Why are actors expected to "Act" like their parents?

Why are actors who in the past or presently have parents in the acting career expected to act or look like their parents?  On the other hand, is it fair to judge an actors son or daughter in this way specially if they began their career with their own abilities, knowledge and qualifications?

It’s natural as a fan or for majority of the hindi films watching public to expect an actor like Abhishek Bachchan to act or look like Amitabh Bachchan.  Growing up those fans have watched and admired the acting of Amitabh and became so interwined and involved with the actor and his acting that public seize to see more of it.  So in this case are we really a fan of just a parent and being ignorant and unfair to young and upcoming actors?  

Recently Shruti Hassan, daughter of legendary actors Kamal Hassan and Sarika was fed up of comparisons with her parents and said ‘it’s impossible for her to perform like my parents.  You can’t compare…be it a father and daughter or anyone else.  Everyone has their own quality, she added’.

Although it’s comes out as a natural reaction to fans that it’s okay to compare an actor to their parents, it’s also good to think out a box and adapt to seek out unique talents in the children’s of all veteran actors and actresses.