The following information may help to address your question:
Down syndrome is usually not inherited, but occurs due to a random event during the formation of reproductive cells (egg or sperm) when an extra copy of chromosome 21 is passed on through the egg or sperm. You may be interested in learning more about general risk factors for this type of Down syndrome at the MayoClinic Web site. To visit the page click here.
Less commonly, a parent may carry a rearrangement of their chromosomes that increases their chance of having a child with Down syndrome as well as fertility problems and miscarriages. Click here to read more about this type of Down syndrome, called translocation Down syndrome.
A genetic professional can review your son and daughter-in-law's family history to help assess their specific chances of having a child with Down syndrome.
Once again a genetic professional can help assess your children's, and their future offspring’s, risk for HSCR. Family history information (and where possible medical records) can be very important in assessing this risk.
These syndromes have many other signs and symptoms. When HSCR occurs as part of a syndrome, inheritance risk will depend on the parent syndrome's mode of inheritance.