Down Syndrome (DS) is the most common disease caused by failure of chromosomes. It occurs in 1 in 650 births. In people with DS on 21 chromozom or part of it occurs in every cell instead of the usual triple-double - ie. trisomy 21st This means that in each cell are 47 chromosomes instead of the 46th Usually there is a tripling of the whole chromosome (free trisomy). Rare is translocations trisomy, in which the supernumerary chromosome 21 segment Bar attaches to another chromosome. Only when translocations trizomy 21 may be a transmitter of the parents as a result, it can be in one family, more children born with DS. Free trisomy but occurs randomly and the probability of the child's illness increases with age mothers. The cause is also considered exposure to exogenous factors (ionizing radiation, viruses, chemicals).