SPOILER WARNING!!! THIS CONTENT IS FROM POTTERMORE.DO NOT READ THIS IF YOU WANT TO FIND THINGS OUT FOR YOURSELF.DON'T READ AND BLAME ME AFTERWARDS!!
McGonagall’s backstory from Pottermore
Minerva McGonagall was the first child, and only daughter, of a Scottish Presbyterian minister and a Hogwarts-educated witch. She grew up in the Highlands of Scotland in the early twentieth century, and only gradually became aware that there was something strange, both about her own abilities, and her parents’ marriage.
Minerva’s father, the Reverend Robert McGonagall, had become captivated by the high-spirited Isobel Ross, who lived in the same village. Like his neighbours, Robert believed that Isobel attended a select ladies’ boarding school in England. In fact, when Isobel vanished from her home for months at a time, it was to Hogwarts School of Witchcraft and Wizardry that she went.