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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.1c
Chapter 9, Problem B.1c

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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1
Identify the genotype of the parents: Since both parents are heterozygous carriers of 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 children using a Punnett square: Cross Aa (mother) with Aa (father) to find the genotypic ratio of offspring. The possible genotypes are AA, Aa, and aa.
Calculate the probability of the second child being homozygous recessive (aa): From the Punnett square, the chance of aa is 1 out of 4, or 25%. This probability is independent of the first child's genotype because each child's genotype is an independent event.
Calculate the probability of the second child being a heterozygous carrier (Aa): From the Punnett square, the chance of Aa is 2 out of 4, or 50%.
Summarize the results: The chance the second child is homozygous recessive is 25%, and the chance the second child is a heterozygous carrier is 50%, regardless of the first child's genotype.

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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 occurs when a trait is expressed only if 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. For two heterozygous carriers, each child has a 25% chance of being affected, 50% chance of being a carrier, and 25% chance of being unaffected.
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Independent Assortment and Probability in Genetics

Each child’s genotype is determined independently according to Mendelian genetics, meaning the outcome for one child does not affect the probabilities for the next. Thus, even if the first child is homozygous recessive, the chance for the second child to inherit the same genotype remains the same as the initial probability.
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Gamete Genetics and Independent Assortment

Genotype Probabilities for Offspring of Carrier Parents

When both parents are heterozygous carriers, the possible genotypes for each child are: 25% homozygous recessive (affected), 50% heterozygous carrier (unaffected but carrier), and 25% homozygous dominant (unaffected, non-carrier). These probabilities apply independently to each child regardless of previous offspring’s genotype.
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Related Practice
Textbook Question

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?

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?

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 term is used to identify a functional protein like this one formed when two identical polypeptides join together?