Analysis of information sources in references of the Wikipedia article "English possessive" in English language version.
In speech the genitive is signalled in singular nouns by an inflection that has the same pronunciation variants as for plural nouns in the common case
In writing, the inflection of regular nouns is realized in the singular by apostrophe + s (boy's), and in the regular plural by the apostrophe following the plural s (boys')
Also see entry of.3 page 680.