Modifying genetic traits have been going on for more than 30,000 years. Since 1973, scientists have sped up the process.
http://sitn.hms.harvard.edu/flash/2015/from-corgis-to-corn-a-brief-look-at-the-long-history-of-gmo-technology/
Most people I know fear GMO crops and I don't understand why. If science could remove the gene from a particular crop so the plant didn't get a disease then pesticide use could be reduced or eliminated. Nutrition, flavor, etc. could all be improved by adding and subtracting genes.
If.
Yes, "IF" only the gene could be inserted or replaced from one to another.
However.
Unless they have dramatically changed things, some years back it wasn't that simple. They took a segment of the DNA, which included the gene desired, and used ballistics or bacteria to insert them into cells of the target organism. If using the special bacteria, this also included a part of their DNA in the process. The target cells were not controlled where the DNA went, but were selected for later. How did they select? If the gene was not readily apparent, they had other genes included, antibiotic resistance, etc. which could be selected for. No use growing a bunch of plants to find out they didn't have the gene.
The gene was either inserted randomly with the ballistic approach or where the bacteria chose. Is that a good place? Could all this "junk" DNA surrounding the desired gene have other functions as they are finding out more and more? Suppose this junk DNA turned on or off other DNA in the target plant? What if the DNA wasn't controlled and expressed all the time or the wrong times?
So yes, you have a modified plant that produces substances that kills corn borers, or what not. Does it do other things? Does it fail to do what it should? What happens if this extra DNA came from an animal or bacteria and created compounds that combined with the target plant compounds to produce what?
What if?
What if we really don't know what we're doing?
I've read (of course that may not be true) that just the process disturbs the plants DNA even if a gene is not inserted.
If it was like computer code, can't really see the problem. And DNA is like computer code, just that we're hiring 5th graders to write a website database for us and .... OOPS, didn't know that would happen.
And comparing Genetic Engineering to classic plant breeding I don't think is really relevant.