Mary Shelley was the only child of notables within their generation’s intellectual class: pioneering feminist Mary Wollstonecraft and the “radical philosopher” William Godwin. Wollstonecraft certainly exerted influence over social considerations of her daughter’s time, but her mother died almost immediately following Mary’s birth, “and the young child was educated through contact with her father’s . . . circle.” (Pamela Bickley, Intro to Mary Shelley’s The Last Man). As her father described her at age fifteen,
She is singularly bold . . . and active of mind. Her desire of knowledge is great, and her perseverance in everything she undertakes almost invincible. (Radu Florescu, In Search of Frankenstein)
Her husband, the poet Percy Bysshe Shelley, also recognized her abilities. Conclusion: Shelley—if only based on her life experience and encompassing imagination—was uniquely able to synthesize scientific novelties of the age, such as galvanism (involving shooting dead things with electricity), with proto-feminism and other post-Romantic anxieties.
Frankenstein came out anonymously during the developmental years of the novel, appearing during the first quarter of the 19th century. Percy Bysshe Shelley, who advocated on behalf of his wife for the novel’s initial publication, was presumed by its publishers to have written it himself, this affront to “conduct, manners [and] morality.” Shelley secured publication before revealing Frankenstein’s true author. Critics questioned "whether the head or the heart of the author be [sic] the most diseased." (Both quotes are from journalists responding to Frankenstein’s first appearance: from Susan Tyler Hitchcock, Frankenstein: A Cultural History)
Some unrelated points follow to further place the characters, their gender, and how Shelley’s experiences aligned with the novel’s major themes:
As of the year 1818, Shelley had already lost the first of three of her infant children. The second one would die that year. It should be apparent to the reader of the novel that both the child William, and, cruelly, his governess Justine, are innocents—really both doomed children—as they are put to death in the story’s machinations. As for Mary Shelley’s avatar within the novel, Elizabeth, she’s described as having “a calmer . . . disposition” (than Victor), but also as a striving, questing spirit; someone enlightened, and prone to bypass society’s gendered limitations. The men, namely Victor and Henry, are instead portrayed as creatures of ambition and assertive purpose. Youthful, headstrong and heedless, Victor lacks an exit plan when things go very wrong, but there’s no question that he will make his masculine mark upon the world.
Mary Shelley first conceived her monster in response to a ghost-tale storytelling challenge in which the two other active participants contributed the vampire plots fashionable at the time. As the female in the challenge, she proposed a protagonist that’s become the model for the sympathetic monster, comparatively more motivated by human virtues—as contrasted to his outward inhumanity—than Victor, his detached male parent. As for the male emphasis within the vampire tale, that mythology is commonly seen as symbolic of sexual predation with the objective of dominion over womanhood.
Finally, Frankenstein is a cautionary tale of man’s scientific overreach, but commentators have also seen it as a study of the perversion of nature; the consequences of male-based "procreation." In this sense, the novel is a feminist critique. A combination of all these elements inform Shelley’s empathy for her characters throughout the novel.