I have read countless articles where bodybuilders lump everyone into 3 groups, ectomorph, mesomorph, and endomorph. They then assume ectomorphs have the worst muscule building genetics and mesomorphs have the best.

While this is generally true, there is many more factors at play, that dictate how well a person can gain muscle. Many ectomorphs are not small framed and skinny because they have inferior genetics for building muscle, but because they have a very fast metabolism. The thyroid plays a role in your metabolism. What would a mesomorph look like if he restricted his calories? Thats right an ectomorph! An ectomorph just burns too many calories, so they stay thin keeping them from building significant muscle.

Now I’m not saying an ectomorph can have just as good of muscle building genetics as a mesomorph. Mesomorphs inherently have good muscle genetics, that allows them to naturally have larger muscles and more symmetrical body. I don’t think a ectomorph could ever challenge a mesomorph for muscle size, if they followed the same workout routines and diets. What I am saying is that, ectomorphs can have good or bad muscle building genetics for their frame. The only way to know this potential is to train and eat 
hard.

For example, some ectomorphs can gain muscle easily with little fat gain, provided they get enough calories.  Other ectomorphs put on too much fat and become “skinny fat” as they struggle for small muscle gains.  How well they gain muscle with a surplus of calories, will depend on their muscle building genetics. Not all ectomorphs are the same! The same could be said for other bodytypes. Some mesomorphs gain muscle easier than other mesomorphs. In each bodytype, there are some that gain muscle(or fat), easier than others of the same bodytype.

Genetic Factors of muscle growth:

There is a few genetic factors that are determined at birth for 
how well you can gain muscle compared to other people. The
most important factors appear to be: muscle fiber type, hormone levels, and myostatin.

A few years ago it was found in mice, that when myostatin went unchecked, the mice grew to huge muscular beasts. This led to a myostatin supplement craze. Myostatin supplements were proved later to be junk science, not because of myostatin, but because their claims of how they changed Myostatin in the body. You can not do anything about Myostatin levels(at least not now).

Hormones undoubtedly play a huge role also. People take steroids to give them supraphysiological levels of hormones to go past their genetics. It’s not just testosterone, but Growth hormone, and IGF-1, that all act simultaneously together in producing muscle gains. Some people have naturally higher testosterone levels than other people.

Fiber type, like myostatin, is apart of your body genetics. There is slow twitch and fast twitch. Slow twitch are very responsive to muscle hypertrophy(increase in diameter). Many ectomorphs can blowup to big muscular guys if they have a lot of the slow twitch fibers. Lifting heavy will change the characteristics of a 3rd type(sort of an in between fiber type) of muscle fiber to act more like slow twitch fibers.