Principles of Plant Genetics and Breeding

(discussed next). In addition to the nucleus, DNA occurs
in some plastids (organelles that are capable of dividing,
growing, and differentiating into different forms).
These plastids are chloroplasts. DNA also occurs in the
mitochondria. The DNA in these organelles is not subject
to Mendelian inheritance but follows what is called
cytoplasmic (or extrachromosomal or extranuclear)
inheritance. The distribution of DNA into gametes following
nuclear division is unpredictable and not equitable.
Molecular techniques may be used to separate nuclear
DNA from non-nulcear DNA during DNA extraction
from a tissue, for independent analysis. Some extranuclear
genes are of special importance to plant breeding.
Some male sterility genes are located in the mitochondria.
As will be described later, cytoplasmic male sterility
(CMS) is used in the breeding of corn and many other
species. It is used to eliminate the need for emasculation
(a time-consuming and tedious operation to prepare
plants for crossing by removing the anthers). Also,
because genes occur in the cytoplasm but pollen grains
(plant male sex units) lack cytoplasm, it is important in
a hybrid program which of the two parents is used as
female (provides both nuclear genes and cytoplasmic
genes) and which as male (provides only nuclear genes).
Genes carried in the maternal cytoplasm may influence
the hybrid phenotype, an effect called the maternal
effect (Figure 3.1). When uncertain about the presence
Pigmented
chloroplast
F 1
F 2
Nuclear
genotype
PLANT CELLULAR ORGANIZATION AND GENETIC STRUCTURE 37
(IjIj) (ijij)
(Ijij)
of any special beneficial genes in the cytoplasm, some
breeders conduct reciprocal crossing in which the parents
take turns in being used as the female parent.
Chromosomes and nuclear division
Genes (DNA sequences) are arranged in linear fashion
in chromosomes, which may be visible as strands in the
condensed stage as the cell prepares for nuclear division.
Each species is characterized by a set of chromosomes
per cell (Table 3.2). On the basis of the number of
chromosomes, there are two kinds of cells in a sexually
reproducing plant. Cells in the gametes (gametic cells)
of the plant (pollen grains, eggs) contain half the set of
chromosomes in the cells in other parts of the body
(somatic cells). The somatic chromosome number is
called the diploid number (2n), while the gametic cells
contain the haploid number (n). Further, the somatic
chromosomes can be arranged in pairs called homologous
chromosomes, based on morphological features
(size, length, centromere position). In sexually reproducing
plants, one member of each pair is derived from
the maternal parent (through the egg) and the other
from the paternal parent (through the pollen). This
occurrence is called biparental inheritance and as a
result each diploid cell contains two forms of each gene
Female Male Female Male
×
IjIj Ijij Ijij ijij
Colorless
chloroplast
Figure 3.1 Maternal inheritance of the iojap (ij ) gene in maize. The wild type gene is Ij. The green color of the leaf is
caused by the chloroplasts, which are maternally inherited. The appearance of the leaf color is determined solely by the
maternal phenotype. (Adapted fromr Klug W.S., and M.R. Cummings. 1997. Concepts of genetics, 5th edn. Prentice Hall.)
F 1
F 2
×
(ijij) (IjIj)
(Ijij)
(a) All white leaves
(b) All striped leaves
(c) 3 : 1 green : white leaves
Ijij