- Classification based on characteristics: Chromosomes are divided into two groups based on characteristics.
(i) Autosomes: All the chromosomes present in the body cells of organisms are called autosomes. The number of autosomes is 22 pairs or 44. It does not carry hereditary characteristics.
(ii) Sex Chromosomes: Chromosomes which determine the sex of an organism are called sex chromosomes. Sex chromosomes are called allosomes/heterochromosomes. The number of sex chromosomes is one pair or 2. X and Y.
In 1891, the scientist Henking observed that certain elements of the nucleus were involved in the production of spermatozoa in insects. He named it X-body. It is recognized as the X chromosome in sex determination. Later another sex chromosome was discovered in males. It is called Y chromosome. Sex chromosomes can be carriers of color blindness or hemophilia. This problem is called Sex Linked Inheritance.
- Classification based on position of centromere: Chromosomes are divided into four parts based on the position of centromere.
(i) Metacentric Chromosome: Chromosome which lies in the center of centromere is called metacentric chromosome. During the anaphase phase of cell division, the metacentric chromosomes look like the English letter V. Amphibians have metacentric chromosomes. All chromosomes of Solanum nigrum are metacentric. Human chromosomes 1, 3, 6, 7, 8, 11, 16, 19, 20 and X are metacentric types.
(ii) Sub-metacentric chromosomes: Chromosome which is slightly away from the center of centromere is called sub metacentric chromosome. During the anaphase phase of cell division, the sub-metacentric chromosomes look like the English letter L. Human chromosomes 2, 4, 5, 9, 10, 12, 17 and 18 are of sub metacentric type.
(iii) Acrocentric Chromosome: Chromosome which is near the edge of centromere is called acrocentric chromosome. In the anaphase phase of cell division, acrocentric chromosomes look like the English letter O. All acrocentric chromosomes are sat-chromosomes. Human chromosomes 13, 14, 15, 21, 22 and Y are acrocentric types.
(iv) Telocentric Chromosome: Chromosome which is located at the extreme end of centromere is called telocentric chromosome. During the anaphase phase of cell division, the telocentric chromosomes look like the English letter I. Telocentric chromosomes are rare in organisms. Humans do not have telocentric chromosomes. Telocentric chromosomes are found in gymnosperms (Welwitschia) and onion (Allium).
[The dark purple variant of Typhonium trilobatum has 11 metacentric, 4 sub-metacentric and 2 acrocentric chromosomes]
- Classification based on the number of centromeres: Chromosomes are of five types according to the number of centromeres.
(i) Acentric Chromosome: A chromosome which does not have a centromere is called an acentric chromosome. It is the fragile part of the chromosome. It does not participate in cell division as it does not have a centromere.
(ii) Monocentric Chromosome: Chromosome having only one centromere is called monocentric chromosome. Chromosomes in most species are monocentric.
(iii) Dicentric Chromosome: A chromosome having two centromeres is called a dicentric chromosome. Wheat, Bhutra, yeast and Dsophila fly have dicentric chromosomes.
(iv) Polycentric Chromosome: A chromosome having more than two centromeres is called a polycentric chromosome. Banana plants, Luzula purpurea, Spirogyra and roundworms have polycentric chromosomes.
(v) Diffuse Chromosomes: If the centromeres are isolated in a chromosome, it is called diffuse chromosome. Banana plants have diffuse chromosomes.
- Classification of dye retention: Chromatin material is divided into two types based on dye retention.
(i) Euchromatin: The part of the chromosome which becomes light colored when stained with alkaline dye is called euchromatin. This region contains a large amount of DNA and plays an important role in heredity. It synthesizes mRNA.
(ii) Heterochromatin: The part of the chromosome which becomes dark when stained with alkaline dye is called heterochromatin. These regions contain small amounts of DNA and do not play a significant role in heredity. It does not synthesize mRNA.