Today’s surname is McFADYEN, which, according to Wikipedia, is a “Scottish patronymic surname meaning ‘son of little Patrick.’ The Celtic prefix ‘Mc’ means ‘son of,’ while ‘Fadyen’ is a derivative of the Gaelic Pháidín, meaning ‘little Patrick.’ ” 
The McFADYEN in our family tree is my husband’s  great-great grandmother Janet McFADYEN, b. 5 Feb 1828 in Kilbirnie, Ayrshire, Scotland. She appears in the 1841 census as Janet McFADGEAN a 13-year-old cotton mill worker. 

In 1846, Janet McFADZEAN marries John MacKIE. (In another abstract, they appear as Janet McFADYEN and John MACKEY.) In 1848, they emigrate to America with their son, Matthew. Their first home in America was in Cass, Schuylkill, Pennsylvania, but shortly after that, they settle in Truro Township, Knox County, Illinois. 

In an profile of John MacKIE published in an 1878 history of Knox County, she appears as Janett McFAYDEN. In her son’s marriage record, she appears as Jennett McFADYEN. And in another son’s death certificate, she’s Janet McFADDEN. 
There is no death certificate for her, but I am hoping to find an obituary.
But that still doesn’t solve that fairly common problem of which spelling, among many choices, to use. So many times in our trees, surnames can be nothing but educated guesses. My next stop is searching for a parish birth record. Any bets on whether this will add a new spelling to the list?