In The Kite Runner, Hassan is believed to be Ali's son. One of the major similarities between the two -- although it comes out later that Hassan is actually Baba's son -- is their physical deformities. Ali had polio as a child, and grew up with twisted bones and constant pain. Hassan, for his part, was born with a harelip (cleft lip or palate), a very common occurrence in which the lip and/or palate is separated. Today, it is easily corrected in infancy, but especially in third-world countries it can be a significant problem. Since Hassan is believed to be the son of a menial servant, other boys take advantage of his friendly nature and disfigurement; in a similar way, the boys and some adults mock and tease Ali for his limp.
In their later lives, both Ali and Hassan are treated to severe hardships; Hassan is raped by a bully and loses his only friend, Amir, who betrays him and accuses Hassan of theft. At that point, knowing that he will have the added stigma of theft, Ali accepts the shame of accusation and leaves with Hassan rather than stay.
"Life here is impossible for us now, Agha sahib. We're leaving." Ali drew Hassan to him, curled his arm around his son’s shoulder. It was a protective gesture and I knew whom Ali was protecting him from. Ali glanced my way and in his cold, unforgiving look, I saw that Hassan had told him. He had told him everything, about what Assef and his friends had done to him, about the kite, about me. Strangely, I was glad that someone knew me for who I really was; I was tired of pretending.
(Hosseini, The Kite Runner, Google Books)