Each of us is born with a complete set of genes we inherit from our parents. Your genotype can refer to either the entire set of genes you carry within you, or any subset of those genes, i.e., the genes that determine eye color. A phenotype is the physical expression of a particular subset of genes. So, in the example of eye color, your genotype would determine that you have brown eyes, but your phenotypes would determine your eyes' exact shade of brown (chestnut, hazel, amber, etc.). Phenotypes are influenced partly by genes and partly by your environment. Consider the flamingo. Many of us think all flamingos are pink, but actually their genotype isn't encoded for pinkness. The food they eat influences whether their phenotype expresses pinkness or whiteness.
So what does this have to do with creating the optimum lifestyle for your health?
Let's say you go to your local gym and ask a trainer to create a customized workout plan for you. That trainer will ask what your end goal is—fat burning, muscle building, endurance, or achieving and maintaining a particular bodyfat ratio—then he or she will arrange a set of exercises that are generally associated with your end goal. Your trainer might even might make a few dietary suggestions to supplement your workout plan. This may sound like you're getting your money's worth, and you might even see some results. But a major factor is missing from this workout plan. You!
Just as your genotype and phenotypes determine your eye color, they also determine the types of fitness training and eating regimens your body will be most responsive to. Back to the customized workout plan the trainer created for you. Let's say you and a friend follow that same exact plan, while eating the same exact food, for three months. Chances are you both won't see the same results. Maybe you'll mostly shed fat while your friend makes significant muscle gains. Why? Both of you don't carry the same identical set of genes within you. That's why any customized workout plan must include a genomic reading, otherwise your fitness training and eating plan could end up being more a waste of time than the life-altering journey you're truly seeking.