Holoprosencephaly is a highly variable condition in mammals, including humans. Its effects range ...

Holoprosencephaly is a highly variable condition in mammals, including humans. Its effects range from an absence of brain areas, mid-face disorders, and cyclopia to slight dental abnormalities. One form, HPE3, is caused by Shh mutations.
 
  Familial cases are inherited as dominants. Which of the following do you expect? A) HPE3 is actually a multigene complex.
  B) Each type of malformation is due to a different HPE3 allele.
  C) HPE3 expression is modified by genetic background.
  D) HPE3 individuals also share a common environmental exposure.
  E) All mutations in Shh that result in HPE3 are due to frameshifts.