The Obama family tree is huge and complex, a plant of extended boughs and multiple offshoots — some of which, like Barack Obama's relatives in the US, have taken root in the most unexpected places.

Mr Obama is not the first presidential candidate to suffer embarrassment on account of his kith and kin. Just about every one has contained at least one black sheep in the family; some have had flocks of them.

Like many African clans, the Kenyan side of Mr Obama's family tree is a thicket of half-siblings, cousins, aunts and uncles. The genealogy is complicated by the fact that both his father and grandfather had several wives and numerous children. Hussein Onyango Obama, the patriarch of the clan, was married three times. He had Barack Obama Sr with his second wife, Akuma, but she left soon after, and the job of raising his son fell to his third wife, Sarah. This is the elderly woman whom Mr Obama refers to as “Granny” although she is, in fact, his step-grandmother.

Uncle Omar and Aunt Zeituni, the two relatives who moved to Boston, are the children of Sarah and Hussein Onyango Obama, and therefore Mr Obama's half-uncle and half-aunt.

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Mr Obama's father had at least eight children by four different women. Intelligent, charming and irresponsible, Barack Obama Sr referred to himself as a “serial polygamist”, and appears to have drifted from woman to woman as he drifted from job to job, fathering numerous progeny along the way. He was already married to a Kenyan woman, Kezia, with whom he had two children, when he met and married Ann Dunham, Mr Obama's mother. In his memoir Dreams From My Father, Mr Obama describes his excitement and confusion on meeting his Kenyan family when he visited the country for the first time in 1988. He attended a family reunion, and was mobbed by his newly discovered kin, “all of them smiling and waving like a crowd at a parade ... we continued around the room, shaking hands with aunts, cousins, nephews and nieces. Family seemed to be everywhere ... all of them fussing, fretting over [my father's] long-lost son”.

Mr Obama also described the new sense of responsibility he felt towards this extended family. “If Jane [another aunt] or Zeituni ever fell ill, if their companies ever closed or laid them off, there was no government safety net. There was only family, next of kin; people burdened by similar hardships. Now I was family, I reminded myself, now I had responsibilities.”

Mr Obama is not the first to find that relatives can be a burden in presidential politics. Jimmy Carter's brother, Bill, smoked pot on the White House roof, urinated in public, launched his own brand of “Billy” beer and entered the world belly-flop competition. He also accepted $220,000 to act as a goodwill agent for Libya, shortly before the US bombed the country.

Lyndon Johnson's younger brother, Sam, was a heavy drinker who offered his brother unsolicited advice. Patti Davis, Ronald Reagan's step-daughter, stripped for Playboy and Abraham Lincoln's brothers-in-law fought for the Confederacy. Yet an embarrassing relative need not be damaging. Jimmy Carter won himself some much-needed popularity when he was asked about his wayward sibling and replied: “I love him like a brother.”

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