Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 11 - Gene Mutation, DNA Repair, and Homologous Recombination

Problem 39

Chapter 11, Problem 39

Thinking back to the discussion of gain-of-function and loss-of-function mutations, explain why gain-of-function mutations are often dominant and why loss-of-function mutations are often recessive. Give an example of a type of gain-of-function mutation that is dominant and of a loss-of-function mutation that is recessive.

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Understand the concept of gain-of-function mutations: These mutations result in a gene product with enhanced or new activity. They often lead to dominant phenotypes because the altered gene product can exert its effect even in the presence of a normal copy of the gene.

Understand the concept of loss-of-function mutations: These mutations result in reduced or completely abolished activity of the gene product. They are often recessive because the presence of one normal copy of the gene can usually compensate for the loss of function in the mutated copy.

Relate these concepts to base substitution mutations: Base substitution mutations can alter the DNA sequence, leading to changes in the amino acid sequence of the protein. This can result in either gain-of-function or loss-of-function effects depending on the nature of the mutation.

Provide an example of a gain-of-function mutation: A mutation in the FGFR3 gene (associated with achondroplasia) can lead to constitutive activation of the receptor, causing dominant phenotypic effects such as abnormal bone growth.

Provide an example of a loss-of-function mutation: A mutation in the CFTR gene (associated with cystic fibrosis) can result in a nonfunctional protein, leading to recessive phenotypic effects because one functional copy of the gene is insufficient to prevent the disease.

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Key Concepts

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

Gain-of-Function Mutations

Gain-of-function mutations result in a gene product that has enhanced or new functions. These mutations often lead to a dominant phenotype because the altered protein can exert its effects even in the presence of a normal allele. For example, a gain-of-function mutation in the Ras gene can lead to uncontrolled cell division, contributing to cancer.

Loss-of-Function Mutations

Loss-of-function mutations result in a gene product that is inactive or absent, leading to a reduction or complete loss of function. These mutations are typically recessive because one normal allele can often compensate for the defective one, maintaining sufficient gene function. An example is the mutation in the CFTR gene that causes cystic fibrosis.

Base Substitution Mutations

Base substitution mutations involve the replacement of one nucleotide with another in the DNA sequence. This can lead to changes in the amino acid sequence of proteins, potentially resulting in gain-of-function or loss-of-function effects. Understanding how these mutations impact protein function is crucial for explaining the dominance or recessiveness of the resulting phenotypes.

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Base Distortions

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Textbook Question

A geneticist searching for mutations uses the restriction endonucleases SmaI and PvuII to search for mutations that eliminate restriction sites. SmaI will not cleave DNA with CpG methylation. It cleaves DNA at the restriction digestion sequence ↓ 5′−CCC GGG−3′ 3′−GGG CCC−3′ ↑ PvuII is not sensitive to CpG methylation. It cleaves DNA at the restriction sequence ↓ 5′−CAG CTG−3′ 3′−GTC GAC−5′ ↑ Explain why CpG dinucleotides are hotspots of mutation.

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Textbook Question

In a mouse-breeding experiment a new mutation called Dumbo is identified. A mouse with the Dumbo mutation has very large ears. It is produced by two parental mice with normal ear size. Based on this information, can you tell whether the Dumbo mutation is a regulatory mutation or a mutation of a protein-coding gene? Why or why not?

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Textbook Question

Considering the Dumbo mutation in Problem 37, what kinds of additional evidence would help you determine whether Dumbo is a mutation of a regulatory sequence or of a protein-coding gene?

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Textbook Question

Common baker's yeast (Saccharomyces cerevisiae) is normally grown at 37°C, but it will grow actively at temperatures down to approximately 25°C. A haploid culture of wild-type yeast is mutagenized with EMS. Cells from the mutagenized culture are spread on a complete-medium plate and grown at 25°C. Six colonies (1 to 6) are selected from the original complete-medium plate and transferred to two fresh complete-medium plates. The new complete plates (shown) are grown at 25°C and 37°C. Four replica plates are made onto minimal medium or minimal plus adenine from the 25°C complete-medium plate. The new plates are grown at either 25°C or 37°C and the growth results are shown.

Which colonies are prototrophic and which are auxotrophic? What growth information is used to make these determinations?

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Textbook Question

Common baker's yeast (Saccharomyces cerevisiae) is normally grown at 37°C, but it will grow actively at temperatures down to approximately 25°C. A haploid culture of wild-type yeast is mutagenized with EMS. Cells from the mutagenized culture are spread on a complete-medium plate and grown at 25°C. Six colonies (1 to 6) are selected from the original complete-medium plate and transferred to two fresh complete-medium plates. The new complete plates (shown) are grown at 25°C and 37°C. Four replica plates are made onto minimal medium or minimal plus adenine from the 25°C complete-medium plate. The new plates are grown at either 25°C or 37°C and the growth results are shown.

Classify the nature of the mutations in colonies 1, 2, and 5.

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Textbook Question

What can you say about colony 4?

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