Figure 2: A healthy male Karyotype
According to “yourgenome.com” a chromosome disorder is described as a change to the structure or order of a chromosome. They go on to say that the loss or gain of a whole chromosome is the most significant chromosome disorder and can affect hundreds if noth thousands of genes, and is often fatal.
Chromosomes viewed in sets are referred to as karyosets, each karyoset contains 23 pairs of chromosomes (see figure two). Structural abnormalities to the karyoset occur when genetic material (DNA) is either missing or present where it should not be.
There are a variety of structural abnormalities and generally come under the following headings; deletions, duplications, translocation, inversion, and ring.
The term deletion describes the absence of some DNA material from any of the given chromosomes, whilst a duplication on the other hand refers to the presence of additional DNA material (additional copies).
In the event of a translocation all of the genetic material is there BUT not always in the correct order, causing a structural change. An inversion also has all full compliment of genetic material, however it may be that a small part of a chromosome is actually inverted (upside down). In the case of a ring, again all of the genetic material is present but a part of one of the chromosomes has broken away and the break away has grown end to end to form, youve got it, a ring.
Furthermore these abnormalities can be categorised into balanced and unbalanced. These terms refer to whether or not the full compliment is present or not. For example (according to your gene.com) a balanced chromosome would have no DNA material missing nor gained (such as in the case of ring, translocation and inversions), and unbalanced would refer to missing or additional DNA material such as in the case of deletions and duplications.
There are a number of ways a karyotype abnormality can occur, such as;
An error in cell division resulting in too many or too few copies of chromosomes, or part chromosomes.
Inherited from one, or both parents.
Exposure to certain drugs (prescribed or otherwise).
So in conclusion a 15q11.2 microdeletion is a chromosome disorder describing the absence of some DNA material on the 15th chromosome of the karyotype thus causing a genetic abnormality.