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

Variation in chromosome number

A summary of the terms used to describe heteroploidy (variation in chromosome number)

 

Term	Type of change	Symbol*
Heteroploid Aneuploid   Nullisomic Monosomic Double trisomic Trisomic B. Euploid Monoploid Haploid Polyploid 1. Autopolyploid Autotriploid Autotetraploid Autopentaploid Autohexaploid Autooctaploid Allopolyploid Allotetraploid Allohexaploid Allooctaploid	A change from 2x One or a few chromosomes extra or missing from 2n One chromosome pair missing One chromosome missing One chromosome from each of two different chromosome pairs missing One chromosome pair extra Number of genomes or copies of a genome more than two One copy of a single genome Gametic chromosome complement More than 2 copies of one genome or 2 copies each of 2 or more genomes** Genomes identical with each other Three copies of one genome Four copies of one genome Five copies of one genome Six copies of one genome Eight copies of one genome Two or more distinct genomes (Generally each genome has two copies) ** Two distinct genomes Three distinct genomes Four distinct genomes	- 2n ± few 2n-2 2n-1 2n-1-1 2n+2 x n 3x 4x 5x 6x 8x (2x1+2x2)** (2x1+2x2+2x3)** (2x1+2x2+2x3+2x4)**

• *2n= Somatic chromosome number (and complement) and n= gametic chromosome number (and complement) of the species, whether diploid or polyplod.
• X = The basic chromosome number (and complement) or genomic number.
• x1,x2,x3,x4 = Distinct genomes from different species.
• ** In general, this condition occurs; other situations may also occur.

• A diploid in which 1 chromosome is missing from the total complement (2n-1) is known as monosomics. A double monosomic individual has two chromosomes missing, but the two chromosomes belong to two different chromosome pairs (2n-1-1). An individual having one extra chromosome (2n+1) is known as trisomic, and that having two extra chromosomes each belonging to a different chromosome pair is called double trisomic (2n+1+1). When an individual has an extra pair of chromosomes, it is known as tetrasomic (2n+2). The same is true about the various features of and the facts accumulated about aneuploidy as well as euploidy. The breeder is generally concerned with monosomics and trisomics, and, in some situations, with nullisomics and tetrasomics.

• In euploids, the chromosome number is an exact multiple of the basic or genomic number. Euploidy is more commonly known as polyploidy. When all the genomes present in a polyploidy species are identical, it is known as autopolyploid and the situation is termed as autopolyploidy. In the case of allopolyploids, two or more distinct genomes are present. Euploids may have 3 (triploid), 4 (tetraploid), 5 (pentaploid), 6 (hexaploid), 7 (heptaploid), 8 (octaploid) or more genomes making up their somatic chromosomes number.

• In case of autopolyploidy, they are known as autotriploid, autotertaploid, autopentaploid, autohexaploid, autoheptaploid, auto octaploid and so on, while in the case of allopolyploidy they are termed as allotriploid, allotetraploid, allopentaploid, allohexaploid, alloheptaploid, allooctaploid etc, Amphidiploid is an allopolyploid that has two copies of each genome present in it and, as a consequence, behaves as a diploid during meiosis. A segmental allopolyploid contains two or more genomes, which are identical with each other, except for some minor differences.