The ACTN3 gene contains instructions for making alpha-actinin-3. Alpha-actinin-3 is a protein associated with fast twitch muscles. I.e. the muscle sprinters, long jumpers and weight lifters use for their explosive powers. You inherit this protein via your DNA from your mother and your father. All Olympians have inherited this protein from at least one parent with one known exception who like 18% of the population has inherited a gene variant from both parents that doesn't make this protein.
I have had a DNA test and my ACTN3 gene doesn't make the protein, meaning I have inherited a variant from both of my parents that lacks it. In all the studies I have read there is a general consensus that the alpha-actinin-3 protein gives athletes about a 3% improvement in performance in explosive sports. There is also a theory that the opposite may be true too, in that the lack of this protein is predominate in endurance athletes and the lack of it seems to help this end of the sporting spectrum. There is, though more research needed as the link is tenuous at the moment.
Now let's say I was a 5km runner and I was lacking this gene and lets also say I want to get faster. If you look at a typical park run the winner is usually around sub 18 minutes. So a top amateur runner can run a 5km in 18 minutes (or less but let's keep the numbers simple) I lack alpha-actinin-3 so therefore I would be 3% slower. 3% of 18 is 0.54 which equals 32.4 seconds. It is therefore still possible for me to run my 5k in 18:34.4. This would put me in the top 3 at a lot of parkruns. I therefore can't blame my genes for not running a sub 20 5k yet.
I have always believed it's hard work to get where you want to be and your genes have little to do with this. Most people believe it's your genes that give you your IQ, your athletic ability, your inability to lose weight, but blaming or praising your genes for your short comings or 'natural abilities' is wrong. It leads you to not putting in the effort required to get where you want to be. Most 'natural' athletes, musicians, chess players, etc etc have spent countless hours and hard work honing their craft. Children of musicians often end up as musicians themselves because they had access to instruments from the day they were born.
It took 10 years to become an overnight successIf you want to run faster, don't blame your genes. Get out there and train properly. Yes you have to do those hill repeats and those intervals.
If you want to speak a foreign language stop saying I don't have that sort of brain, start believing and start learning, we all have the right sort of brain if we want to do it.
If you want to lose weight, be honest with yourself and don't blame the 'fat' gene for the chocolate biscuits you just ate.