Junk "DNA" is proving to be pretty damn cool ... As simon mentioned, we now know that some,enviroments can trigger dna from our past. We know very little about it currently, but it is exciting.
Children born and malnourished in the early weeks will initiate genes that increase fat retention, they gain fsr more weight than nourished neonates. This is the only example I kmow of, I read about it 3yrs ago, I am sure they would,know more by now. Exciting though, because if we can predict the dna change outcome and find beneficial genes to activate, we could! There is a good chance they are 'off' for a reason however, so lots of research needed first.
rheumatoid arthritis is linked to a gene that protects women from specific infections in youth (years of procreation) and so is preserved in our genes, but now we live longer, the same gene is responsible for joint inflammation in older age. Likewise we need to know why certain genes have been switched off first, as ususlly there is a cost.