Here is something everyone hears all the time and gets thrown around too much, genetics. There is not enough time in a day to actually break this down but I will use it for bodybuilding purposes.
I did a lot of studying on genetics just so I can have an idea of what I was working with, Did you also know that when you take AAS before the age of 25 you will never know if you reached your genetic potential?
Look it up; Google is your friend.

People who say they have bad genetics are just plain lazy lets be honest.

Think of your body as a tree. Given the right stimulus, a tree will grow. If it doesn’t, that means something is wrong, disease, not enough light, too much water, The same thing will apply to your body: There is always a reason you’re not growing. And No; it is not that you haven't injected enough test.

To be successful has three tiers: training, recovery, nutrition. You can't expect to build muscle if you skimp out on any or all of those.
Most of us will never be Arnold , but it doesn’t mean we can't come close.

Body Types:

Mesomorphs experience greater muscular hypertrophy and fat loss. Mesomorphs have decent metabolisms, and have a naturally nice build.

Ectomorphs carry little fat and little muscle. Fast metabolisms are the problem, while it's a gift it's also a curse. Ectomorphs are hardgainers due to difficulty gaining muscle and trouble retaining muscle. Ectomorphs have naturally skinny builds.

Endomorphs carry more fat and muscle. Endomorphs struggle trying to lose fat and gain fat easier then gaining muscle. An endomorph will have wide hips, wide waist and a stocky build.

Muscle Growth Potential:

The growth of your muscle is slightly determined by the amount and length of muscle fibers.
The length of those muscle fibers is critical to growth, those with long muscles, or a larger cross- sectional area grow a lot more muscle. Here is an example--- Flex your bicep and then see how many fingers you can put between the bicep and the forearm "gap." For example I can barely fit 2 fingers unless I squeeze them inside so my muscle growth ability is pretty damn good, If you can fit more then 2 fingers then you will have a very limited muscle growth ability. In other words the greater the gap between your elbow joint and bicep, the less potential for growth, this is true for every muscle throughout the body. And No, No amount of training or AAS will fix that so if you believe it will, whatever helps you sleep at night.

Stop working out and start training. Training means, “Increase the capacity to perform a skill or work.” If you are still training with the same weights after a year, you suck. Push yourself to the limit every workout.

I used a calculator to calculate my genetic potential based on my dimensions: Currently I am 5' 6" and 209 pounds at 19% BF.

Your estimated maximum muscular bodyweight at ~19% bodyfat is: 218.6 lbs

Your estimated maximum bulked bodyweight at ~19% bodyfat is: 227.3 lbs

Your estimated maximum muscular measurements (@ ~8%-10% bodyfat) are:

Chest: 47.9 in Biceps: 17.2 in
Forearms: 13.7 in Neck: 16.7 in
Thighs: 25.4 in Calves: 17.1 in

Thats pretty fkn big considering my Biceps are 17.1 now so I am almost maxed with my BF being as high as it is; if I drop to 10% BF I would be a monster.

Here is the link to the maximum potential http://www.weightrainer.net/bodypred.html

Would you like to research more on my topic? Take a look at some of these links
to give you a better understanding of what the term Genetics really means as far as your potential.

http://www.unm.edu/~lkravitz/Article%20folder/genetics.html

http://bodybuilding.about.com/od/bodybuildingfaq/a/How-Much-Of-A-Factor-...