Principles of Plant Genetics and Breeding

the same plant. However, the dual color is impossible
to reproduce by either sexual or asexual propagation.
Commercial use of chimera is not attractive because the
vegetative propagules must, of necessity, comprise both
kinds of tissues in order to reproduce the maternal
features.
Gene action: dominant versus recessive mutations
As previously indicated, mutations may cause a dominant
allele to be changed into a recessive allele (recessive
mutation), or a recessive allele to be changed into a
dominant allele (dominant mutation). Open-pollinated
species may accumulate a large amount of recessive
mutant alleles without any adverse effects. However,
upon selfing, the recessive alleles become homozygous
and are expressed, leading to the phenomenon of
inbreeding depression. Using recessive genes in breeding
takes a longer time because it requires an additional
step of selfing in order to identify and select the desired
recombinants. On the other hand, dominant mutations
manifest in the current generation, needing no additional
regeneration to be observable.
Structural changes at the chromosomal level
Three types of structural changes in the chromosome
can occur as a result of mutation.
Gene mutation
Kind of mutation Gene mutations entail a change in
the nucleotide constitution of the DNA sequence,
adding or deleting nucleotides.
1 Transitions and transversions. As previously
described, the DNA consists of four bases – A, T, C,
and G – that pair in a specific pattern, G–C and A–T.
G
C
(a)
G
C
A
C
Normal
A
T
G
C
MUTAGENESIS IN PLANT BREEDING 201
Mutant
Normal
Transition
Transversion
A T A C
A T C G
T A G C
Figure 12.1 Mutations may occur by transition or
transversion.
The structure of the DNA may be modified in two
ways – transition or transversion of the bases (Figure
12.1). Mutation by transition entails the conversion
of one purine base to another purine (or a pyrimidine
to another pyrimidine). During replication, the second
purine (a different purine), which has altered
base-pairing properties, guides an incorrect base into
position. Consequently, one normal base pair is converted
to another pair that is genetically incorrect.
An agent of mutation (a mutagen) such as nitrous
acid has been known to cause deamination of adenine
to hypoxanthin, cytosine to uracil, and guanine to
xanthine, the net effect being a replacement of A–T
with G–C in the DNA structure. A transversion
involves the substitution of a purine by a pyrimidine
and vice versa.
2 Tautomeric shifts. It is known that each of the bases
of DNA can exist in rare states as a result of the redistribution
of electrons and protons in the molecule,
events called tautomeric shifts. When this occurs, the
base sometimes is unable to hydrogen bond with its
complementary base. Instead, some of these altered
bases succeed in bonding with the wrong bases,
resulting in mutations when, during replication, one
purine (or pyrimidine) is substituted for the other
(Figure 12.2).
3 Effect of base analogues. Certain analogues of the
naturally occurring bases in the DNA molecule have
Figure 12.2 Mutations may be caused by tautomeric shifts: (a) shift involving cytosine, and (b) shift involving thymine.
A
T
(b)
A
T
G
T
Normal
G
C
A
T
Mutant
Normal