hilary_step...I think most agree that "Cainan" in the LXX of Genesis 11:12 is a secondary insertion, duplicating the "Kenan" of Genesis 5:12. This makes the genealogy ten generations in length from Shem to Terah, just as it was ten generations from Adam to Noah. It also pushes back the date of the Flood another 130 years, which adds to the LXX's scheme (motivated by synchronistic chronography?) of making the Flood date as early as the fourth millennium BC by adding 100 years to the ages of the postdiluvian patriarchs (up to Nahor, who gets an additional 50 years) when they had their first son.
I would say that in principle, omitted generations may well lie in the original oral tradition that the genealogies are rooted in, but the present schematic form of the genealogies in the Priestly narrative does not really allow it. It is worthwhile to compare the Sethite genealogy in ch. 5 (P) with the Cainite one in c. 4 (J), as these are clearly variants of the same lineage but drawn from independent oral traditions. Some names differ in spelling (e.g. qyn | qynn, `yrd | yrd, mchwy'l | mhll'l, mtwsh'l | mtwshlh), while some are identical (hnwch | hnwch, lmk | lmk), some swap positions in the lineage (i.e. hnwch and mchwy'l | mhll'l), while others occur in the same positions (e.g. qyn | qynn, mtwsh'l | mtwshlh, lmk), and some occur in one lineage but not in the other (e.g. 'nwsh and nwch in the Cainite line, such that the threefold mention of sons at the terminus of the lineage involves the sons of lmk in the Cainite line and the sons of nwch in the Sethite line).