In the middle of The Kite Runner, protagonist Amir and his wife Soraya discover that they cannot bear a child. Because they both want a child, they consider adoption. Her father, General Taheri, says this:
"Take Amir jan, here. We all knew his father, I know who his grandfather was in Kabul and his great-grandfather before him, I could sit here and trace generations of his ancestors for you if you asked. That's why when his father -- God give him peace -- came khastegari, I didn't hesitate. And believe me, his father wouldn't have agreed to ask for your hand if he didn't know whose descendant you were. Blood is a powerful thing, bachem, and when you adopt, you don't know whose blood you’re bringing into your house."
(Hosseini, The Kite Runner, Google Books)
For the Afghan people, blood-relations are as important as morality and honor. Their understanding of relationships comes from the knowledge of a person's heritage and background. This becomes even more poignant later when Amir discovers that Hassan, who he failed to protect in his youth, was actually his half-brother; this leaves him with even more guilt and shame, because he let fear overcome his responsibility to "his own blood."