<P> Seamus and Maggie McFly (portrayed by Michael J. Fox and Lea Thompson) are Irish immigrants and the paternal great - great - grandparents of Marty McFly . In Part III, Marty is befriended by Seamus and Maggie . While Maggie does not trust the "strange young man", Seamus has a familiar feeling about him and believes that helping him is the right thing to do . They have a son named William (Marty's great - grandfather). Much like his descendants, Seamus is harassed by a member of the Tannen family, Buford Tannen . </P> <P> Maggie McFly is played by Lea Thompson, who also plays Marty's mother Lorraine, even though Maggie is not an ancestor of Lorraine; in a DVD commentary track for Part III, Bob Gale states that the creative team considered it important to include Thompson in the film, and he imagines that McFly men are simply "genetically predisposed" to be attracted to women who look like her . </P> <P> Marty (Jr .) and Marlene McFly (both portrayed by Michael J. Fox) are Marty McFly and Jennifer Parker's future fraternal twin son and daughter in Part II . </P> <P> Originally Marty (Jr .) was to be arrested for joining a robbery initiated by Griff and his gang . Marlene attempted to help Marty Jr. break out of jail but failed and was sentenced to 20 years in a woman's prison . Doc and Marty prevented the event from ever happening . </P>

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