Singular possessives of nouns always use the apostrophe S regardless of their spelling.
The possessive would be genius's (e.g. a genius's IQ is very high).
The plural possessive form is colonists'.
The possessive form for the noun colonist is colonist's.Example: A colonist's day was filled with hard work.
The possessive form of the noun historian is historian's.Example: The historian's presentation was very interesting.
spies
The possessive form of the singular noun yesterday is yesterday's.Example: There was an article about that in yesterday's newspaper.
The plural possessive of "library" is libraries'.
The possessive form of the singular noun toy is toy's.example: The toy's inventor was a genius.
The possessive form of the plural noun dependents is dependents'.
The possessive form of else is else's, as in someone else's possessions.
Hayden's
The possessive form for the noun man is man's.example: It is a man's right to be happy.
The possessive of the singular noun alto is alto's.
The possessive form of "Zeus" is spelled as "Zeus's" (pronounced "Zoo-siz").
The possessive for a single country is country's (e.g. a country's borders).
Cowhand's
The possessive form for the noun leaf is leaf's.Example: You can usually identify a tree by the leaf's shape.
The plural is composers and the plural possessive form is composers' (just an apostrophe added).