When is "'s" needed after a possessive "of"?

Question

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?

Answer 1

Consider the following sentences -

1. A friend of Tom
2. A friend of Tom's
3. Tom's friend

The above three sentences are equivalent. But in some cases they mean differently. Consider the next example sentences carefully -

1. A picture of Bob
2. A picture of Bob's
3. Bob's picture

While the example sentences with "friends" are equivalent, the example sentences with "picture" is not. The first one mean The picture depicts Bob. The second sentence says the picture belongs to Bob, while the last one can mean both, depending on the context.

Is your doubt clear? Now I hope you got the answer of your question.