[...] The boy, who was otherwise healthy, is one of only a handful of known true human chimaeras - people carrying tissues that originated in two separate embryos. More common are mosaics, who have patches of tissue that differ genetically from the rest of their body, thanks to a mutation or chromosomal anomaly that arose early in embryological development. [...]
Chimaerism affecting a variety of tissues can also result from other events. In 1995, for instance, Bonthron described another boy who was partially parthenogenetic: cells from his blood and certain other tissues contained none of his father's chromosomes; instead, they featured a duplicated set of one half of his mother's. Although it is not unknown for an egg to start developing without being fertilized, fully parthenogenetic human embryos cannot develop to term. Bonthron believes that the partially parthenogenetic boy owed his unusual genetic constitution to an egg that spontaneously divided into two cells, one of which was fertilized. The second cell then copied its maternal chromosomes, allowing the resulting chimaera to form a viable embryo.