Eh not to rain on the parade, but we've beat this to death. There are too many ways we can argue for this given the lack of empirical studies. There's the presence of dominant and recessive genes too, just because type could be inherited does not mean your parents were the same type/letter etc. For once I want to see some well researched study, and I'm not just talking about a collection of data involving families and types. Has anyone actually researched into the science of MBTI? Maybe different genes/chemicals combine to make different letters/functions etc.? There's a reason this always gets brought up but we never get anywhere.
Big 5 is taken far more seriously than MBTI academically in general. So, when searching for studies it's usually much easier to find quality Big 5 studies than to find quality MBTI studies.
Still, as far as a personality-related question, the heritability of personality factors has been researched far more than most. There are lots of relatively consistent Big 5 studies (more on that in a moment). The the one
MBTI study I found indicates in its abstract:
Twins’ scores on the continuous scales were subjected to behavior genetic model-fitting procedures. Extraversion-Introversion and Thinking-Feeling yielded heritabilities of about .60, consisting largely of nonadditive genetic variance. Sensing-Intuition and Judgment-Perception yielded heritabilities of about .40, consisting largely of additive genetic variance.
I can only see that abstract for that study (the rest is hidden behind a pay wall), so I can't read the full study. It's interesting that this study found non-additive genetic variance (which means the effectives dominant/recessive traits and other cases where one gene affects another) for E/I and T/F. The E/I being non-additive fits well with Big 5 studies (which have generally found that E/I is non-additive). Studies seem to disagree a bit on the additiveness of other Big 5 traits. I wish I could find more than just a single MBTI study, since it would be nice to have confirmation across a larger pool and with different methodologies.
Also, if one reads some of the Big 5 studies, there are some interesting tidbits scattered about. For example from
one Big 5 study:
Our results suggest that genetic and environmental effects are not uniform across all facets of a dimension. For example, our data suggest that not all facets of Conscientiousness are influenced to the same degree by genetic factors. Individual differences in Order, Self-Discipline, and Deliberation appear to be largely determined by environmental influences. The implication is that some of the broad dimensions may not be etiologically homogeneous. The results of even more detailed analyses, such as the item level analyses of Heath et al. (1989) of the EPQ, may be fruitful and reveal additional information about the genetic structure of personality. If this tentative general finding is replicated on a larger sample, it may provide a different perspective on the structure of personality and help resolve some of the controversies surrounding the number of broad dimensions required to conceptualize personality (e.g., Zuckerman, 1992; Zuckerman, Kuhl- man, & Camac, 1988).
so, that kind of thing suggests that aspects of J/P might be very directly related to upbringing (or other aspects of environment), even thought on the whole it might be genetically influenced.
So, the 40% to 60% genetic rule of thumb may not fall consistently across all the sub-scales of each trait, even though it's been fairly consistently born out over all.. Looks like the answer to the question of the heritability of personality traits (or type) is complicated, and it's unlikely to to become crystal clear any time soon.