By genetic syndrome we mean a disease due to a permanent alteration of the DNA, called a mutation. Depending on the type of mutation that occurs in the DNA, genetic diseases are divided into three broad categories. The first includes the so-called monogenic or Mendelian diseases, caused by mutations that occur in single genes. These diseases can in turn be divided into autosomal dominant (such as Huntington’s chorea), autosomal recessive (cystic fibrosis, sickle cell anemia, thalassemia) and X-linked diseases (haemophilia). The second category is that of chromosomal diseases. These are due to a mutation that occurs in the structure of chromosomes (chronic myeloid leukemia) or to an alteration in the number of chromosomes, both sexual (Turner syndrome, due to the presence of only one X chromosome instead of two) and autosomes (syndrome of Down, characterized by the presence of three chromosomes 21 instead of two).