From a dictionary and a usage guide, I learnt that these phrases are OK:
the home of a schoolteacher
the sister of the Duke of Urbino
a friend of my mother's
a cousin of Lorna Cook's
a close friend of the President's
The usage guide says that sometimes the 's can be omitted, especially in AmE. So my first question is
1. Are the first two phrases from their 's versions, i.e.
the home of a schoolteacher's
the sister of the Duke of Urbino's
2. If so, then why do
a cousin of Lorna Cook
a friend of my mother
whose 's are omitted seem to be uncommon ? Or they aren't? Is there any rule, pattern or tendency to which I could stick regarding this issue?