Mutations have different phenotypic effects

Mutations are classified generally by the type of cell in which they occur:

Does every mutation have a phenotypic effect? Not necessarily. Some mutations have effects on proteins and their function, and some do not (Focus: Key Figure 15.1):

focus: key figure

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Figure 15.1 Mutation and Phenotype Mutations may or may not affect the protein phenotype.

Question

Q: For a protein coding gene, which of the three types of mutation will be most common, and why?

Silent mutation, because the genetic code is redundant (many mutations do not change the amino acid translated) and because many amino acids in a protein are not essential for the activity of the protein (e.g., do not affect the active site).

Activity 15.1 Gene Mutations and Function Simulation

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Activity 15.2 Point Mutations Simulation

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Figure 15.2 Point Mutations When they occur in the coding regions of proteins, single base changes can cause silent, missense, nonsense, or frame-shift mutations.

Some mutations have effects on phenotype only under certain conditions. For example, a conditional mutation affects phenotype only under restrictive conditions and is not detectable under what are called permissive conditions. Many conditional mutations are temperature-sensitive, resulting in proteins with reduced stability at high temperatures. For example, the point restriction phenotype in rabbits and Siamese cats (see Figure 12.13) is due to a temperature-sensitive (conditional, loss-of-function) mutation in a coat color gene. At body temperature, the protein encoded by the gene is unstable and nonfunctional, so that the animal has dark fur only in its cooler extremities.

Most point mutations can be reversed; reversion mutations result when a gene is mutated a second time so that the DNA reverts to its original sequence or to a coding sequence that results in the non-mutant phenotype. When reversion mutations occur, the phenotype reverts to wild type.

Now let’s look more closely at the DNA level to see how various kinds of mutations play out.