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No change of genetic material occurs in which of the following cytogenetic abnormalities? (AIIMS May 2012)

A Deletion

B Insertion

C Translocation

D Inversion

Ans. D Inversion (Ref. Robbins 8th/138, 160)

Inversion refers to a rearrangement that involves two breaks within a single chromosome with reincorporation of the inverted, intervening segment. It can be of the following two types:

a. An inversion involving only one arm of the chromosome is known as paracentric.

b. If the breaks are on opposite sides of the centromere, it is known as pericentric.

Inversions are often fully compatible with normal development. It is not associated with change in genetic material.

Deletion refers to loss of a portion of a chromosome. Similarly, insertion would lead to increase (and not loss) in the genetic material.

In a translocation, a segment of one chromosome is transferred to another. It can be of the following types:

In balanced reciprocal translocation, there are single breaks in each of two chromosomes, with exchange of material. There is no loss of genetic material and so, the affected individual is likely to be phenotypically normal. A balanced translocation carrier, however, is at increased risk for producing abnormal gametes.

In robertsonian translocation (or centric fusion), a translocation between two acrocentric chromosomes. Typically the breaks occur close to the centromeres of each chromosome. Transfer of the segments then leads to one very large chromosome and one extremely small one. Usually the small product is lost. This loss is compatible with a normal phenotype because it carries only highly redundant genes.

Therefore, deletion, insertion and Robertsonian translocation would lead to change in genetic material.

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