That issue of Harriet "passing as a white woman" is quite ironic and reveals how pernicious and corrupting was the system of racial slavery in the US. Harriet was legally a slave and "negro" because her great-grandmother was an African slave who had had at least one child (Harriet's grandmother) with a white sea captain. The captain allegedly tried to purchase her great-grandmother and his daughter, but the owner wouldn't sell for whatever reason (maybe the owner had children by her as well). All Harriet's grandfathers in her maternal line were white men and, of course, all her paternal grandmothers and grandfathers were white.
But it goes beyond that: Harriet's mother Sally Hemmings, Jefferson's life partner since the death of his wife, was Jefferson's sister-in-law. She was the half-sister of his wife by their father, John Wayless. Wayles was the ultimate owner of Harriet's grandmother, Betty, and her great-grandmother - his wife received Betty, as a wedding present from her father. Since it was legally stipulated in the transfer that Betty was always to be the legal property of Wayles's wife or her children, it seems that Betty was considered family. After his wife's death, Wayles and Betty, Harriet's grandmother remember, had six surviving children (including Sally who was inherited by her half-sister, Jefferson's wife).
All this demonstrates that slavery in US, and then in the South where it persisted, involved generations-long bondage of people by their intimate relatives.
Since the importation of slaves was banned by Federal Law in 1808 (the US Constitution prohibited the government from banning it any earlier), all slaves born after were born in America. Had state laws not been imposed to prevent estates from freeing their slaves if they were not entirely debt free and other hurdles, it is hard to imagine that slavery could have survived beyond the 19th century.There could have been good motivations for these laws as well as bad ones. Imagine an unscrupulous owner who opted to make his farm more efficient by "benevolently" freeing slaves who were too old or infirm to work. A law ostensibly intended to protect such people, however, harmed young, hale, family-aged slaves, who coincidentally were those that Southern established institutions feared most. This was referred to at the time as The Problem of Slavery: How to free all the slaves without any socio-economic-political disruption of current norms - which of course was not possible.
In the early 1830s, chronicler Alexis de Toqueville, encountered an owner who had spent his waning years trying fruitlessly to free his children before his death.
I happened to meet an old man, in the South of the Union, who had lived in illicit intercourse with one of his Negresses and had had several children by her, who were born the slaves of their father. He had, indeed, frequently thought of bequeathing to them at least their liberty; but years had elapsed before he could surmount the legal obstacles to their emancipation, and meanwhile his old age had come and he was about to die. He pictured to himself his sons dragged from market to market and passing from the authority of a parent to the rod of the stranger, until these horrid anticipations worked his expiring imagination into frenzy. When I saw him, he was a prey to all the anguish of despair; and I then understood how awful is the retribution of Nature upon those who have broken her laws.