Spider as being inherited as an extra chromosomal disorder would actually make perfect since, and could easily explain the presence of the neurological issues given better as well (spider wobble). The reason that you couldn't transfer two copies of the chromosome is perhaps it only exists in a single chromotid state, and when two copies of the chromosone present itself one just deteriates... It would then agree with the 75% odds, and not the 67% odds expected if it was lethal in its "super form". I think this is probably the best understanding we can make of it, and if a geneticist put a LITTLE bit of work on it, could easily be proven IMO.