The biblical story of Cain and Abel plays a central role in the novel East of Eden. In "Genesis", Adam and Eve's sons, Cain and Abel, offer sacrifices to God. The shepherd Abel sacrifices his best lamb, while the farmer Cain offers grain. Because God prefers Abel's gift over Cain's, Cain becomes infuriated and kills his brother in a jealous rage. When God inquires into Abel's whereabouts, Cain retorts, "Am I my brother's keeper?" God tells him that his brother's blood cries out to him from the ground, and that he is cursed. God marks Cain, and banishes him to wander the earth.
The characters of Charles and Adam Trask (who share the initials C&A with their biblical forebears) closely follow the Cain and Abel paradigm. Cyrus Trask favors Adam's birthday gift of a puppy over Charles' gift of an expensive knife, and Charles almost beats his brother Adam to death in a jealous rage. However, unlike Charles (Cain), Adam (Abel) becomes the wanderer, first in the army, then as a vagabond, and then in South America. Finally, Adam moves to California, where he settles and raises his twin sons. His bitter brother remains on their Connecticut farm. One day he has a farm accident which results in a large facial scar, a mark akin to Cain's. When Charles dies, he leaves his fortune to his brother: clearly, although jealousy drove them apart, the brotherly bond is still there.
Years later, Adam's wife Cathy ("Kate") Trask gives birth to the next generation of Trask brothers, the dark-haired Caleb Trask (Cal) and the fair-haired Aron Trask - another set of brothers who share the initials C&A. Unsurprisingly, these brothers perpetuate the Cain and Abel motif. After the birth of their sons, Cathy abandons Adam, and he sinks into such a severe depression that he neglects to even name his children. When Samuel Hamilton hears of this, he rushes to the Trask ranch and finds the children asleep on the warm ground (recalling God's declaration that Abel's blood cried out from the ground). Samuel suggests to Adam that he name the children Cain and Abel, but Adam won't hear of it.
Adam favors Aron over Cal, much as Cyrus preferred Adam over Charles and God preferred Abel over Cain. When Cal offers Adam a birthday present of $15,000, Adam berates him for having taken advantage of farmers during war-time. Unable to contain his jealousy, Cal takes Aron to their mother's brothel, destroying Aron's belief that Cathy is dead. Aron is completely devastated and joins the army, resulting in his death in France. The "Genesis" tale thus plays out in the Trask family, with Cal, albeit inadvertently, killing his brother.