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Chromosome Abnormalities : Overview


A chromosomal disorder, anomaly, aberration, or mutation may be a 
missing, extra, or irregular portion of chromosomal DNA. It are often from a typical number of chromosomes or a structural abnormality in one or more chromosomes. Chromosome mutation was formerly utilized in a strict sense to mean a change during a chromosomal segment, involving quite one gene. The term "karyotype" refers to the complete set of chromosomes from an individual; this will be compared to a "normal" karyotype for the species via genetic testing. A chromosome anomaly could also be detected or confirmed during this manner. Chromosome anomalies usually occur when there's a mistake in cellular division following meiosis or mitosis. There are many sorts of chromosome anomalies. They will be organized into two basic groups, numerical and structural anomalies.


Numerical abnormality

This is called aneuploidy (an abnormal number of chromosomes), and occurs when a private 
either is missing a chromosome from a pair (monosomy) or has quite two chromosomes of a pair (trisomy, tetrasomy, etc.).
An example of trisomy in humans is mongolism 
, which may be a developmental disorder caused by an additional copy of chromosome 21; the disorder is therefore also called mongolism . Having an additional copy of this chromosome means individuals have three copies of every of its genes rather than two, making it difficult for cells to properly control what proportion protein is formed . Producing an excessive amount of or insufficient protein can have serious consequences. Genes on chromosome that specifically contribute to the varied symptoms of mongolism are now being identified. The frequency of mongolism has been determined to be a function of advanced maternal age.


Structural abnormalities

When the chromosome's structure is altered, this will 
take several forms:


Ø     Deletions: some of the chromosome is missing or deleted. Known disorders in humans include Wolf-Hirschhorn syndrome, which is caused by partial deletion of the short arm of chromosome ; and Jacobsen syndrome, also called the terminal deletion disorder.

Ø     Duplications: some of the chromosome is duplicated, leading to extra genetic material. Known human disorders include Charcot-Marie-Tooth disease type 1A, which can be caused by duplication of the gene encoding peripheral myelin protein on chromosome.

Ø     Translocations: some of chromosome is transferred to a different chromosome. There are two main sorts of translocations:

Ø     Inversions: some of the chromosome has broken off, turned the wrong way up, and reattached, therefore the genetic material is inverted.

Ø     Insertions: some of chromosome has been deleted from its normal place and inserted into another chromosome.

Ø     Rings: some of a chromosome has broken off and formed a circle or ring. This will happen with or without loss of genetic material.

Ø     Isochromosome: Formed by the reflection copy of a chromosome segment including the centromere.



Most chromosome abnormalities occur as an accident within the 
ovum or sperm, and thus the anomaly is present in every cell of the body. Some anomalies, however, can happen after conception, leading to Mosaicism (where some cells have the anomaly and a few do not). Chromosome anomalies are often inherited from a parent or be "de novo". This is often why chromosome studies are often performed on parents when a toddler is found to possess an anomaly. If the oldsters don't possess the abnormality it had been not initially inherited; however it's going to be transmitted to subsequent generations.



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