@i:
Hodi is Arlong's son, calling it now. Makes sense because why else would Oda go out of his way for an elaborate (albeit interesting) discussion on the lineage of fishmen? At first I thought Hodi was going to be an Arlong clone but the more I thought about it the more I started to realize that maybe the reason Hodi is so much like Arlong is because they're Father and Son.
I think the focus on fishman/merman genetics is rather part of the buildup to ultimately shattering the Arlong-esque "fishman-supremacy" psychology that Hodi and his gang have latched on to.
After all, according to Den's explanation, fishmen/mermen have long had a implicit understanding of the capacity for two individuals to be wildly different from one another, even when closely related. As Den mentioned, they don't really understand why humans focus so much on categorizing people. But then, in an admittedly understandable backlash to their own people's long persecution at the hands of humans, many fishmen (Arlong and Hodi being notable examples) turn around and effectively do the same in reverse.
I think therefore that, ultimately, Hodi is going to be confronted with the fact that, while not physically as outlandish as it can be with fishmen/mermen, humans are much the same: even two humans who are directly related to one another can be vastly different. Even the child of a callous and hateful Celestial Dragon could turn out to be a strong supporter of the Equality of Fishmen and Mermen.
That's one of the main reasons I see Hodi as a misguided - and ultimately redeemable - character.
As to his potential relation to Arlong, its certainly possible, but I doubt it. Arlong never struck me as that old (I'd say mid-30s tops… Was his age ever mentioned?), and Hodi seems early to be in his early-20s. Similarly, there's more than enough connection with just the ideology. Hodi may be misguided, but he's willing to sacrifice his life for his beliefs. Making his coming conflict with Luffy (and yes, I fully expect that they will clash before this arc is over) personal via a direct familial relation to Arlong actually takes away from Hodi's character, I feel...
EDIT: Hey there, ninjas! waves