[Pilsner]

There was a story a few years ago about a couple whose child had a very rare genetic disease. The docs did genetic testing, and delivered the unsettling news that the father of the child was not the husband, but a very close male relative of the husband -- father, brother, perhaps uncle. The husband took this in stride, saying it was not possible, as he was an only child, his father was an only child, and had been dead for many years.

Turns out the husband had a fraternal twin brother, briefly, while in utero, and when that brother's embryo had spontaneously stopped developing and broken up, parts of it had been incorporated into the husband's embryo, so that his blood and semen did not have identical DNA.

[IYAS9YAS]

The original C.S.I. show dealt with this in an episode back in 2004. A rapist is identified in a line-up but his DNA swab doesn’t match the rape kit sample. It shows a close male relative, but doesn’t match any of the brothers either.