I have a great interest in this subject, I am crap at putting things into words so i took this from a site which should maybe clear up where the Doubt dervied from....
MARY QUEEN OF SCOTS
Mary was born at Linlithgow in 1542, the daughter of James V and Mary de Guise (who had been courted by Henry VIII of England). The princess became queen at the age of six days upon the death of her father. At age six, Mary was betrothed to Henry VIII's son, the ill-fated Edward, but with what results history will never disclose, the proposed union was nullified by a pro-French and Roman Catholic faction. The ire of the English king, exemplified by the period of invasions of his Scottish neighbours known as "the rough wooing," resulted in the defeat of the Scots at Pinkie (1547) and Mary's being sent to France.
THE LINK TO ELIZABETH
Mary returned to Scotland.
The plot then thickened. When Elizabeth I became queen, Mary became heir presumptive to the English throne as the granddaughter of Margaret Tudor. Not only that, but Roman Catholics throughout Europe considered her to be a better claimant to the Crown of England than the queen herself, for they believed that Elizabeth's mother Ann Boleyn had been married illegally to King Henry.
James VI & I was the only heir in line as Elizabeth died without child and out of wedlock.
I took this info from http://www.englishmonarchs.co.uk/stuart.htm
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