At these page are answers to frequently asked questions.
Please send a mail if you miss something
FAQ's - Frequently Asked Questions
- What is the Chromosome Help-Station doing?
- What are chromosomes?
- What are chromosome disorders?
- How do you get a chromosome disorder?
- What is your future with such a disorder?
- When is something rare?
- Where can I find information on
chromosomedisorders?
- Where can I find other parents or patients?
The Chromosome Help-Station wants to promote the interest of people with rare
chromosome-disorders, see Goal. This website is a good
tool in that.
Top
- Other links:
- Introduction to chromosome abnormalities at
CDO (English) and a translation in German at Leona
- The Little
Yellow Book, published by Unique
- Pictures of chromosomes:
- Sketches
of all chromosomes, information and links at the Human Genome
Research website
- Drawing
of a chromosome, Nat. Human Genome Research Institute
- Graphic
of an unbalanced translocation 11;22
(from: An introduction
on chromosome disorders, presentation at the conference of European Chromosome 11q Network,
1998, Ede-Lunteren, Netherlands, by Dr. Conny van Ravenswaaij, Klinisch Genetisch Centrum St Radboud
ziekenhuis, Nijmegen, Netherlands.)
The human body is built out of billions of bricks: the cells, Different
types of cells do exist: blood cells, skin cells, muscle cells and so on. All these cells
have a nucleus that includes chromosomes. Normally the chromosomes in all cells are alike.
In all cells 46 chromosomes occur in 23 identical pairs. The pairs are numbered from big
to small. The pairs 1 to 22 are the same for man and woman. The 23 pair are the sex
chromosomes: a woman has two X-chromosomes a man has one X-chromosome and one
Y-chromosome.
The chromosomes show an incision and it is possible to make a
distinction between a short leg (=p) and a long leg (=q). By special staining bands on the
chromosomes can be made visible as a kind of bar-code. Internationally agreements have
been made about the numbering of the bands. This makes the medical equation of chromosome
disorders the same in whole the world.
Chromosomes contain genetic information. It has been estimated that on
the chromosomes 50,000 to 100,000 genetic recipes (genes) are found. Although chromosomes
can be studied well under a normal microscope, genes cannot be seen. A child receives one
half of its genetic information (with half of its chromosomes) from its mother and the
other half from its father. This is due to the fact that in the development of germ-cells
and spermatozoa all pairs of chromosomes split and the half from each pair is confined in
the germ-cell or spermatozoon. After impregnation of the germ-cell by the spermatozoon
there are 23+23 or 46 chromosomes in the impregnated ovum. This impregnated ovum
partitions many times and all chromosomes partition as well.
- Other links: Introduction to chromosome abnormalities
at CDO (English) and a translation in German at Leona
eV.
(from: An introduction
on chromosome disorders, presentation at the conference of European Chromosome 11q Network,
1998, Ede-Lunteren, Netherlands, by Dr. Conny van Ravenswaaij, Klinisch Genetisch Centrum St Radboud
ziekenhuis, Nijmegen, Netherlands.)
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).
In Table 1 the main chromosome disorders are summarised.
Table 1, chromosome disorders
Different number of chromosomes
-
trisomy, three chromosomes the same
-
monosomy, missing of a chromosome
Disorder in chromosome structure
-
deletion, part is missing
-
duplication, extra part
-
inversion trisomy, upside down
-
ring-chromosome, ends are fused
Translocation, exchange of parts of chromosomes
-
balanced, no problems for health
-
unbalanced, miscarriage or congenital problems
- Other links: Introduction to chromosome abnormalities at
CDO (English) and a translation in German at Leona
(from: An introduction
on chromosome disorders, presentation at the conference of European Chromosome 11q Network,
1998, Ede-Lunteren, Netherlands, by Dr. Conny van Ravenswaaij, Klinisch Genetisch Centrum St Radboud
ziekenhuis, Nijmegen, Netherlands.)
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.
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Will it go away?
No, it will not go away. Despite of the many researches on genes and
chromosomes (like the Human Genome Project)
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).
Consequences of a chromosome disorder
(from: An introduction
on chromosome disorders, presentation at the conference of European Chromosome 11q Network,
1998, Ede-Lunteren, Netherlands, by Dr. Conny van Ravenswaaij, Klinisch Genetisch Centrum St Radboud
ziekenhuis, Nijmegen, Netherlands.)
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.
(from: An introduction
on chromosome disorders, presentation at the conference of European Chromosome 11q Network,
1998, Ede-Lunteren, Netherlands, by Dr. Conny van Ravenswaaij, Klinisch Genetisch Centrum St Radboud
ziekenhuis, Nijmegen, Netherlands.)
About three percent of all children are born with a congenital
abnormality. About one per 200 new born (0.5%) has a chromosome disorder. The major part
of these children has Down's syndrome or a sex-chromosome too much or missing one. This
other 0.2% has a relative rare chromosome disorder. This is a very diverse group with much
variation in congenital problems and delay in development.
- When you meet your doctor or medical specialist and when a chromosome
disorder is identified, you usually are put in contact with a geneticist.
- In books and magazines. A good library or bookshop has information on DNA,
chromosomes and anything that has a connection with it.
- At organisations of people with chromosome
disorders or general groups.
- Within the Internet; you can browse with a search engine, but you can also
go
directly to one of these databases.
- In organisations of people with chromosome
disorders or general groups.
- Within the Internet; you can browse with a search engine, but you can also
go
directly to one of these databases.
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