I'm not the best expert... but as I understand it there is some belief that a gene can be present... but not 'expressed'... i.e., the effect of the gene doesn't really kick in until some unknown "kickstarter" flips the switch to have it express itself.
For me, my theory is that while that may be true? It might also be more complex than that.
The body is filled with systems that have checks and balances... and they 'kick' in as they are signaled to do so. Sometimes they malfunction, but there are other 'check points' in the processes that tend to keep the entire process on track. I'm learning this is very much the truth as part of the Methylation process. And I had previously studied the blood clotting cascade process which had similar multiple checks and balances (i.e., too little and you tend to have bleeding issues and too much you tend to develop blood clots).
I think that when it comes to things like the methylation process we'll eventually learn that certain genes in and of themselves that have a defect may be fine, but if you combine defects in a specific combination of matches, you start having issues... BUT that what might happen is that it is also combined with environmental and dietary/stress/etc factors... Think of it like a sewage pipe, if the sewage flowing through is at normal mass and flow rates - even a defective process may still get it to the sewage treatment plant without too many problems. BUT, if the mass/volume/incoming speed of the sewage increases dramatically, then the system issues will become more obvious. The more defects in the system, the lower that 'capacity' levels are... the fewer defects in the system, the more 'capacity' works with only minor issues - and those minor issues may even be at a point of being symptom-free.
That said... genes are supposed to cover many factors - like of appearance. I, personally, was born with red hair... but it fell out when I was an infant and regrew as blond. By the time I was 18, my hair started to darken until I was a brunette (or well, until I stopped dying it blond to stay the color it had been when I was younger). My brother, and both my sons were very blond as children - but are growing out of blond and their hair is now quite brown. The eldest (the youngest doesn't have facial hair yet) - has brown hair with red facial hair. My grandson was born with auburn hair, but at six months it's now growing back in as light blond. I think most people's hair might change colors slightly - as in lighten more or darken more, but rarely change to multiple different actual colors (well, not counting changing to gray). I've been told it's a change in our gene expression - but no one can explain to me how these changes are getting triggered for all of us.