Ticklishness can be genetic, and run in families, or even sides of familes. In the case of my best friend's family, he has told me that his father and sister are not really ticklish, whereas, he, his mother, and his aunt (mother's sister) are all very to extremely ticklish, so, from this, I would say that not only is ticklishness genetic, and runs in familes, it can also be isolated to sides of families, where one side might be really ticklish, and another side isnt.
Mitch