Some people carry two sets of DNA in their body, as if they were two people fused into one. Because, in a way, they are.
A human chimera has two cell populations coming from more than one zygote. The classic kind is tetragametic: two fertilized eggs (future fraternal twins) merge very early into a single embryo. Other routes: twins sharing blood in the womb, fetal cells staying in the mother for decades (and the other way round), or a bone marrow transplant. It’s not the same as mosaicism, where both cell lines come from a single fertilized egg.
Most chimeras never find out. Lydia Fairchild and Karen Keegan did, the hard way: DNA tests said they weren’t the biological mothers of their own children.
Can a chimera have two differently coloured eyes? Yes, but it’s a rare cause. Heterochromia usually comes from chance in how melanocytes reach the iris, from mosaicism, from syndromes like Waardenburg, Horner or Sturge-Weber, or from injury, disease and medication. Odd eyes prove nothing; only a genetic test does.
Only around a hundred tetragametic cases are documented. Probably many more walk around unaware, since an estimated 20 to 30% of single pregnancies start out as multiples.