What you are about to read is absolute truth, and anyone seriously interested in knowing the real answer to a question should never be offended by the truth.
There are no such things as merfolk, and there is no such thing as a mergene. We have already completely sequenced the human genome. No such thing exists.
A sentient species always leaves behind artifacts of its existence. Merfolk could not exist at depth and be even vaguely similiar in appearance to humans, so they would have to live in fairly shallow depths. Artifacts would include architecture, articles of day to day life, such as tools, jewelry, and possibly written records, carved onto stone slabs. Merfolk items would be so different from anything a land-based lifeform could produce that it would be instantly identifiable.
No such evidence has ever came to light - and in our time of submarines, underwater sonar imaging, remote camera robots, bathyspheres, privately funded treasure hunts, scuba divers, underwater nuclear tests, etc. it is impossible for a shallow-depth culture of "mermaids" to exist and not be found.
The human body, with its specialized developments, has designed itself for land living in a gravitational environment. A merfolk would not need the body to be so bipedally designed and the buoyancy of the water itself offsetting the pull of gravity allows for a more symmetrical alignment of the organs from front to back instead of from side to side.
You cannot "evolve" backwards. By the time Homo Sapiens had evolved from Homo Neanderthalis, there was a millions of years interval between us and the time our mammalian ancestors differentiated from amphibian life. No such "regressive mutation" is possible - therefore human/fish "merfolk" are impossible.
Aquatic creatures, as stated above, evolved along far different lines than land-based animals. Even if a branch of fish could have developed human level intelligence (which is not possible - our brain development is a direct result of our environment and cannot be duplicated in the ocean) the closest possibility is the dolphin, which is the only sea creature that shows evidence of cognitive function anywhere near our level. There is absolutely no environmental reason for a cognitively developing sea creature to physically develop into a half human. It doesn't work like that.
As to "magic" - if "spells" actually worked to transform humans into their wishes, the world would be Post too long. Click here to view the full text.