OF course they are not, however this doe not mean, that they can't look as real a possible, and I respect the artistic visions.
However if you're a biologist, so it would be easier for me to justify my point of view, mermaids are believed to he based on human anathomy in the upper part, and fish in the lower part, you should know how fish spine looks, so it would mean, that the spine is extended to the end of the tail, thus it would be more suitable if the tail bent like a spine, rather like synovial join.
Here is a good example, [link] [link] The only remaining issue is whether the tail moves vertical or diagonal, technically if it is extension of human spine, the tail should make left to right moves, instead of up and down...
I think it is irrelevant. Since mermaids are not real no interpretation of them can be considered more correct just because it supports a different mythology than the one you prefer. While it is true that traditionally a mermaid is considered half human and half fish there is no reason to make that the only way to interpret the mermaid. this is like those people who pitch a fit over sparkly twilight vampires.
ok if you look at the issue of a spine being in the tail rather than legs in isolation ignoring any other factors then I would say the spine would probably result in a faster more efficient swimmer. however you are assuming a mermaid needs to be a fast and efficient swimmer. its not like humans are fast and efficient runners. there are so many factors at play in how a creature evolves again I say realism of a mermaid depends entirely on the mythology behind it.
They couln't have retained the 'knee gene' because it is impractical from evolutionary point of view, better adaptation to environmental conditions would be extended spine, thus stiff long bones would had been supplanted by flexible spine.
I dont usually do it. It was just an impulse to do something different......and I like it for her, but I probably wont do on the next mermaid. usually I like the more dynamic curvy fishy tails, but this one just seemed to want legs in there