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.