No it simply happens inbred, same as hunchback and other negative threat , go check the defines LUA .
there is always a chance of inbred , it is never 0% .
Trait chances aren't in defines.lua, they're in common/traits/traits.txt.
The chance of any character getting any trait randomly at birth is set by the line "birth = n", with n being a number between 1 and 10,000 and determining how many characters, on average, out of every 10,000 will be born with the trait randomly regardless of their parents' traits.
The "inbred" trait lacks the line "birth = n", meaning that it cannot be assigned randomly at birth; it will only come about as the result of, you guessed it, inbreeding.
OP: My guess would be that one of your wife's relatively close male relatives is not being honest about their tumbling, or that she has inbred ancestors, or that you have inbred ancestors.