Is this an exception in the use of 的?
When 的 is used for possession or 's, it is often omitted in speaking when it's the subject of a sentence.
For example:

她(的)家很漂亮/她(的)家很漂亮 = Her house/home is beautiful.
你(的)電腦很慢/你(的)电脑很慢 = Your computer is really slow.
他(的)新車很貴/他(的)新车很贵 = His new car is quite expensive.

But 的 is usually kept when it's the object of a sentence.

我喜歡他的車/我喜欢他的车 = I like his car.

But then, again as in any language, there is always exceptions :(

For example:
我去過他(的)家/我去过他(的)家 = I've been to his house.

And because of this omission, interesting sentences like this one can come up occasionally in daily conversation:

他媽的洋裝很高雅/他妈的洋装很高雅 = His mother's dress is very elegant.

If you say this out loud, most Chinese people would pick up "他媽的" (damn, sh*t, bloody, f*cking depending on context) and think you are cursing for a split second before you finish the sentence. While this is not a serious problem in casual conversation, one way to avoid this awkward confusion is to explicitly say "他媽媽的".


  Added simplified Chinese after the original examples in Traditional Chinese as requested.