In As I Lay Dying, Jewel’s biological father is Reverend Whitfield, the man who had an affair with Addie before the story begins. Jewel’s father is not Anse Bundren, the biological father of all the other siblings (Cash, Darl, Vardaman, and Dewey Dell).

The fact that Jewel has a different father than all the other siblings influences the story primarily by creating tension and conflict, throwing into constant uncertainty the role that Jewel plays in the family. The family certainly doesn’t talk openly or clearly about Jewel having a different father, not even when Darl asks about it explicitly (section 49), so the tension remains roiling under the surface of the plot.

How does this happen, specifically?

First, notice how Addie gives Jewel preferential treatment, but treats the other family members with resentment, especially her husband Anse: “I told Addie it want any luck living on a road when it come by here, and she said, for the world like a woman, ‘Get up and move, then’” (section 9). Addie treasures Jewel above all the other siblings, which adds more tension to the family dynamics. Given that Jewel is the most precious and valuable of the children to his mother Addie, you can see why his name is Jewel, and why the other siblings have plenty of reason to resent him and treat him like an outsider.

Second, notice how Jewel acts as though he is not really part of the Bundren family—but only sometimes. He adores his horse, but not his siblings. He acts devoted to his mother Addie, but not to the others; he wishes that as she dies, he alone could stand by her: “It would just be me and her on a high hill” (section 4). He scorns Cash’s work on the coffin and vaguely wishes that Cash, and the others, would just disappear, as further evidenced here: “If it had just been me when Cash fell off of that church and if it had just been me when pa laid sick with that load of wood fell on him." Jewel doesn’t even get to narrate nearly as many chapters of the story as his siblings do. He’s an outsider in his own family.

As such, the rest of the Bundren family can’t depend on Jewel to stick with them throughout their difficult journey. Will Jewel’s love for Addie, even after her death, inspire him to feel devoted to the Bundren family and make sacrifices for them? As readers, we won’t know until we finish the story—this tension, this question of how Jewel will behave keeps us hooked on the story, eager to see what happens.

To sum that up, Jewel’s status as a true member of the family is up in the air, which creates conflict between him and the other Bundrens. This conflict helps propel the story.

Let's go a bit further, and consider why all this conflict and tension really matters. Because the lack of clarity surrounding Jewel’s status creates chaos in the story, Faulkner achieves his goal of creating a modernist novel. For a bit of historical and literary context, Faulkner (1897-1962) and writers like him grew up witnessing the horrors of the first World War, and they poured their disillusionment into stories, such as As I Lay Dying, that express their despair and their sense of the world falling apart and of human lives meaning nothing. (This is a simplified idea of literary modernism, and you can read more about it here: https://www.enotes.com/topics/modernism/in-depth.)

If we’re reading this story, trying to figure out why it matters that Jewel has a different dad, then we’re trying our best to wrestle with the plot and get it to make sense. None of the narrators actually comes out and gives it to you straight, telling you that Jewel has a different dad. Darl comes close when he asks the question in section 49, but instead of answering honestly, Jewel insults Darl angrily. Readers have to figure out Jewel’s parentage from the clues. We have to ask ourselves: what other important facts or dark secrets are the narrators hiding from us? We can’t trust them, and unreliable narrators make the story highly complex. Reading becomes hard work. Confusion, haziness, fragmentation, the numerous narrators, the lack of numbers that would normally label the chapters, the stream of consciousness passages, the question about whether Jewel really belongs—all of this uncertainty forces us to really think to understand the book. That’s what modernist literature is all about.