In this method, one cell becomes two cells, which have the exact same number and type of chromosomes as the parent cell. The second method of cell division occurs in the ovaries and testicles (meiosis) and consists of one cell splitting into two, with the resulting cells having half the number of chromosomes of the parent cell (haploid). So, normal eggs and sperm cells only have 23 chromosomes instead of 46. During cell division, many errors can occur. In meiosis, an event called "disjunction" may occur when the pairs of chromosomes are supposed to split up and go to different spots in the dividing cell. "However, occasionally one pair doesn't divide, and the whole pair goes to one spot. This means that in the resulting cells, one will have 24 chromosomes and the other will have 22 chromosomes (Cicchetti 33)." This misfortune is called "nondisjunction." If a sperm or egg with an atypical number of chromosomes merges with a normal mate, the ensuing fertilized egg will have an irregular number of chromosomes. In DS, 95% of all cases are caused by one cell having two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. For this reason, the scientific name is trisomy 21 (Cicchetti 121). .
Approximately 90% of the abnormal cells are the eggs as shown in recent research in these cases. The cause of the nondisjunction error is not known, but there is without doubt a connection with maternal age. Research is aimed at determining the cause and timing of the nondisjunction occurrence. Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation. "In this case, two breaks occur in separate chromosomes, usually the 14th and 21st chromosomes. There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosome (Leshin, www.ds-health.com, 2000)." While the number of chromosomes remains normal, there is a triplication of the 21st chromosome.