Principles of Plant Genetics and Breeding

82 CHAPTER 5
both DNA strands; and (v) inversion of a sequence of
nucleotide base pairs within the DNA molecule. These
mechanisms are discussed further in Chapter 12 on
mutation breeding.
Mutations may also be induced by plant breeders
using agents such as irradiation and chemicals. Many
useful mutations have been found in nature or induced
by plant breeders (e.g., dwarfs, nutritional quality genes).
However, many mutations are deleterious to their
carriers and are hence selected against in nature or by
plant breeding. From the point of view of the breeder,
mutations may be useful, deleterious, or neutral. Neutral
mutations are neither advantageous nor disadvantageous
to the individuals in which they occur. They persist in
the population in the heterozygous state as recessive
alleles and become expressed only when in the homozygous
state, following an event such as selfing.
Transposable elements
The phenomenon of transposable elements (genes
with the capacity to relocate within the genome) also
creates new variability. Transposable genetic elements
Ds Wx
Ac Ds Wx
Ac Ds Wx
Ac Ds Wx
Ac Ds
Wx
Ac
Ds
Wx
Ac Ds Wx
(transposable elements, transposons, or “jumping genes”)
are known to be nearly universal in occurrence. These
mobile genetic units relocate within the genome by the
process called transposition. The presence of transposable
elements indicates that genetic information is
not fixed within the genome of an organism. Barbara
McClintock, working with corn in the 1940s, was the
first to detect transposable elements, which she initially
identified as controlling elements. This discovery was
about 20 years ahead of the discovery of transposable
elements in prokaryotes. Controlling elements may be
grouped into families. The members of each family
may be divided into two classes: autonomous elements
or non-autonomous elements. Autonomous elements have
the ability to transpose whereas the non-autonomous
elements are stable (but can transpose with the aid of an
autonomous element through trans-activation).
McClintock studied two mutations: dissociation
(Ds) and activator (Ac). The Ds element is located on
chromosome 9. Ac is capable of autonomous movement,
but Ds moves only in the presence of Ac. Ds has
the effect of causing chromosome breakage at a point on
the chromosome adjacent to its location (Figure 5.5).
Ac is absent; Ds is not transposable;
wild-type W phenotype
Ac present; Ds transposable
Ds transposed
Chromosome breakage;
fragment is lost; no expression
of W; mutation produced
Ds transposed into W gene;
W gene incapacitated;
mutation produced
Ds jumps out of W, restoring
W activity
Figure 5.5 In the Ac–Dc (activator–dissociation) system of transposable elements in maize, the transposition of the Ds to
Wx causes chromosome breakage, leading to the production of a mutant. In another scenario, the Ds is transposed into
Wx, causing a mutant to be produced.