded. This requires calculating the conditional probability of being heterozygous given the child is not affected, adjusting the original genotype probabilities accordingly.
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Related Practice
Textbook Question

Answer the following questions for autosomal conditions such as PKU.

If the first child of parents who are both heterozygous carriers of a recessive mutant allele is homozygous recessive, what is the chance the second child of the couple will be homozygous recessive? What is the chance the second child will be a heterozygous carrier of the recessive mutation?

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Textbook Question

Answer the following questions for autosomal conditions such as PKU.

If both parents are heterozygous carriers of a mutant allele, what is the chance that their first child will be homozygous recessive for the mutation?

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Textbook Question

Some proteins are composed of two or more polypeptides. Suppose the DNA template strand sequence 3′-TACGTAGGCTAACGGAGTAAGCTAACT-5′ produces a polypeptide that joins in pairs to form a functional protein. What is the amino acid sequence of the polypeptide produced from this sequence?