What are human chimeras and how common are they?

The answer

Human chimeras possess cells with two or more distinct genetic profiles, originating from different zygotes. This occurs naturally through several mechanisms: vanishing twin syndrome (one twin absorbs the other in utero, occurring in up to 1 in 8 pregnancies), maternal-fetal microchimerism (fetal cells crossing the placenta and persisting in the mother for decades), and blood chimeras (fraternal twins exchanging blood through shared placental circulation). In a famous 2002 case, Lydia Fairchild nearly lost custody of her children because DNA tests showed she wasn't their genetic mother - her reproductive organs contained different DNA from her blood. A 2015 study found that 63% of mothers carry male fetal cells (from sons) in their brains decades after pregnancy. Artificial chimerism occurs through bone marrow transplants, which can completely replace a patient's blood-cell DNA.