Parental origin of the extra chromosome in Down's Syndrome

In 11 studies of patients with Down's syndrome and their parents non-disjunction was maternal in 306 cases and paternal in 77 cases. In 258 families, parental origin could not be conclusively determined. The percentage of paternal failures varied between 4.3 and 35.3% in different studies. When the type of non-disjunction was compared to the type of meiotic error, more males were born when the error was maternal (sex ratio 0.55) and more females when the error was paternal (sex ratio 0.43). An overall sex ratio of 0.52 was found. De novo translocation trisomy of type (21;21) originated paternally in one third of the cases. In (14;21) de novo translocation trisomy only one of eight cases was of paternal origin.