If you were to group genres according to a set of binary opposites you would likely see science fiction and fantasy linked this way; though the second link should also show how horror is but one muffled heartbeat away from fantasy too.
We've discussed the way in which fantasy and sci-fi are bosom buddies and then opposites in a crystalised breath and perhaps one more example should prove this: take the Golden Age hero Jack Frost a humanoid ice elemental according to the history, and compare him with Hal Jordan. The first is fantasy, right enough, with no better explanation than a secretive Arctic origin but consider the superhero most comprehensively buggered by the writers - worse than Batman's broken back, Superman's "death", Aquaman's loss of a hand, Iron Man's alcoholism - and whom I've lost all interest in.
Subsequent writers may have departed from John Broome's skilful use of the powers to craft storylines, rather than some character flaw in the wielder of those powers, but the silver age Green Lantern does what most revamps of golden age characters do; fills in the blanks by clearing the tommy rot and giving things a more, erm, scientific orgin. Much of the magic is removed and we have the power ring and lamp being handed by an alien who has crashed. That isn't a dramatic departure from the Alan Scott version since much of the space origin is also part of it.
The interesting point of comparison here is in just what a fantastic thing the power ring is; beyond the parameters of science fiction really. Yet that is technically where it resides where Jack Frost's more modest powers are fantasy only.
It's worth noting too that, while the blur of fantasy and SF works in the superhero's favour, the presence of a figure like Killraven or Machine Man while entertaining enough in their own right, do nothing but detract and distract from the powerful original concepts of a War of the Worlds or 2001: A Space Odyssey.
I have never heard of horror being a subset of fantasy, yet it's hard to imagine something that would be a hybrid; they're hyphen worthy
Science fiction, on the other hand, is worlds away in the dark of space.
Romance begs the question as to the manner of fantasy; but it's lonely in outer space.
The major houses have touched on the grand spectacle of wars between alien races: Marvel its classic Kree - Skrull War and DC the current Rann - Thanagar War.
Fantasy is an escape from the reality of war but clashes between various fairy folk are legion in modern tellings.
Science Fiction resides at two poles: there's the weighty intellectual ruminations on how society would develop given a set of circumstances and there's the dogfights in space. Guess which side contains the action. The classic protagonists: Flash Gordon and Buck Rogers of the 25th Century. File under: 'humans conquer space'
And we can look at how action works within fantasy. Fantasy sets up its own constructs but requires movement, even if just repetitive movement.
As with its presence in the other genres surveyed,Action is an element within sci fi and fantasy rather than a cohort.
Could this also apply to drama? Science fiction which bears down on its characters and affects their interactions is the dominant form. We don't think of a sci-fi tinged drama so much as a dramatically-based science fiction story. Ditto fantasy. This could be argued but, clearly, they appear wrapped together rather than being seen as discrete forms.
Adventure in fantasy takes the form of a quest while for techies [cough] the definition of adventure is to boldly go where no fan has gone before
What clues can be found in intergalactic crime or monstrous scientific experimentation for the mystery of space and the space for mystery? Ming the Merciless is a villain and a worthy adversary to the star of the book so he shares as much with Sax Rohmer's creation. DC has reliably flagged mystery in space.
Fantasy puts a microscope to the effect that it has on 'reality'; shows the effect they have on each other.
Science fiction permeates the popular comics and sometimes it even IS comic. A fantasy doesn't need to be amusing, it just needs to be good.