Hermione's possible genotypes are MMss or MM'ss. What are possible genotypes of Hermione's parents who are muggles?

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