The possessive singular of all English nouns is formed by adding -'s: Alexis's. If Alexis's seems awkward to you, you may use the Frenchified possessive "of Alexis," but never use the plural possessive form, which is to add an apostrophe after the final -s of the plural, for a singular noun.
Use whichever form you use to say it Alexis' or Alexis's. Use the form the way you pronounce it.
The singular possessive form is week's.
The singular possessive form of "classmate" is "classmate's."
The possessive form is Amos's.
The singular possessive form of "gypsy" is "gypsy's."
The singular possessive form of "countries" is "country's".
The singular form is goose; the singular possessive form is goose's.The plural form is geese; the plural possessive form is geese's.
The singular possessive form of county is county's.
Quantum is singular, not singular possessive. The singular possessive form is quantum's.
singular possessive: boy'splural possessive: boys'
The singular possessive form is speculum's.
The singular possessive form is frontiersman's.