Maybe none. If I am understanding the question correctly, you're asking if someone who has a child with holoporsencephaly has genetic problems themselves. As dr ferfuson said, this problem is sometimes associated with genetic problems *in the kid.* children with these types of problems can be born from perfectly normal parents. It just happens. To be sure, parents can get tested if they want.
Multiple. Holoprosencephaly is seen frequently with trisomy 13. It is also found in partial chromasome deletion syndromes for 11, 13 &18. One of my few cases was an affected fraternal twin that also had 21 trisomy down syndrome (the other twin was normal).