@lovelivelaugh I've been thinking about this topic a while. Some say genetics is a fact, some others say is just BS.
In my opinion, both are right. In engineering, when we make an "energy balance" we make it using "optimal" parameters, but we know that those paarmeters aren't true for every machine. So, to make a point, let´s imagine two different machines (A and B) that are used to do the same task. Machine A uses an amount of energy and Machine B uses a bit more energy to do the same task. Why is this?? Well, because the machines are not the same, they have different parameters, sizes and so on. But, in another scenario, even if Machine A and Machine B are the same, they will not use the same amounts of energy to do the same task. Why is that?? Well, basically because there are some variables that affect the machines but we don't notice them until a detailed analysis.
In conclussion, genetics affect in certain degree, because, maybe, two people will need a different amount of energy to do the same activity, and that's something we can't control. So, if these two people had the same cal intake... Well, you can imagine the rest.