English speakers preferentially use the possessive apostrophe when the possessor is a living entity.
When the owner is a living entity, the prepositional phrasing is technically correct, but the possessive apostrophe is highly preferred. Cerberus's answer was well-researched, and illustrated to me that most of the rules/exceptions for inanimate objects are idiomatic and may be specific to a time period. However, I wanted to describe why the possessive apostrophe is useful. It allows the speaker to:
- Specify the owner without a prepositional phrase.
- Represent ownership in a natural order.
- Minimize number of words between the subject and the verb.
Consider the following sentence:
The cat of my sister ate the gerbil of my brother.
It has eleven words, two prepositional phrases, and three words separate the subject and the verb. This is technically correct, but very awkward. Consider this re-write:
My sister's cat ate my brother's gerbil.
The re-write has only seven words, no prepositional phrases, and the verb comes just after the subject. Additionally, the possessive apostrophe allows ownership to be listed from least specific to most specific (i.e. of the many things my sister owns, I'm talking about her cat). That's not a major problem in the context above, where there are two independent statements of ownership, but becomes really important in cases of nested ownership.
Consider the following sentence:
The toy of the cat of my sister is red.
This ten-word sentence is technically correct, but awkward. It has two prepositional phrases, six words between the subject and the verb, and nested ownership (i.e. my sister owns a cat, and that cat 'owns' a toy). Nested ownership isn't bad per-se, but the prepositional phrasing again forces an unnatural order - ownership is listed from most specific (this one toy) to least specific (things my sister owns).
The same idea expressed with two possessive apostrophes:
My sister's cat's toy is red.
This is a lot more concise, in that there are now only six words, no prepositional phrases, and the verb comes just after the subject, so the audience doesn't have to comprehend six qualifying words while remembering that the subject is "toy". It's also more clear in that the nested ownership is listed from the least specific (things my sister owns) to the most specific (this one toy), as you would expect from an outline:
cast( )
, wresting the word into nominal. The genitive clitic would be like a backwards looking parse.