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Have there been any Space Marine Chapters out there that have different gene-seeds from various first founding chapters aside from Deathwatch? Is it possible to have such a mixture of Space Marines formed into one Chapter? And would that cause any significant infighting over "who is better" sibling rivalry?

There are chapters with supposedly "chimeric" aka mixed geneseed, but the specific legions are never told.

Some first founding chapters have in extremeis 'dragooned' marines from their successors (the fists were even completely reconstituted). That'd all be from the same 'legion' strain, so there'll be limited divergence.

The only occasion where that would be necessary and desirable is if a task force from several Chapters was cut off from the Empire for a long time. Each different heritage would have their own Apothecaries retreive geneseed, select candidates, train and implant them. The training might be shared - Space Marines are proud but can often see the value in learning things outside their particular dogma, if only to better recognize it.

No Chapter would be founded with geneseed from different strains.

Sure, the Sons of Medusa was formed from chunks of several other chapters. It might seem like it doesn’t count because trust all have gene seed from Ferrus Manus, but the codex orders that from the second founding, all individual chapters’ gene lines are isolated, and genetic drift that happens after that means they’re all genetically different, and ofc culturally too. Reference the angels of absolution who think they aren’t really unforgiven anymore, being merged with a different unforgiven chapter.

But otherwise, new chapters each have their geneseed reproduced from one single set of zygotes. A new chapter isnt just “crimson fist” geneseed, it has “brother steve” geneseed, where all 1000 sets stem from one marine’s worth of organs. Of course, brother Steve is hypothetical, the magi pick out the zygotes from storage.

Not entirely sure if this is what you mean, but in the second Fabius Bile novel there is a chapter called the Red Scimitars. Their geneseed is a mix of (at least) two Legions (though which ones it is not named). Bile ponders what drastic lengths the Ad Mech must have gone to, to keep the formula from being watered down too much (though this is all moot in the present of 40k).

You're more likely to find that in a warband than an actual 'chapter'.