Genetic diseases classify into chromosomal abnormalities and genetic mutations. Among chromosomal abnormalities there are the aneuploidies, diseases caused by alterations of the normal (euploidy) number of chromosomes of the species. An example of aneuploidy is Down syndrome, or trisomy 21, in which there are three chromosomes 21 instead of the normal pair. In the group of chromosomal abnormalities there are also the deletions (absence of part of a chromosome), the inversions (in which a chromosome breaks and its pieces reconnect in inverse manner) and the translocations (pieces of a chromosome that exchange positions). In the genetic mutation group there are the deletions (one or more DNA nucleotide absent), the substitutions and the insertions.