Board Thread:Warhammer 40k General Discussion/@comment-91.224.27.226-20131031140001/@comment-24383134-20140117204953

Always important to note, that in this instance, the word Gene-Seed means the Progenoid Glands, (the 18th and 19th organs implanted in Neophytes).

While the Progenoid Glands do alter the physical looks of an Astartes, they do retain most of their original appearance, otherwise we'd have legions of Sanguinius and Guilliman flying across the galaxy and nothing would ever get done. It's also noted on a few pages in this archive that the Luna Wolves had a marked resemblence to Horus, for some reason, as other legions weren't known for being so similar in appearance.

Support for the theory of the missing legions being folded into the Ultramarines is evident in the recent Space Marine game. Titus (the player character) looks significantly different from Leandros, another Ultramarine. If you accept the game as canon, then it could be possible that all Ultramarines DON'T have Guilliman's Gene-Seed, since simple things such as hair-color would be carried over to the 'progeny'.

Another theory I had is that after the millenia of use, the Gene-seed is wearing out and is no longer as powerful as it was, (use a tool long enough and if wears down), which accounts for a lack of physical differentiation in the space marines of each chapter/legion with the exception of the Salamanders. Support for this holds that more and more space marines are growing different in appearance in the ending years of M41.