C

An affected parent (aa) must pass a recessive allele to every child.  When the other parent is a carrier (Aa), each child has a 50 % chance of inheriting the second recessive allele (aa → affected) and a 50 % chance of inheriting the dominant allele (Aa → unaffected carrier).  Thus 50 % of offspring are affected and 50 % are carriers.  [Gelatt & Gum, 1981, PMID 7325430]