The Real Life comparison:Despite what movies would have you think, the area of genetics is vastly more complicated than "Add a pinch of X" or "Swap Y with Z". The basis of such inaccuracies come from the simplified way genetics is taught in middle and high school: The double-helix is like a ladder, with the rungs being nucleobases Adenine, Cytosine, Thymine, and Guanine. Each rung fits together, A to T, and C to G. Mix each rung around and presto! You got a tail! This is also how it's portrayed in movies with CG effects showing the double helix spinning around, taking out one rung and putting another in, with the whole thing glowing to show how it's been affected. This trope gets even stranger when the genes in question come from somewhere besides Earth. DNA is not just "genetic material." It's a specific molecule that we humans, and almost every living thing on Earth, use. It's one of boundless possible candidates for the job. Viruses may use DNA or its sister RNA for their genome. There's no reason to think extraterrestrial genetic material should be the same as ours. Additionally, DNA contains lots of special little sequences besides genes. These include viruses that simply hopped onto our DNA and get copied with it (endogenous retroviruses), sequences which are apparently just "junk" we can't get rid of, and sequences which tell cells how often to turn on each gene and when to shut it down. This also requires that almost all living cells on Earth interpret a gene the same way. That's the genetic code; almost all cells interpret a particular sequence of three "rungs" to mean the same amino acid when they turn the gene on and start making protein. Yet when the plot demands it, an alien has DNA and all the ancillary stuff to be compatible with our genetic heritage. That's so absurdly improbable that if an organism from another planet were to have DNA at all, rather than some other possible molecule, most scientists would immediately start wondering if we had a common origin (or scream "Hoax!") In fiction, aliens have DNA and interpret it just as things which evolved on Earth do. No Biochemical Barriers indeed! Just slap it into our cells and away we go making a Half-Human Hybrid and more. It should be noted that gene therapy, the alteration, addition or removal of existing genes in an adult organism, exists. Yes, we no longer need the more atrocious method of Eugenics to eradicate debilitating genetic diseases. However, it can be more problematic than presented in fiction, and can wear off after a little while. Additionally, it normally requires very complicated surgery (or the use of gutted viruses) to carry out; you can't simply inject some foreign DNA into your blood and wait for the mutations to take place. However, an evidence for the plausibility of gene therapy lies in the existence of Endogenous retrovirus DNA. One of our ancestors got infected by a retrovirus, which made its way into the said ancestor's germ cells. Bingo! Now his/her children inherit the virus DNA with the parent DNA, until this day. Some of the endogenous retrovirus DNA has since mutated and became junk DNA, but some of those viruses are actually still active and facilitating mutations and evolution, for better or for worse (like helping the embryo implant in the womb, or playing a role in several diseases). Addition of genes via plasmids (segments of DNA or RNA transplanted between organisms) also exists; it is how viruses reproduce and how bacteria and small beings can adapt. In genetic transfer, plasmids are faster and more efficient in propagating beneficial genes between species than sexual reproduction (which was only useful due to multicellular organisms lacking plasmid capabilities), and this is why bacteria can share antibiotic-resistance genes so fast and easily. Human experimentation has reproduced this, but this is limited to adding new traits to cells. So far, said cells may propagate, successfully integrate into an organism and produce a blood vessel network, but this can only happen in the embryonic stage, and in most cases are yet incapable of forming complicated structures typical of most organs by themselves. However, genetically altered cells may still be used to produce proteins and hormones an organism wouldn't be able to create naturally due to genetic illness, for example giving diabetics the capability to produce insulin and live normally. For more information, try UsefulNotes.Genetics or (if you can handle the Techno Babble) The Other Wiki.