• Chromosomal deletions, sometimes known as partial monosomies, occur when a piece or section of chromosomal material is missing. Deletions can occur in any part of any chromosome. When there is just one break in the chromosome, the deletion is called a terminal deletion because the end (or terminus) of the chromosome is missing. When there are two breaks in the chromosome, the deletion is called an interstitial deletion because a piece of chromosome material is lost from within the chromosome. Deletions that are too small to be detected under a microscope are called microdeletions.[1][2][5] A person with a deletion has only one copy of a particular chromosome segment instead of the usual two copies. Some examples of more common chromosome deletion syndromes include cri-du-chat syndrome and 22q11.2 deletion syndrome. 


• Chromosomal duplications, sometimes known as partial trisomies, occur when there is an extra copy of a segment of a chromosome. A person with a duplication has three copies of a particular chromosome segment instead of the usual two copies. Like deletions, duplications can happen anywhere along the chromosome.[1][2][5] Some examples of duplication syndromes include 22q11.2 duplication syndrome and MECP2 duplication syndrome. 


• Balanced translocations occur when a chromosome segment is moved from one chromosome to another. In balanced translocations, there is no detectable net gain or loss of DNA.[1][2][5]


• Unbalanced translocations occur when a chromosome segment is moved from one chromosome another. In unbalanced translocations, the overall amount of DNA has been altered (some genetic material has been gained or lost).[1][2][5]


• Inversions occur when a chromosome breaks in two places and the resulting piece of DNA is reversed and re-inserted into the chromosome. Inversions that involve the centromere are called pericentric inversions; inversions that do not involve the centromere are called paracentric inversions.[1][2][5]


• Isochromosomes are abnormal chromosomes with identical arms - either two short (p) arms or two long (q) arms. Both arms are from the same side of the centromere, are of equal length, and possess identical genes. Pallister-Killian syndrome is an example of a condition resulting from the presence of an isochromosome.[2][5] 


• Dicentric chromosomes result from the abnormal fusion of twp chromosome pieces, each of which includes a centromere.[5] 


• Ring chromosomes form when the ends of both arms of the same chromosome are deleted, which causes the remaining broken ends of the chromosome to be "sticky". These sticky ends then join together to make a ring shape. The deletion at the end of both arms of the chromosome results in missing DNA, which may cause a chromosome disorder. Genetics Home Reference (GHR) provides a diagram of the steps involved in the formation of a ring chromosome.[1][2][5] An example of a ring condition is ring chromosome 14 syndrome.  

What causes chromosome disorders?
The exact cause is unknown, but we know that chromosome abnormalities usually occur when a cell divides in two (a normal process that a cell goes through). Sometimes chromosome abnormalities happen during the development of an egg or sperm cell (called germline), and other times they happen after conception (called somatic). In the process of cell division, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division, called nondisjunction, can result in cells with too few or too many copies of a whole chromosome or a piece of a chromosome,[1][6] Some factors, such as when a mother is of advanced maternal age (older then 35 years), can increase the risk for chromosome abnormalities in a pregnancy.[1]


What is mosaicism?
Mosaicism is when a person has a chromosome abnormality in some, but not all, cells. It is often difficult to predict the effects of mosaicism because the signs and symptoms depend on which cells of the body have the chromosome abnormality.[2][7] Genetics Home Reference (GHR) provides a diagram of mosaicism.


How are chromosome disorders diagnosed?
Chromosome disorders may be suspected in people who have developmental delays, intellectual disabilities and/or physical abnormalities. Several types of genetic tests can identify chromosome disorders: 

• Karyotyping
• Microarray (also called array CGH)
• Fluorescence in situ hybridization (FISH)

What signs and symptoms are associated with rare chromosome disorders?
In general, the effects of rare chromosome disorders vary. With a loss or gain of chromosomal material, symptoms might include a combination of physical problems, health problems, learning difficulties and challenging behavior. The symptoms depend on which parts of which chromosomes are involved. The loss of a segment of a chromosome is usually more serious than having an extra copy of the same segment. This is because when you lose a segment of a chromosome, you may be losing one copy of an important gene that your body needs to function.[2]

There are general characteristics of rare chromosomal disorders that occur to varying degrees in most affected people. For instance, some degree of learning disability and/or developmental delay will occur in most people with any loss or gain of material from chromosomes 1 through 22. This is because there are many genes located across all of these chromosomes that provide instructions for normal development and function of the brain.[2] Health providers can examine the chromosome to see where there is a break (a breakpoint). Then they can look at what genes may be involved at the site of the break. Knowing the gene(s) involved can sometimes, but not always, help to predict signs and symptoms. 


Can chromosome disorders be inherited? 
Although it is possible to inherit some types of chromosomal disorders, many chromosomal disorders are not passed from one generation to the next. Chromosome disorders that are not inherited are called de novo, which means "new".[6] You will need to speak with a genetics professional about how (and if) a specific chromosome disorder might be inherited in your family. 

• The National Society of Genetic Counselors provides a searchable directory of US and international genetic counseling services.
• The American College of Medical Genetics has a searchable database of US genetics clinics
• The University of Kansas Medical Center provides a list of US and international genetic centers, clinics, and departments.
• The American Society of Human Genetics maintains a database of its members, which includes individuals who live outside of the United States. Visit the link to obtain a list of the geneticists in your country, some of whom may be researchers that do not provide medical care.

• National Human Genome Research Institute (NHGRI)
• Genetics Home Reference (GHR)
• MedlinePlus