26/07/2026
Chimerism is a rare genetic condition in which a person has two genetically distinct types of DNA in different cells or tissues of the body. It can occur when two separately fertilized eggs (embryos) fuse very early in development, resulting in one individual with two genetic cell lines.
A well-known case involved Lydia Fairchild, a woman from Washington State whose DNA tests unexpectedly showed that her children did not genetically match her. Even testing around the birth of her third child produced the same result. Further investigation revealed that DNA from her blood and cheek cells differed from DNA in her reproductive tissues. The reproductive tissue carried the DNA type that had been passed to her children. This explained the apparent DNA mismatch and led to her being cleared of all accusations.
Key clinical point: In suspected chimerism, DNA testing from only one tissue may be misleading. Testing samples from multiple tissues may be necessary for an accurate diagnosis. Many people with chimerism are otherwise healthy and may never know they have the condition