Chromosome Abnormalities

Below is an explanation of different chromosome abnormalities and the nomenclature used to describe them. Any of these chromosome rearrangements can involve large or small segments of the chromosomes.  If the involved segment(s) are large enough they can be visualized by routine chromosome analysis were chromosomes are stained with a special stain, called Giemsa, which gives each chromosome a characteristic banding pattern. This type of staining is called G-banding. Small rearrangements involving the telomere region may be below the resolution of routine chromosome analysis and may only be detected with subtelomere analysis, which uses Fluorescence In Situ Hybridization, also known as FISH, which is explained in What is FISH

We have 46 chromosomes, arranged into 23 pairs. One pair of chromosomes, the sex chromosomes, are labeled XX for female and XY for male. The remainder of the chromosome pairs is numbered 1-22.  This normal set of 23 pairs of chromosomes is referred to as the diploid number of chromosomes.  An individual pair of chromosomes is referred to as a disomy. The image below shows a G-banded normal male karyotype, which is a figure where the chromosomes are paired together and lined up in order from 1-22, X and Y.
 

In the normal situation one chromosome from each pair is inherited from the mother and the other from the father.  However, abnormalities can occur in which a whole chromosome or a portion of a chromosome can be added or deleted.  This occurrence results in addition or subtraction of genes, which cause can cause significant problems with growth and development.

Several different terms are used to describe these abnormalities of genetic material.  A trisomy or duplication occurs when 3 copies of a chromosome or chromosome segment is present instead of the normal 2 copies.  A monosomy or deletion occurs when only 1 copy of a chromosome or chromosome segment is present. 

A well-known example of a chromosome trisomy is Down syndrome. As shown in the karyotype image below, Down syndrome, also called trisomy 21, occurs when a person has 3 copies of chromosome 21 instead of the usual 2 copies.

The terms monosomy and deletion or trisomy and duplication are also used to describe only a partial deletion or duplication of a chromosome.  For example, only one arm of the chromosome may be affected.  The absence of a short arm of chromosome 4 may be called monosomy 4p, or a 4p deletion.  The presence of an additional long arm of chromosome 3 may be called trisomy 3q or a 3q duplication.  Smaller regions of a chromosome, such as individual or multiple bands, may also be involved in duplications or deletions. These rearrangements are designated by the specific chromosome bands involved in the rearrangement.

A translocation involves the exchange of chromosome pieces between two chromosomes. A translocation can be balanced, in which all the genetic material is present, but in the wrong location, or a translocation can be unbalanced, which results in the loss of a chromosome segment and the gain of second chromosome segment.

(Insert figure of chromosome translocation)

Because a balanced translocation carrier has the correct amount of genetic material, the individual is usually normal and does not have any problems with growth or development. The loss and gain of genetic material in an unbalanced translocation carrier often results in these problems. A person who has a child with an unbalanced telomere rearrangement is at risk of carrying a balanced translocation, and therefore having another affected child.

As previously stated some chromosome monosomies, trisomies, or translocations can be detected via routine cytogenetic analysis, however smaller rearrangements involving the telomere region may only be detected using a higher resolution test, such as subtelomere FISH analysis.