Biology Reference
In-Depth Information
Figure 1.15
Movement (transposition) of transposable elements into chromosomes can result in
mutations that inactivate genes or alter their expression. Transposable elements are also known as
jumping genes.
change in the nucleotide sequence. In reversions, the original mutation is not
restored to its previous unmutated form; rather, the second mutation restores
the code for the original amino acid because the code is degenerate.
Regulatory
mutations
are mutations that affect the ability to control expression of a gene.
The movement of a
transposable element
(TE) into a gene can also cre-
ate mutations in genes. TEs are segments of foreign DNA that can move into
genomes. When TEs move into a gene, as is shown in
Figure 1.15
, the gene will
be inactivated or the gene product will be altered and produce in a visible phe-
notype (mutation). TEs can cause other types of mutations, including inversions
(
Caceres et al. 1999
). TEs are found in most eukaryotic organisms, and there are
many types. TEs are important for understanding genome evolution and for
genetic engineering, and they are discussed further in Chapters 3, 4, 9, and 14.
1.23 Common Genetic Terminology
A
wild-type gene
is normally identified only after a mutation has disrupted the
phenotype of an organism. Mutations commonly are given a descriptive name,
such as “white eyes.” The name of the gene usually is italicized (
white
) and is
abbreviated using one, two, or three italicized letters (such as
w
). If the mutation
is dominant, the name and abbreviation are capitalized (
White
and
W
); they are
in lowercase (
white
) if the mutation is recessive. Individuals that are homozygous
for the recessive
w
mutation are
w/w
and have white eyes. Heterozygous flies are
w/w
+
, with the wild-type allele designated as
w
+
, and their appearance (pheno-
type) should be wild type. The gene product is called the white product or white
protein and is not italicized. The term for the gene product may be abbreviated
as the w protein. Sometimes the protein product is designated by the gene name
but capitalized to distinguish it from the gene (WHITE).
