Thief of Time - Genetics

Then she thought: my grandfather is Death. He adopted my mother. My father was his apprentice for a while. That's all that happened. They were both human, and I turned up in the normal way. There is no way I should be able to walk through walls and live outside time and be a little bit immortal, but I am, and so this is not an area where logic and, let's face it, basic biology have any part to play.

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The Thief of Time - p181