MM'Ss or MMSs are the possible genotype of Hermione's parents.
Explanation:
Muggles have a gene for magic but not expressed in them, S represents silencing, s is no silencing).
If Hermione's genotype is MMss or MM'ss
then Hermione's parents would have Ss (but did not express S in Hermione and parents were heterozygous)
Hermione's parents could have M' M alleles but M' could or could not have appear in this case
so parents genotype are
MM'Ss or MMSs
It can be checked by crossing the possible genotype of parents
M M' s S
M MM MM' s ss Ss thus Hermione has this possible combination
M MM MM' S Ss SS as MMss , MM'ss
