linked together on the same piece of DNA. The only changes that could occur in the DNA
sequence would be caused by mutation, and each mutation would stay on the same piece of
DNA forever. If this were the case, then plant improvement via breeding would be imposs-
ible. In both nature and agriculture the “goal” is to combine advantageous alleles together
within the same breeding line to improve a plant for natural or agricultural settings. Without
recombination, the target of selection would be the chromosome with the allele of interest,
and there would be a limited number of chromosome combinations from which to make
selections. Luckily for crop breeders, mutation is not the only process that generates
genetic diversity.
Recombination allows for alleles to be shuffled during every meiotic division (Fig. 2.9).
It has been estimated that crossing over occurs during every meiotic division for each
chromosome, and therefore the lifespan of any chromosomal sequence is actually only
one generation. This allows for different alleles at different chromosomal loci to reshuffle
and land on the same chromosome. Crop breeders rely on this process because they attempt
to select for recombination events that liberate the specific allele from a genetic background
to improve the crop line without having to select for chromosomes. Often, crop plants have
been highly selected to obtain a group of alleles that help the crop perform well under
specific agricultural conditions. A single new allele may improve the crop, but the
breeder needs to retain all the original genes of that crop. The process of recombination
allows the breeder to try to find specific recombination events where the one allele has
crossed over to join all the other original crop-selected alleles (see the next chapter for
an in-depth description of plant breeding).
2.3.4 Cytogenetic Analysis
Scientific methods to observe chromosomes have improved greatly since Mendel outlined
the laws that describe chromosome movement across generations. The easiest way to
observe chromosomes is via chromosome staining during mitosis. Many readers can
remember back to their high school biology classes where they observed stained onion
(Allium cepa) root tips with the microscope. In these lab exercises, condensed chromo-
somes were stained with a DNA-specific dye (a fuchsin-based DNA-specific stain devel-
oped by Feulgen in 1914), and the different stages of cellular mitotic division
determined by observing the patterns of the chromosomes in each cell. Chromosome
viewing by simple light microscopy is, however, limited to those plant species with large
chromosomes in which single layers of actively dividing cells can easily be attained.
These conditions are not common to most tissue types in adult plants.
Figure 2.9.Recombination occurs when homologous chromosomes trade DNA sequences, thus
generating genetic diversity.
2.3. MITOSIS AND MEIOSIS 33