How can magical testicles produce DNA that is different from its host?

Question

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?

Answer 1

Genetic chimera

Part of the dying twin was incorporated into the surviving twin.

In mundane births, this happens in the zygote stages, but if you want it later, one of the magical abilities was to send off his DNA into his brother's body.

Answer 2

https://en.wikipedia.org/wiki/Chimera_(genetics)

In particular,

In 2002, Lydia Fairchild was denied public assistance in Washington state when DNA evidence appeared to show that she was not the mother of her children. A lawyer for the prosecution heard of a human chimera in New England, Karen Keegan, and suggested the possibility to the defense, who were able to show that Fairchild, too, was a chimera with two sets of DNA, and that one of those sets could have been the mother of the children.

So rather than one twin dying, making it so their embryos merged in some manner. Mana / wild magic could be enough of an explanation.

Then as the remaining embryo develops into a human, the majority of the child can have come from the mudblood embryo while the reproductive organs can have come from the one with magical ability.