Principles of Plant Genetics and Breeding

Purpose and expected outcomes
Hybridization, as previously discussed, is a means of reorganizing genes from the parents involved in the cross in
a new genetic matrix. Whereas the contents of the chromosomes may change because of the phenomenon of genetic
recombination, normal hybridization does not alter the chromosome number of the species. However, certain
natural processes can result in altered chromosome numbers. Similarly, the breeder may develop new variability by
altering the number of chromosomes in the species through various processes. Furthermore, a number of the major
crop species contain altered chromosome numbers. After studying this chapter, the student should be able to:
1 Define the term polyploidy.
2 Discuss the variations in chromosome number in plants.
3 Discuss the effects of polyploidy on plants.
4 Discuss the importance of autoploidy in crop production.
5 Discuss the genetics of autoploidy.
6 Discuss the implications of autoploidy in plant breeding.
7 Discuss the occurrence of alloploidy in nature.
8 Discuss the genetics and breeding of alloploidy in plant breeding.
9 Discuss the applications of aneuploidy.
Terminology
Ploidy refers to the number of copies of the entire chromosome
set in a cell of an individual. The complete
chromosome set is characteristic of, or basic to, a species
(see Table 3.2). A set of chromosomes (the genome) is
designated by “x”. Furthermore, the basic set is called
the monoploid set. The haploid number (n) is the
number of chromosomes that occurs in gametes. This
represents half the chromosome number in somatic cells,
which is designated 2n. A diploid species, such as corn,
has n = 10 and 2n = 20. Also, a diploid species has 2n = 2x
in its somatic cells, and n = x in its gametes. Some species
have a higher ploidy, for example an autotetraploid
(four basic sets of chromosomes) which has somatic cells
13
Polyploidy in plant
breeding
with 2n = 4x and gametes with n = 2x. For corn, for
example, 2n = 2x = 20, while for wheat, a hexaploid with
42 chromosomes and a basic set of seven, 2n = 6x = 42.
Sometimes species that have more than two genomes
comprise sets from different origins. To distinguish the
source, each genome is designated by a different letter.
For example, wheat has chromosome sets from three
different origins and hence has a genetic designation
(genomic formula) of AABBDD (Figure 13.1). To
indicate the number of haploids derived from individuals
of different ploidy levels for a single genome, a prefix is
added to the term “haploid” to denote the number of
sets (x) of the basic genome present. For example a
monohaploid (n = 1x) is derived from a diploid, while a
dihaploid (n = 2x) is from a tetraploid, and so on.