Chromosomal Abberation

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Chromosomal aberrations are changes in the structure or number of chromosomes. These changes can have a significant impact on an individual's development, health, and fertility.


There are two main types of chromosomal aberrations:


Numerical Aberrations: Changes in the number of chromosomes.

Structural Aberrations: Changes in the structure of one or more chromosomes.

1. Numerical Aberrations

These involve having an incorrect number of chromosomes. They typically arise from errors during cell division (meiosis or mitosis).


Aneuploidy:


This is the most common type of numerical aberration, involving the gain or loss of a single chromosome or a small number of chromosomes. It results in a chromosome number that is not an exact multiple of the haploid set (n).

Monosomy (2n-1): The loss of one chromosome from a diploid set.

Example: Turner syndrome (45, X) where a female has only one X chromosome instead of two.

Trisomy (2n+1): The gain of one extra chromosome, resulting in three copies of a particular chromosome instead of the usual two.

Example: Down syndrome (Trisomy 21), Patau syndrome (Trisomy 13), Edwards syndrome (Trisomy 18).

Nullisomy (2n-2): The loss of both chromosomes from a homologous pair. This is usually lethal.

Tetrasomy (2n+2): The gain of two extra chromosomes from a homologous pair.

Polyploidy:


Involves the presence of one or more entire extra sets of chromosomes, meaning the chromosome number is an exact multiple of the haploid set (e.g., 3n, 4n).

Triploidy (3n): Three sets of chromosomes. This is effectively three times the haploid number (e.g., 69 chromosomes in humans). It is usually lethal in humans.

Tetraploidy (4n): Four sets of chromosomes (e.g., 92 chromosomes in humans). Also typically lethal in humans.

2. Structural Aberrations

These involve changes in the architecture of one or more chromosomes, often resulting from breaks in the DNA followed by abnormal rejoining.


Deletions:


A portion of a chromosome is missing or "deleted." This can be a small segment (microdeletion) or a larger part.

Example: Cri-du-chat syndrome (deletion on the short arm of chromosome 5), Prader-Willi syndrome (deletion on chromosome 15).

Duplications:


A portion of a chromosome is repeated, leading to extra genetic material.

Example: Charcot-Marie-Tooth disease type 1A (duplication on chromosome 17).

Inversions:


A segment of a chromosome is inverted (reversed end-to-end) and reinserted. The genetic material is present, but in the wrong order.

Pericentric inversion: Includes the centromere.

Paracentric inversion: Does not include the centromere.

Translocations:


Genetic material is exchanged between two non-homologous chromosomes.

Reciprocal Translocation: Two non-homologous chromosomes exchange segments. The total chromosome number usually remains 46, but the arrangement of genetic material is altered.

Robertsonian Translocation: Involves fusion of two acrocentric chromosomes (chromosomes with centromeres near one end—chromosomes 13, 14, 15, 21, 22) at their centromeres, with the loss of their short arms. This typically results in a chromosome count of 45 but with all essential genetic material.

Example: A common cause of familial Down syndrome.




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