Chromosomal Aberrations Overview

Chromosomal Aberrations

• Definition: Deviation from a normal number or structure of chromosomes.
• Common Cause: Often linked to mental retardation (MR) and developmental issues.
• Structural aberrations are mutations.
• Balanced aberrations: Chromosomes have the normal amount of genetic information.
• Unbalanced aberrations: Chromosomes have missing or extra genetic information.

Types of Structural Aberrations

1- Break

• A break in the chromosome is followed by the reunion of the sticky ends, potentially causing a rearrangement.

2- Deletion

• Terminal: Loss of a chromosomal segment from one end.
• Interstitial: Loss of a chromosomal segment from within the chromosome.
• Examples: Wolf-Hirschhorn syndrome (deletion of short arm of chromosome 4), Cri-du-chat syndrome (deletion of short arm of chromosome 5).
• A deleted segment's genetic material is lost, potentially causing serious health issues.
• A ring chromosome is also a type of deletion where the ends of a chromosome fuse to form a ring.

3- Duplication

• Addition of an extra piece of homologous chromosome due to unequal crossing over. This results in two copies of the same gene on the chromosome.

4- Inversion

• Two breaks within a chromosome are followed by the reunion, but in an inverted order..
• Pericentric: The break includes the centromere.
• Paracentric: The break does not include the centromere.

5- Translocation

• Transfer of a chromosome segment to a non-homologous chromosome.
• Robertsonian translocation: Fusion of the long arms of two acrocentric chromosomes (typically chromosomes 13, 14, 15, 21, or 22). Short arms are usually lost. This is significant because it can lead to conditions like Down syndrome.
• Reciprocal translocation: Exchange of segments between two non-homologous chromosomes creates a balanced translocation as no genetic information is gained or lost. One example is the Philadelphia chromosome, (translocation between chromosomes 9 and 22), which is associated with chronic myeloid leukemia (CML).

6- Isochromosomes

• Result of a peculiar division of a chromosome during anaphase of mitosis.
• One chromosome has two long arms, while the other has two short arms.
• This occurs because the chromosome divides transversely instead of longitudinally during cell division.

Important Note (NB)

• If a parent carries a translocation of chromosome 21, the risk of having a child with Down syndrome can be as high as 100%.

Explore the fascinating world of chromosomal aberrations, where you'll learn about deviations in chromosome structure and number. This quiz covers types of structural aberrations, such as breaks and deletions, as well as their implications for health and development. Test your knowledge on key examples like Wolf-Hirschhorn and Cri-du-chat syndromes.