Not legit. People used to believe this because they identified a gene correlated with MPB that was passed on via the X chromosome only (Mother to all her children, Fathers pass on an X to daughters and a Y to sons. Men only get an X from their Mothers lineage.). But further research has lead to people ditching this theory. It's a lot more complex, but yes if your Mothers Father/Brother and so on not balding, it increases your odds of being safe.