Children whose seizures have some of these features are less likely to have the syndrome of Childhood Absence Epilepsy. 1  An absence seizure with mild clonic components involves mild, often subtle twitching of the eyelids, the corners of the mouth, and sometimes the arms.
1 Trisomy 21 â More than 90% of Down syndrome cases are caused by trisomy 21. 2  An extra chromosome (chromosome 21) originates in the development of either the sperm or the egg. 3  When the egg and the sperm unite to form the fertilized egg, three (rather than two) chromosomes 21 are present.
Down syndrome - trisomy 21 - occurs when there are 3 rather than 2 number 21 chromosomes (hence, trisomy 21.) The most common type of Down syndrome - complete trisomy 21 - occurs as a result of a process called nondisjunction. When a cell divides, one of each pair of chromosomes goes to each daughter cell.