>>215947178 (OP)
1% means about one great-great-great-great-great-grandparent, or possibly one great-great-great-great-grandparent. If you're 25 and your ancestors on average had children at 25, this was someone born in the first half of the 19th century. Moreover, Jewish status is only inherited matrilineally, so you only have 1 in 64 or 1 in 128 odds of being halakhically Jewish without knowing it (the odds that it's your mother's mother's mother's mother's mother's mother, or your mother's mother's mother's mother's mother's mother's mother respectively). If you're concerned about the possibility, talk to your local rabbi about it I guess?
