Medical Genetics In Down syndrome, 95% of al cases are caused by this event: one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes, hence the scientific name, trisomy 21. Recent research has shown that in these cases approximately 90% of the abnormal cells are the eggsMedical Genetics In Down syndrome, 95% of all cases are caused by this event: one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. Hence the scientific name, trisomy 21. Recent research has shown that in these cases, approximately 90% of the abnormal cells are the eggs