Principles of Plant Breeding (2+1)

Polyploid Breeding

Introduction

• The change in number of chromosomes is an important source of genetic variation that resulted in the evolution of a number of crop species. The genes, which produce enzymes and finally traits are located on the chromosomes as a result of which the change in chromosome number leads to observable change in the trait of an individual.
  
• Each chromosome exists as a member of the pair and the number of such pairs of chromosome is specific to a particular species. As an instance the human beings have 23 pairs, maize have 10 pairs and Arabidopsis has only 5 pairs of chromosomes. A basic set of chromosomes constitute a genome containing one number of each pair of chromosomes and represents the minimum number of chromosomes that has survived in any species.
• Individuals having chromosome number more than two sets referred as polyploids. The individual having two sets of chromosomes is called as diploid while individual with only single set is called monoploid. The somatic chromosome number of an individual i.e. Sporophyte is always designated as 2n and the chromosome number of the gametes as n whereas x is basic chromosome of the genome. The basic number of any species is designated as x, which represents a complete set of non-homologous chromosomes of one genome.
• The basic number of any species is designated as x, which represents a complete set of non-homologous chromosomes of one genome. Genome of monoploid is designated by symbol x, and that of haploid with symbol n and that of sporophyte with symbol 2n. In true diploids e.g. maize x=n=10: 2n=20. But in Polyploids the monoploid and haploid chromosome number is different. For instance in common bread wheat which is polyploid, the somatic chromosome number is 42 and haploid number 21 and the monoploid basic number is 7 Thus wheat x=7, n=21,2n=42 or n=3x and 2n=6x=42. The haploids of diploids are called monohaploids, and the tetraploids are called dihaploids and those of higher polyploids are known as poly haploids. The doubling of chromosome number of haploid results into a plant called doubled haploid.