Breeding of Vegetable, Spice and Tuber crops (2+1)

Types of changes in chromosome number

• The somatic chromosome number of any species, whether diploid or polyploid, is designed as 2n, and the chromosome number of gametes is denoted as n. An individual carrying the gametic chromosome number, n, is known as haploid.
• A monoploid, on the other hand, has the basic chromosome number, x. In a diploid species, n = x; one x constitutes a genome or chromosome complement.
• The different chromosomes of a single genome are distinct from each other in morphology and/ or gene content and homology; members of a single genome do not show a tendency of pairing with each other.
• Thus a diploid species has two, a triploid has 3 and a tetraploid has 4 genomes and so on.
• The terminology of heteroploidy is summarized.
• Individuals carrying chromosome numbers other than the diploid (2x, and not 2n) number are known as heteroploids, and the situation is known as heteroploidy.
• The change in chromosome number may involve one or a few chromosomes of the genome; this is known as aneuploidy.
• The aneuploid changes are determined in relation to the somatic chromosome number (2n and not 2x) of the species in question. Therefore, the terminology for aneuploid individuals arising from diploid and polyploidy species is the same.
• Heteroploidy that involves one or more complete genomes is known aneuploidy.
• By definition, therefore, the chromosome numbers of euploids are an exact multiple of the basic chromosome number of the concerned species, while those of aneuploids are not. Aneuploid individuals from which one chromosome pair is missing (2n-2) are termed as nullisomic