Down syndrome is a genetic disorder, occurring when an individual has all or 
part of an extra copy of chromosome 21. Parents of children with Down syndrome 
are often confused by the term genetic disorder because they associate the term 
with inheritance but have also learned that Down syndrome is not typically 
inherited. These parents may have questions about the nature of chromosomes, how 
Down syndrome occurs, recurrence risk and more. This article attempts to address 
many of the common questions parents of children with Down syndrome express 
regarding the genetics of the disorder including the mechanisms by which Down 
syndrome occurs: nondisjunction, translocation and mosaicism, as well as 
providing information about prenatal testing options, how the diagnosis is made 
and where parents may go for further information.