Yes, but for any disease there are a number of known gene polymorphisms that predispose you to the disease. These genes may interact with the rest of your genome in incredibly complex ways, but I bet all it would take to cure a disease in most cases would be to target the one or two most prevalent polymorphisms.
"You bet"? This is one of the core questions in human genomics/health research today and nobody has been able to make a convincing argument in general that your proposed approach would work.
Would you agree: The question being specifically whether in most cases necessary-even-if-not-sufficient genetic traits can be found ("genetic traits", as opposed to "genetic illnesses" since illness may not be the most likely result.) The problem being that although so far most of our exciting findings are of at-least-necessary genes (or plural genes, etc); obviously it's just these sort of cases that will produce high correlations and grab attention, so there should be a presumption of the equivalent of "publication bias." Necessarily, these will be the low-hanging fruit even if such cases are actually the exception, overall.
Of course, mere and relatively small correlations between disease and genetic traits are extremely common; but there are lots of possible explanations of such findings. In some cases 16 percent of all genes have a small positive correlation to a given disease, and "if everybody is at fault nobody is at fault!"
unfortunately that usually isnt the case. there are very few diseases where one mutation or molecule is truly causative. in many areas, weve approached the limits of how much we can treat disease by just targeting one molecule or mutation. this is an area where $50-70B of pharma R&D spend goes each year for the last few decades, and the yield of that research in terms of approved drugs is declining exponentially
While it may sound easy to change a polymorphism (and even there only for a single base, don't get me started on larger structural variants), do not forget that you have to apply this one single change to 10 ^ 14 cells (or even just 1 in 10000 cells is 10 ^ 10), while evading the immune system and not inducing any detrimental other mutations. For a software developer it always sounds so simple just to fix a single spelling mistake but that really isn't it.
(+ obvoiusly the stuff from the other comments about how hard it is to determine the cause to begin with)
