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.
Who Discovered Down Syndrome. Down syndrome is a chromosomal disorder that produces a set of mental and physical symptoms. It is caused by an extra copy of chromosome 21. In a person with Down syndrome there are three copies of the chromosome 21, one from the father, one from the mother and a third copy.The extra copy of this chromosome changes the development of the brain and body.Down syndrome affects about 1 in every 800 babies.r Down could not identify what caused the condition and it was not until 1959 that a Professor named Jerome JÃ©rome lejeune discovered That down syndrome was caused by a chromosomal. Irregularity he found that in people With down syndrome an extra chromosome was. present