Recently there has been a study done on the "vegetarian" gene. Those who do not eat meat have an additional gene that helps create the fatty acids that they may be lacking from their plant based diet. The gene is found in the DNA of people who come from historically plant based diet countries such as India. However, beneficial the gene is to helping build fatty acids, the gene has leaves them more vulnerable to high inflammation and certain types of cancer. A team from Cornell University looked into this and found that the gene itself produces arachidonic acid which helps absorb more fatty acids from plants. However, when this is combined with modern day cheap cooking oils such as sunflower oil, the body actually produces too much of this acid, which in turn leads to "dangerously high inflammation and colon cancer". They also found the gene limits production of omega-3 fatty acids (which protect against heart disease) and is why they generally have more omega-6 fatty acids. What was advised for vegetarians is to simply use oils lower in omega-6's, such as olive oil.
This article stood out to me because it shows how diets have impacted genetics. This was a case where something seemingly healthy actually can cause more harm down the line. I think it would be interesting to look at how this gene developed. One could look into if this develops over a lifetime of someone being a vegetarian, or does take multiple generations for it to be fully expressed. I think another study on a meat eaters diet would also be beneficial. You could compare the two to see how diets of past generations have impacted today's genetics.