Principles of Plant Genetics and Breeding

228 CHAPTER 13
1st meiotic
division
2nd meiotic
division
(Abnormal
disjunction)
(Normal
disjunction)
Zygotes 2n + 1 2n + 1 2n − 1 2n − 1
2n
(trisomic) (trisomic) (monosomic) (monosomic) (normal)
(a) (b)
Figure 13.7 The origin of anueploidy. Abnormal disjunction may occur at the first meiotic division (a) or at the second
meiotic division (b) producing gametes with a gain or loss in chromosomes.
is increased by one complete chromosome. A secondary
trisomic is one in which the extra chromosome has
identical arms (i.e., one particular arm occurs as a
quadruplicate). Such a chromosome is called an
isochromosome. Sometimes, the extra chromosome
added is derived from parts of different chromosomes.
Such additives arise from chromosome breakage–fusion
events.
Primary trisomics may be used to assign genes to
chromosomes. Theoretically, there are as many possible
trisomics as there are chromosome pairs. Scientists can
generate trisomic stocks for a species. To assign a gene,
the mutant (e.g., a) that is homozygous for the allele
of interest is crossed to all the trisomic tester stocks.
Assuming that all stocks are homozygous for the wild
type and assuming normal meiotic segregation, two F 1
types will be produced. Those produced from the union
of normal gametes (n) will segregate with the normal
diploid ratio 3A :1aa. However, where a trisomic plant
(n + 1 gamete) is involved, an aberrant ratio would
result. Because a trisomic stock is unique, the gene of
interest would be located on the chromosome that the
trisomic stock represents. These results assume random
segregation of the three chromosomes of the trisomic
2n
(normal)
2n + 1
(trisomic)
2n − 1
(monosomic)
plant and equal viability of pollen regardless of genetic
constitution. In reality there is a preponderance of n
gametes and reduced function of n + 1 gametes. The
consequence of reduced functionality is that, sooner
or later, a trisomic would revert to a diploid, unless the
scientist makes special efforts to maintain it. Trisomics
have been applied in creative ways in plant breeding,
including their use in hybrid seed production in barley,
using a balanced tertiary trisomic that carries a recessive
male-sterility gene. The addition of chromosomes from
other species (called alien addition lines) has been
explored in interspecies crosses such as wheat × rye.
Chromosome addition lines may be unstable enough to
be developed as cultivars.
Chromosome deletions
Unlike chromosome addition in which gene duplication
occurs (hence an implied duplication in function), chromosome
deletion leads to a loss of function. The consequence
of a deletion depends on the functional roles
of the genes of the chromosome that is lost. Invariably,
surviving plants have less vigor and more sterility problems.
However, in polyploids, the presence of homeologous