Chromosomal+aberrations

=Chromosomal aberrations=

**Chromosomal aberrations** (or **abnormalities**) refer to the group of conditions where there is an anomaly, either in the number or the structure of chromosomes in a cell. This might be seen as a large-scale mutation, occurring above the level of change in nucleotide sequences. A number of genetic diseases are linked to chromosomal aberrations.

Numerical abnormalities are typically referred to with the suffix //-somy//. The usual situation in a diploid cell is disomy: having two copies of each chromosome. When a homologous pair of chromosomes either loses or gains a chromosome, this disrupts the total complement of chromosomes in that cell such that it is no longer a multiple of the monoploid number (x). This phenomenon is called aneuploidy. A monosomy is the loss of a chromosome, while trisomy, tetrasomy, etc. involve gaining chromosomes. Examples of genetic diseases where aneuploidy has occurred are trisomy 21, the presence of three copies of chromosome 21, or //Down's Syndrome//, and monosomy X, the loss of an X chromosome in a human female, or //Turner's Syndrome//.

Possible structural aberrations in chromosomes are diverse, and include:


 * **Deletions**, where part of a chromosome is lost completely. This loss of genetic material may have devastating impacts on the individual: known human disorders include Wolf-Hirschhorn Syndrome, caused by a deletion on chromosome 4, and Jacobsen syndrome, caused by a deletion at the terminus of the long arm of chromosome 11.


 * ** Duplications, ** where part of a chromosome is copied, causing an addition of genetic material (the copy may be a paralog). An example human disease is Charcot-Marie Tooth Disease type 1A, which may be caused by a gene duplication on chromosome 17.


 * ** Translocations, ** where part of a chromosome is excised and moved to another chromosome. Translocations may be subdivided into two types: //reciprocal translocation,// where two chromosomes exchange segments, and //Robertsonian translocation//, where an entire chromosome attaches to another at its centromere, forming a dicentric chromosome. In humans, Robertsonian translocations only occur between the acrocentric chromosomes 13, 14, 15, 21 and 22; any Robertsonian translocation involving chromosome 21 may cause Down's Syndrome in the offspring of that individual.


 * ** Inversions, ** where a portion of chromosome is excised, flips around, and is reinserted back-to-front. Comes in two types: paracentric and pericentric.


 * ** Rings//,// ** where a portion of chromosome breaks off and forms an independent circular structure, either with or without the loss or gain of genetic material


 * ** Isochromosomes, ** where a chromosome arm is lost and replaced by an exact mirror image of the remaining arm.