Page 61 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

220 Chapter 7 Anatomy and Function of a Gene: Dissection Through Mutation

Figure 7.1 The DNA of each human chromosome contains hundreds to
thousands of genes. The DNA of this human chromosome has been spread out and
magnified 50,000×. No topological signs reveal where along the DNA the genes reside.
The darker, chromosome-shaped structure in the middle is a scaffold of proteins to which
the DNA is attached.
© Dr. Don Fawcett/J.R. Paulson & U.K. Laemmli/Science Source

influences phenotype. One mutation in the rhodopsin gene, for example, causes
the substitution of one particular amino acid for another in the construction of
the rhodopsin protein. This single substitution changes the three-dimensional
structure of rhodopsin and thus the protein’s ability to absorb photons, ultimately
altering a person’s ability to perceive light.

7.1 Mutations: Primary Tools mutation. The resulting novel mutant allele can be either
of Genetic Analysis recessive or dominant to the original wild-type allele.
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Geneticists often diagram forward mutations as A → a
when the mutation is recessive to the wild-type allele, and
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as b → B when the mutation is dominant to the wild-type.
learning objectives Mutations can also cause a novel mutant allele to revert
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1. Distinguish between the effects of mutation in somatic back to wild type (a → A , or B → b ) in a process known
and germ-line cells. as reverse mutation, or reversion. In this chapter, we des-
2. Describe four types of point mutations: transitions, ignate wild-type alleles, whether recessive or dominant to
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transversions, deletions, and insertions. mutant alleles, with a plus sign ( ).
3. Summarize the factors associated with differences in Mendel originally defined genes by the visible pheno-
mutation rate. typic effects—yellow or green, round or wrinkled—of
4. Explain how the fluctuation test and replica plating have their alternative alleles. In fact, the only way he knew that
shown that mutations arise randomly and genes existed at all was because alternative alleles for
spontaneously. seven particular pea genes had arisen through forward
mutations. Mutations can occur in somatic cells or in
germ-line cells. The mutations in Mendel’s pea plants
We saw in Chapter 3 that genes with one common allele are were heritable because they occurred in the germ-line
monomorphic, while genes with several common alleles in cells of the plants and were thus transmitted through
natural populations are polymorphic. The term wild-type gametes. Close to a century later, knowledge of DNA
allele has a clear definition for monomorphic genes, where structure clarified that such mutations are heritable
the allele found on the large majority of chromosomes in changes in DNA base sequence. DNA thus carries the
the population under consideration is wild type. In the case potential for genetic change in the same place it carries
of polymorphic genes, the definition is less straightforward. genetic information—the sequence of its bases.
Some geneticists consider all alleles with a frequency of
greater than 1% to be wild type, while others describe the
many alleles present at appreciable frequencies in the popu- Mutations May Be Classified
lation as common variants and reserve wild-type allele for by How They Change DNA
use only in connection with monomorphic genes.
A substitution occurs when a base at a certain position in
Mutations Are Changes in DNA one strand of the DNA molecule is replaced by one of the
Base Sequences other three bases (Fig. 7.2a); after DNA replication, a new
base pair will appear in the daughter double helix. Substi-
A mutation that changes a wild-type allele of a gene (regard- tutions can be subdivided into transitions, in which one
less of the definition) to a different allele is called a forward purine (A or G) replaces the other purine or one pyrimidine