This is one of those stories you check multiple times to make sure you’ve seen it right.
I read this on Time magazine’s daily news brief awhile back, and I didn’t believe it either. Basically, here’s what happened —
After fertility treatments had produced a son, the couple had a paternity test. Imagine their surprise when the test revealed the father shared only 10% of the child’s DNA. Had the clinic made a mistake?
More-detailed testing placed the DNA commonality at 25%, which was consistent with being the child’s uncle, as opposed to a normal fatherly commonality of 50%. But how could a father be an uncle?
Have you ever heard of “vanishing twin syndrome”? This man had a fraternal twin with him in the womb. But his twin died early and was “absorbed” into his body. A test of the man’s sperm revealed 90% was his DNA and 10% was his twin’s.
Yes, these cases are very rare. What is even rarer is that some of the cells from his twin were apparently germline cells, which develop into sperm. Hence the child he fathered actually ended up with the absorbed twin’s DNA.
At least that’s how I understand it. For a more-complete explanation, the original report is here —