Congenital abnormalities can be caused by an anomaly in the number or structure of the chromosomes. Disorders in the gametogenesis, that is, in the first and second meiotic divisions in the formation of oocytes and sperm cells, lead to such defects.
Chromosomal mutations can be made visible with staining methods (banding techniques). Following the staining the chromosomes are analyzed using a 1000x enlargement. With this one can recognize the chromosomes as striped cords and display them in aafter they have been ordered according to their size and the positions of the centromeres. Various groups of similar chromosomes are created in this way.
oday, small aberrations are examined using a molecular cytogenetic (FISH) approach. Among other things, the FISH (fluorescence-in-situ-hybridizing) method makes possible the targeted identification of partial aneuploidies such as deletions, duplications and unbalanced translocations (see below) that cannot be resolved with a light microscope. A comprehensive array of DNA probes and techniques are today available from which one can choose for utilization, depending on the diagnostic problem being worked on. With the FISH method chromosomal alterations can be analyzed down to a size of ca. 5 million nucleotides.
Smaller defects lie below the resolution limit of this approach - today they are analyzed using DNA analysis techniques.