In this article we will discuss about the Variations in Chromosome Number to Plant and Animal.
Every cell in the body of living organism is provided with a packet of genetic material (the nucleus) which contains chromosomes. The number of chromosomes in the nuclei of a given species is fixed.
Some species of plants and animals have identical chromosome numbers.
In such cases one should not think that these organisms will show similar characters, because it is not the number of chromosomes which differentiates various species from one another, but rather the nature of hereditary material in chromosomes determines the characters of species. In most of the plants and animals the chomosome number ranges between 10 and 50.
Numbers above and below this range are comparatively rare.The highest number of chromosomes reported so far is about 1100 found in Radiolaria, an unicellular marine protozoan. In plants, chromosome number ranges from two pairs (in a member of compositae, Haplopappus gracilis) to several hundred found in ferns.
In Ophioglossum 768 chromosomes have been counted in each nucleus. In higher plants and animals, body cells normally have two identical sets of chromosomes (2n), i.e., there are two identical members for each type of chromosomes in the nucleus.
In 14 chromosomes pea, for example, there are 7 chromosomes of different shapes, sizes and structures and each kind of chromosome has its own homologue. So, fundamentally there are 7 pairs of chromosomes in pea. The minimum number of chromosomes, though all are different which function as a harmonious and integrated unit, is called basic chromosome number. This basic number is denoted by the letter ‘x’.
The individuals in which the somatic cells have chromosomes in pairs are called diploid (2n). When the reduction division takes place in a diploid cell, the daughter nuclei so formed will have chromosome number that is just half the diploid chromosome number, i.e., each daughter cell now gets one member of each chromosome pair.
The chromosome number present in the product of meiosis of a diploid cell is referred to as haploid or gametic number of genome. When two gametes, each carrying haploid number of chromosomes (n) fuse they produce a diploid cell. The diploid cell produces the diploid body.
In this way, in the life cycle of an individual, the haploid and diploid conditions come regularly one after another and the same chromosomes number is maintained from generation to generation. Several cases, however, are known in which chromosome number becomes changed in the somatic cell.
This condition is known as ploidy. The change in the chromosome number involves addition or elimination of individual chromosomes or of complete chromosomal set (genome).
On this basis two main classes of ploids can be recognised:
1. Aneuploids or Heteroploids.
1. Aneuploidy or Heteroploidy:
It is a condition in which nuclei contain chromosomes whose number is not true multiple of basic chromosome number or genome. The organisms with such abnormal conditions are called aneuploids.
2. Euploidy (GK. Eu = true; and ploid = unit):
This is a condition in which the change in the chromosome number involves additions or elimination of complete chromosome set or genome. The basic chromosome number of euploids is represented by the haploid (n).
The euploids above the haploid or monoploid level are called polyploids. They may be diploid (2n), triploid (3n), tetraploid (An), pentaploid (5n), hexaploid (6n) and so on. In all these cases of euploids the chromosome numbers is exact multiples of original haploid chromosome number (n).