Board Thread:Warhammer 40k General Discussion/@comment-5846164-20130516215639/@comment-6078851-20130517025331

the ultramarines' gene-seed, while being of a particularly high purity, is not - i repeat is not - the purest. the purest astartes gene-seed belongs to the dark angels (source - codex: dark angels, i think).

the ultramarines boast the most successors due to the purity of their gene-seed. why the dark angels' gene-seed is repeatedly passed over for the creation of new chapters is known only to those within the inquisition. it's probably influenced by the events on caliban during the heresy but it doesn't change the fact that dark angel gene-seed is second to none (except maybe the grey knights).

it also points to the fact that when the emperor created the gene-seed for the legions he got it spot-on with the first legion and the gene-seed for every other chapter is not as pure as that of the dark angels - including the smurfs.