Chromosomes are structures are present in all cells consist of deoxyribonucleic acid (DNA) and proteins. DNA contained in chromosomes is divided into parts called genes, acts to the maintenance and transmission of hereditary characteristics of the individual. In each cell contains two copies of each chromosome for a total of 22 pairs plus one pair of sex chromosomes (XX for females, XY for males).

Chromosomal abnormalities are alterations in the number or structure of chromosomes that occur in about 7.5% of conceptions. In most cases, though they are incompatible with life and lead to early miscarriage, it follows that only 0.6% of live births are affected by chromosomal disorders. Numerical abnormalities of chromosomes are divided in monotony, when there is only one copy of chromosome instead of two, and trisomies when three copies of the chromosome (although there are, in rare cases, women with four or five X chromosomes) . As for structural abnormalities, they may consist of more or less extensive loss of a fragment of a chromosome (deletions and microdeletions), the existence of two copies of a fragment in the same chromosome (duplication and microduplicazione), the repositioning of a fragment into place different from the original one after a rotation of 180 ° (inversion) or transfer of material between two or more different chromosomes (translocations).