Ah the old tired Nature versus Nuture debate.
Even if you had 100% Introvert genes, If you had to be extroverted to survive, you would become extroverted, and after a while, internalize those extroverted values, claim as your own for a sense of pride and personal dignity and then raise your offspring in the tradition of extroversion and within a few generations you would have the religion of extroversion, where the congregation might still be predominately introverted genetically, but the man-made institution of the ancestor required extroverted behavior...
The consensus on personality disorders is that they are symptomatic of behaviors that are considered quite normal at certain stages of developmental growth. However, for some reason those who suffer from P. D.s don't develop past a certain point, in a certain area of their personality.
It may be assumed that all differences in personality (MBTI types included) are due to the same process. There are some areas where it is easy to excel and other areas where it is difficult not to fail - when developing a social Identity. The main difference is that development is not halted at a certain level (as in the case of a personality disorder) but merely retarded for a relatively short time so that more or less "Well Rounded" personalities still exhibit quirks, eccentric-ness and weirdness on occasions.
The development of a social identity/personality is primarily a case of social adaptation, which of course there are genetic pre-dispositions as factors, but depending on the type of society or culture one deals with in, the personality varies, even when the genetics do not.