Every single person in my immediate family--me, mother, father, brother, and sister--is a J. I'm by far the weakest J, but that probably came over time as I saw how retarded it made everyone else (and me, for a long while).
Me, my brother, and my dad are all ENTJs.
My mom's the only introvert and the only sensor. Poor her..
I'm one of nine kids born to an ENFJ and an ENTJ. I am the only other extrovert in the whole family. The only one out of nine kids.
Wow, your parents would have taken you kids all over the place, much to the dismay of your introverted sublings I bet LOL!
A genetic theory, potentially tossed out the window already ... BUT WAIT - imagine if you looked at inheritance here the same as eye color. One gene for I, one for E. You get one or the other from each parent to make a pairing. The combos result in actual expression. (OF COURSE, I'm just playing here, all you T's out there, I am not trying to prove anything LOL!)
If you buy the 75% E population vs 25% I, it would mean that E is the dominant trait. Therefore:
I (from your dad) + I (from your mom) = I
I + E = E
E + I = E
E + E = E
3 out of 4 people, E. 75%
Cool..........I just cracked the genetic code for E vs I. My scientific study and accolades forthcoming. You read it here first!
And for tinyarmy's example above, her parents would have been dominant E, but have had E + I for both of their inherited genes. It would have been improbable for so many I + I pairings, but not impossible by any stretch.
Now, I hear you asking, what about I + I = E - same result as other unexpected expressions - the result of natural mutation & deviation.
"There is considerable evidence that whatever the biological foundations to introversion and extraversion are, they have a genetic basis." Link goes on to explain research done on the matter.
And the final paragraph ... interesting!
"Jung suggested that introversion and extraversion themselves had their roots in two basic ways of procreating. One was a broadcasting of many offspring with the hopes that a few would survive, while the second was the birth of a few offspring raised in highly protective conditions. In short, introversion and extraversion would represent two kinds of evolutionary adaptation, two viable ways of surviving in this world. And this kind of inheritance would guarantee that neither one of these patterns would die out, and as a result mankind would maintain the plasticity and adaptability, which is one of its most valuable attributes."