How does Amir's infertility relate to the theme of fatherhood in The Kite Runner?

Through-out the book the relationship between Amir and his
father has been a significant theme.  Amir loves his father but can find no approval
from him until he is an adult and in college. 

Amir had
spent his whole life trying to be like his father.  In a way his inability to have a
conceive a child reflects as another way that Amir is not able to be the man or son his
father wants him to be.  His father often looked at Amir as being different from him ad
not living up to his expectations of him.

Baba was prolific
and had been able to have two sons, but Amir can not produce one.  It is only through
taking on the child of his half brother, and his father's sin, that he is able to become
a parent and also redeem his own soul.