Aberrations of the normal number and structure of chromosomes can cause mental, growth and developmental delay, defects of sexual development, congenital defects, abnormal facial features, deformation of extremities, defective intrauterine development, spontaneous miscarriages, and infertility. Expressive changes of the karyotype are used to be found in cancer cells. In majority of cases a lot of genes are abundant or deleted and then many organs are affected – therefore disorders are described as „syndromes“. Down and Turner syndromes are the best known syndromes caused by numerical aberrations of human chromosomes. In the diagnostics of structural aberrations, the use of methods of classical cytogenetics becomes limited. Fluorescence in situ hybridisation (FISH) is therefore used more frequently. FISH method has several advantages: rapidity, sensitivity, specificity and possibility to apply this method also in interphase nuclei. Application of FISH allows us to detect submicroscopic deletions and amplifications and to explain the aetiology of inborn developmental defects and cancer diseases (including familiar cases). The newest FISH modifications perform one step analysis of multiple chromosomal rearrangements and help us to ascertain the diagnosis and the prognosis of cancer diseases. The use of special computer software for chromosomal and FISH analysis is the most important part of the current cytogenetic diagnostics.

        Key words: chromosomes, chromosomal rearrangements, cytogenetic diagnosis, fluorescence in situ hybridisation (FISH).