Chromosome analysis involves looking at cells under a microscope to see if the chromosomes are normal. Chromosomes are tiny structures inside the nucleus of every cell that contain the unique genetic information for an individual, in the form of DNA.
People who have physical features suggesting a chromosome abnormality are candidates for this test. It is also done for people who have certain types of cancer of the white blood cells. A child can be tested while still in the womb if there is a reason to believe that he or she may have a chromosome abnormality. In some cases, relatives of people with particular chromosomal syndromes are tested to see if the problem may run in the family.
Chromosome analysis is performed on cells taken from the person to be tested. The cells can be from blood or biopsy tissue. To test an unborn child, the amniotic fluid in the womb or the placenta, the organ that attaches the baby by its umbilical cord to the wall of the womb, can be used.
Cells from any of these sources are put into containers in a laboratory and allowed to grow and multiply. When there are enough cells to study, the chromosomes are removed. Special stains are used to color the chromosomes and make them visible. Then, they are studied under a microscope to see if their number and structure are normal.
Pictures of the chromosomes that have been arranged by size and shape are often called "karyotypes." If there are too many or too few chromosomes, or if one is abnormally shaped, the problem can more easily be seen on a karyotype.
No special preparation is needed if the cells will be obtained from blood or biopsy material. Cells from the amniotic fluid or the placenta are obtained by amniocentesis or chorionic villus sampling (CVS).
These procedures must be done using special methods and only at certain times during pregnancy, to avoid harming the unborn child. Cells from bone marrow are obtained with bone marrow biopsy. The preparations for each of these will be explained by the healthcare professional.
Abnormal numbers of chromosomes cause patterns of physical features, called syndromes (Down Syndrome is the most familiar example). Abnormalities of number do not usually run in families.
Abnormal size or shape of chromosomes can also cause syndromes, and may run in families. Complex rearrangements of chromosomes are often linked with certain types of cancer of the white blood cells.
Catalogue of Chromosome Aberrations in Man, 1984, Schinzel.