FAQ's - Frequently Asked Questions

What are Chromosone Disorders?

Disorders in the number of chromosomes

If a pair of chromosomes consists of three in stead of two, it is named trisomy. The most general is trisomy 21 (Down syndrome) or a trisomy of the sex chromosomes (XXX, XXY or XYY). Also a trisomy of chromosome 13 or 18 can occur, but these are much more rare. A trisomy of one of the other chromosomes is rarely viable and causes spontaneous miscarriages.

If one of the chromosomes is only present once it is called monosomy. A monosomy in all body cells is mostly not viable except for monosomy of the X-chromosome (Turner syndrome). If only a par of the cells shows a disorder in chromosome number it is named a mosaic pattern. Examples are mosaic pattern trisomy 8 or a mosaic monosomy 21.

Disorders in chromosome structure

If a part of a chromosome is missing we call this a deletion. In fact a part of the genetic information is than only available once and we call this a partial monosomy. A deletion can be described rather precise by indicating the bands of a chromosome that are missing. Deletions are mostly found on the terminal ends of chromosomes, but also can occur on other sites in the middle of the long or short leg.

In case of a duplication a part of the chromosome is available triple. Because of the presence of genetic information in triple this is called partial trisomy. In trisomy a part of the chromosomal material is found triple. This can be on the same chromosome or on another chromosome (translocation, this will be discussed later). Sometimes doubling occurs in the same chromosome by doubling a piece upside down. This is called inverse duplication.

Sometimes a small extra chromosome is found next to the 46 normal chromosomes. This is called a marker. Also in the case of a marker too much genetic information is present (partial trisomy).

A ring-chromosome can be develop when from both ends of a chromosome parts break off and then melt together. So to say the repair has been done wrong. In a ring-chromosome a part of the genetic information is lacking (deletion). Moreover such a ring chromosome can cause problems in cell-division. This can cause cells that lack the ring chromosome (mosaic monosomy).

How do you get a chromosome disorder?

Repetition risk

The risk that the disorder occurs once again depends from the moment on which the chromosome disorder evolved. In many mosaic patterns cells with chromosome disorders will only develop after impregnation. There is no increased risk that a child will have the same disorder in a next pregnancy.

A complete trisomy (for instance trisomy 18 in all cells) is always caused by an disorder in the germ-cell or spermatozoon. In a germ-cell or spermatozoon accidentally two chromosomes of one pair ended in one cell when they split. After impregnation there are three chromosomes the same: trisomy. The risk that this repeats, or the chance that a next child will also have trisomy is small, 1:100.

In case of a deletion or duplication it is always important to carry out an investigation of the chromosomes of the parents, because it is possible that the chromosome disorder is caused by a "translocation". In case of a translocation parts of chromosomes have been exchanged. If in a situation of translocation no parts of chromosomes are found extra or missing, this is called a "balanced translocation". There is only a different ordering of the chromosomes. In the development of germ-cells or spermatozoon problems can occur. The translocation can be passed on unbalanced, that means that the translocated part of the chromosome is moved to the same germ-cell or spermatozoon as is the complete chromosome where it originates from. Then cells develop with too much (duplication or partial trisomy) and too few genetic information (deletion or partial monosomy). This can cause a spontaneous miscarriage or to the birth of a child with congenital problems. The probability of the chance of an unbalanced translocation is different for each translocation.

New research techniques

Since a few years it is possible to map changes in chromosome orders much more precisely. It can be useful to repeat chromosome investigations that have been carried out before 1980 and that had unclear results.

It is remarkable to realize that we discovered only forty years ago that man has 46 chromosomes. Only since thirty years staining techniques are good enough to let us recognize all 24 pairs of chromosomes (1-22, X and Y). Through new specific staining techniques we are now able to trace very small deletions, that could not have been made visible with standard microscope techniques. The expectation is that also the number of children will increase of which we can detect chromosome disorders.

What is your future with such a disorder?

Will it go away?

No, it will not go away. Despite of the many researches on genes and chromosomes it is obvious that chromosome-disorders cannot be repaired. People who seem to have or will have benefit from gene therapy for their (rare) disease, often have a gene defect. At chromosomes are tens of thousands of different genes (see: what are chromosomes). Learn more about medical research.

Consequences of a chromosome disorder

The impact of a chromosome disorder can be rather variable. The retardation in development can vary from mild learning problems to not being able to talk and/or walk. A chromosome disorder can also cause congenital heart kidneys and brain problems. Moreover external characteristics can be found such as in the Down syndrome a round face, broad nose and slanting eyes. The character and the seriousness of problems due to a chromosome disorder depend among others from the following:

• Is there too few or too much genetic material? Too few genetic material (deletion) often causes more serious problems than too much (duplication).
• How much genetic material is too much or to few available?
• Which part of a chromosome is too much or too few present?
• In a mosaic situation: in how many cells and in which cells does the disorder occur?
• Individual differences, for instance children with Down syndrome have all the same chromosome disorder. However, only a part has a congenital heart disease and the intellectual development differs from child to child.

Down syndrome occurs rather often. That makes it more or less possible what can be expected from a child with trisomy 21. For most other rare chromosome disorders this is much more difficult. The physician will make a careful prediction on the basis of data from international medical literature. Unfortunately there is insufficient follow-up and documentation of children with rare chromosome disorders. This means that available data are restricted. This was one of the reasons why the clinical genetic centre of the University hospital St Radboud started a so-called chromosome clinic in 1994. In this polyclinic facility children with chromosome disorders are followed every 2 or 3 year.

Where can I find medical information?

1. When you meet your doctor or medical specialist and when a chromosome disorder is identified, you usually are put in contact with a geneticist. 
2. In books and magazines. A good library or bookshop has information on DNA, chromosomes and anything that has a connection with it.  
3. At organisations of people with chromosome disorders or general groups such as www.chromodisorder.org
4. Within the Internet; you can browse with a search engine.