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 is an example of a condition that occurs due to nondisjunction in autosomal (non-sex) cells. Individuals with Down syndrome have an extra chromosome on autosomal chromosome 21. A chromosome mutation that results in individuals with more than one haploid set of chromosomes in a cell is termed polyploidy.