Board Thread:Theories and Speculation/@comment-1512913-20161119171442/@comment-1512913-20161119183504

Having genes for 2 nature tranformation kekkei genkai doesn't mean you have a kekkei tota. Kekkei Tota or rather nature transformation Kekkei Tota needs a gene or set of genes that allow one to merge the 2 kekkei genkai.

Which means it definitely isn't a 100% success rate. Considering how rare Kekkei Tota is, not only do the right individuals have to breed, but on top of that a specific mutation might have to occur, which is random.

Mei has gene/s that allow her to mix Fire with Earth and gene/s that allow her to mix Fire with Water, so 2 different genes or 2 different sets of genes = 2 different kekkei genkai, but she has no genetics to mix the Lava with Boil.