Chromosomes alter due to structural changes. These changes are called mutations. The change is usually irreversible and is kept throughout all the following generation, with the original hereditary information being lost. There are 4 types of mutation
1. Deletion – This is the loss of chromosome segments. It can occur terminally or internally. When the end fragment is lost, it is known as deficiency. Consequences – Almost always lethal when there are large deletions. For single deletion, it depends on which segment has been deleted. Examples – Mental retardation, poor suckling as infants, endocrine gland dysfunction, weak muscle tone, poor sexual development in males etc.
2. Duplications – This is the duplication of one or more chromosome segments. There are two types: tandem and non-tandem duplication, based on whether the location of duplication is adjacent, or occurs elsewhere in the string.
Consequences – Duplications usually do not have phenotypic consequences, but play and important role in evolution due to increased gene numbers and evolution of new genes.
3. Inversions – This is the reversal in orientation of the chromosome segment. Depending on the points of breakage, it can be divided into pericentric and paracentric inversions.
Consequences – This does not usually have any phenotypic consequences. Inversions also prevent genetic recombination by crossing-over, and so they are used to balance chromosomes.
4. Translocations – This is the transfer of a chromosome segment into non-homologous chromosomes. There are two types: reciprocal and non-reciprocal, depending on whether it is an exchange of chromosomes between two segments or an attachment of a chromosome into another segment. Consequences – In homozygotes genetic material is neither lost nor gained. In heterozygotes, nothing happens in case of a balanced translocation; however unbalanced translocation might have an additional effect of deletion which might be very harmful for e.g. it might lead to semi sterility or Down syndrome
Figures of the four types of Chromosomal mutations
“Types of Chromosomal mutations”,
“Chromosome mutations”, http://comp.uark.edu/~mlehmann/rearrangements.pdf
Sengbusch P, 2007, “Chromosomal Mutations”, Article retrieved on 10th May 2007,