Unlike most conditions or diseases that are defined by a symptom or a group of medical issues, the conditions of chromosome 18 are defined by the underlying cause of a person's medical and developmental issues: a missing or extra piece of chromosome 18.
 
Each of the conditions of chromosome 18 has a wide variety of characteristics and severity.  The medical or developmental issues an individual has as a result of a chromosome 18 change are entirely dependent on the precise location of the breakpoint and the genes involved in the chromosome change. However, in general, these genetic changes fall into one of five major conditions which are described below.
In this section, we provide information about each of the conditions.  We discuss the genetic basis as well as the features of each of these conditions.  Although every child with a chromosome change is different, these pages provide a general idea of the medical and developmental concerns that are associated with the conditions.
 
Some individuals have chromosome changes involving chromosome 18 that do not fit neatly into one of these five conditions.  For example, individuals with an unbalanced translocation may have a change in another chromosome in addition to the change on chromosome 18.  Therefore, these pages may not be completely specific to all chromosome changes involving chromosome 18.  However, the information provided in this section can act as a starting point for information about the effects of the chromosome change.
 
If you are unfamiliar with genetic concepts, we encourage you to watch a series of podcasts written and narrated by the Registry's president and the director of the Chromosome 18 Clinical Research Center.  Understanding basic genetics will help give the information provided here more meaning.

The diagram below illustrates a pair of normal chromosome 18's.  The conditions of chromosome 18 occur when there are changes in one of these two chromosomes.

Normal Chromosome 18

The most frequent abnormalities of chromosome 18 are:

18 p-
There is a missing piece from the short arm of chromosome 18.
 

18q-
There is a missing piece from the long arm of chromosome 18.  If the missing piece is close to the centromere, it is called proximal 18q-.  If the missing piece is close to the end of the chromosome, it is called distal 18q-.
 

Ring 18
One of the copies of chromosome 18 forms a ring, and material is lost from both the long and short arm.
 

Tetrasomy 18p
An extra chromosome is present. This chromosome is made up of two copies of the short arm of chromosome 18.
 

Trisomy 18
There are three copies instead of the usual two.