Mana is the substance that makes a human capable of performing spells. In the magitocracy, mages rule the kingdom and lord their power over filthy mudbloods. However, there are areas of the world in which Mana roams free and unbound by humans. This wild magic is unrestrained and leads to these areas being high in radiation.
Pregnant mothers are advised to stay away from these areas, as the radiation can have various dehabilitating effects on both parent and child, often leading to death of one or both. One mudblood ignored this advice. Among the twins she was carrying, one was affected by the wild Mana and gained magical properties. However, this twin died in utero while the other surviving twin was born a mudblood.
This twin grew up to be a typical mudblood with no magical abilities and produced children. However, it was determined that none of his kids were biologically his. Rather, they contained his dead brother's DNA. Even stranger, these children all contained magical abilities that mages possess.
How could this be the case?