Researchers have created embryos that are part-human and part-monkey

THE ANCIENT GREEKSDNADNA were good at inventing fantastical animals. The chimera, for instance, was “a thing of immortal make, not human, lion-fronted and snake behind, a goat in the middle”. It was eventually slain by Bellerophon, with help from his flying horse.Not all chimeras are mythological. To biologists, the term describes organisms whose bodies consist of cells from two distinct lineages. In twin pregnancies, for example, one twin can occasionally absorb the other. The resulting individual is built from cells with separate genomes. A 2019 forensic-science conference discussed the case of a man who had received a bone-marrow transplant. Since bone marrow produces blood cells, subsequent tests on the man’s blood matched his donor’s genome, not his own. (More unexpectedly, the donor’s also turned out to be present in swabs taken from the man’s cheeks, and in his semen.)