I do believe to large extent it is hereditary but also a learned function, as well. The fact that five children can be brought up in the same loving home and one of them does not have good middos, can only be explained as that child having inherited certain bad characteristics from other members of the family. But there is a great inyan of Nature Vs. Nurture, and even a child who might have a bad disposition can be taught to do chessed. Interesting question.
I do see that my husband is almost exactly like his father O”H, middos-wise, and that is most fortunate as my dear FIL was a tzaddik bein adam l’chaveiro. Unlike his father however, my husband chose to be frum. People tell me that my personality and actions are just like my mother’s O”H. I wish! She was a great lady.