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.
Chromosome 21 is one of the 23 pairs of chromosomes in humans. People without Down syndrome have two copies of this chromosome. The trisomy of the 21st chromosome causes Down syndrome. Chromosome 21 is the smallest human chromosome, with 48 million nucleotides (the building material of DNA) representing about 1.5 percent of the total DNA in cells. In 2000, researchers working on the Human Genome Project announced that they had determined the sequence of base pairs that make up this chromosome.