Dna

red eyes are considered the wild-type eye color; any other genetically determined eye color in fruit flies is considered to be a mutant. A mutation that alters the wild-type phenotype is called a forward mutation, where as a reverse mutation (a reversion) changes a mutant phenotype back into the wild type.

Geneticists use special terms to describe the phenotypic effects of mutations. A base substitution that alters a codon in the mRNA, resulting in a different amino acid in the protein, is referred to as a missense mutation (< Figure 17.7a). A nonsense mutation changes a sense codon (one that specifies an amino acid) into a nonsense codon (one that terminates translation; Figure 17.7b). If a nonsense mutation occurs early in the mRNA sequence, the protein will be greatly shortened and will usually be nonfunctional. A silent mutation alters a codon but, thanks to the redundancy of the genetic code, the codon still specifies the same amino acid (Figure 17.7c). A neutral mutation is a missense mutation that alters the amino acid sequence of the protein but does not change its function. Neutral mutations occur when one amino acid is replaced by another that is chemically similar or when the affected amino acid has little influence on protein function.

Loss-of-function mutations cause the complete or partial absence of normal function. A loss-of-function mutation so alters the structure of the protein that the protein no longer works correctly or the mutation can occur in regulatory regions that affect the transcription, translation, or splicing of the protein. Loss-of-function mutations are frequently recessive, and diploid individuals must be homozygous for the mutation before they can exhibit the effects of the loss of the functional protein. In contrast, a gain-of-function mutation produces an entirely new trait or it causes a trait to appear in inappropriate tissues or at inappropriate times in development. These mutations are frequently dominant in their expression. Still other types of mutations are conditional mutations, which are expressed only under certain conditions, and lethal mutations, which cause premature death.

Suppressor mutations A suppressor mutation is a genetic change that hides or suppresses the effect of another mutation. This type of mutation is distinct from a reverse mutation, in which the mutated site changes back into the original wild-type sequence (Figure 17.8). A suppressor mutation occurs at a site that is distinct from the site of the original mutation; thus, an individual organism with a suppressor mutation is a double mutant, possessing both the original mutation and the suppressor mutation but exhibiting the phenotype of an unmutated wild type.

Geneticists distinguish between two classes of suppressor mutations: intragenic and intergenic. An intragenic suppressor is in the same gene as that containing the mutation being suppressed and may work in several ways. The suppressor may change a second nucleotide in the same codon that was altered by the original mutation, producing a codon that specifies the same amino acid as the original, unmutated codon (Figure 17.9). Intragenic suppressors may also work by suppressing a frameshift mutation. If the original mutation is a one-base deletion, then the addition of a single base elsewhere in the gene will restore the former reading frame (see Figure 17.9). Consider the following nucleotide sequence in DNA and the amino acids that it encodes:

DNA AAA TCA CTT GGC GTA CAA

Amino acids Phe Ser Glu Pro His Val

Suppose a one-base deletion occurs in the first nucleotide of the second codon. This deletion shifts the reading frame by one nucleotide and alters all the amino acids that follow the mutation.

ij A forward muta"tiö^^^B A reverse mutation changes the wild type into a mutant phenotype.

Genotype:

restores the wild-type gene and the phenotype ij A forward muta"tiö^^^B A reverse mutation changes the wild type into a mutant phenotype.

restores the wild-type gene and the phenotype

Genotype:

Red eyes White eyes Red eyes

^ A suppressor mutation occurs at a site different from that of the original mutation.

... and produces an individual that possesses both the original mutation and the suppressor mutation.

.but has the wild-type phenotype.

... and produces an individual that possesses both the original mutation and the suppressor mutation.

.but has the wild-type phenotype.

Red eyes White eyes Red eyes

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