The process that produces an alteration in DNA or chromosome’s structure or number is known as mutation.
The term ‘mutation’ was coined by Hugo Marie de Vries in 1901.
Any agent that cause mutation or increase the rate of mutation is known as mutagen.
Chromosomal Mutation
Chromosomal mutation is a missing, extra, or irregular portion of chromosome. It is also known as chromosomal anomaly, chromosomal abnormality, chromosomal aberration or chromosomal disorder.
It can be classified into following three categories:
- Polyploidy
- Aneuploidy
- Chromosomal translocation
1. Polyploidy
‘Ploidy’ is the number of set of chromosomes in the nucleus of a cell.
When an organism or a cell contains more than two paired sets of chromosomes (>2n), then the condition is known as polyploidy. It may be tripolidy (3n), tetraploidy (4n), pentaploidy (5n) and so on.
This condition is mostly observe in plants.
Autopolyploids = Polyploids with chromosomes sets, derived from single species.
Allopolyploids = Polyploids with chromosome sets, derived from different species.
2. Aneuploidy
When an organism or a cell contains more than two paired sets of chromosomes, but not a complete set, then the condition is known as aneuploidy.
Aneuploidy originates during anaphase of meiosis when the chromosome(s) do not separate properly between the two daughter cells, a condition known as non-disjunction.
Few examples of aneuploidy are given below:
Nullisomy (2n-2) – The loss of both pairs of homologous chromosomes.
Monosomy (2n-1) – The loss of a single chromosome.
Example – Monosomy of sex chromosome (45, X) causes Turner syndrome or gonadal dysgenesis.
Trisomy (2n+1) – The gain of an extra copy of chromosome.
Example – The presence of an extra chromosome 21 which is found in Down syndrome is called trisomy 21.
Tetrasomy (2n+2) – The gain of an extra pair of homologous chromosomes.
Example – Tetrasomy 9p is caused by the the presence of two extra copies of the short arm of chromosome 9.
3. Chromosomal Translocation
It is caused by rearrangement of parts between non-homologous chromosomes.
Example – Chronic Myelogenous Leukemia (CML)