My best guess:
A schizophrenic man had a sex change and married his adopted brother. The "marriage" is a genuine marriage, but is between a man, a (newly-made) woman with the same parents but no blood relation, and the male alter-ago of the transexual. No clue about the chromosome part, maybe part of the surgery involved cutting one off.
Just in case this is somehow not the correct answer,
I do think two of the 'men' are the same person, somehow. I think in fact 46 of their chromosomes are identical and the 45 reference is a red herring. If they're identical to the atomic level surely it's impossible for them to be two different people?