Scientists are finding that it’s quite common for an individual to have multiple genomes. Some people have groups of cells with mutations that are not found in the rest of the body. Some have genomes that came from other people. - Mitchell Tsai

In 1953, for example, a British woman donated a pint of blood. It turned out that some of her blood was Type O and some was Type A. The scientists who studied her concluded that she had acquired some of her blood from her twin brother in the womb, including his genomes in his blood cells. - Mitchell Tsai

One woman discovered she was a chimera as late as age 52. In need of a kidney transplant, she was tested so that she might find a match. The results indicated that she was not the mother of two of her three biological children. It turned out that she had originated from two genomes. One genome gave rise to her blood and some of her eggs; other eggs carried a separate genome. - Mitchell Tsai

In 2012, Canadian scientists performed autopsies on the brains of 59 women. They found neurons with Y chromosomes in 63 percent of them. The neurons likely developed from cells originating in their sons. - Mitchell Tsai

In The International Journal of Cancer in August, Eugen Dhimolea of the Dana-Farber Cancer Institute in Boston and colleagues reported that male cells can also infiltrate breast tissue. When they looked for Y chromosomes in samples of breast tissue, they found it in 56 percent of the women they investigated. - Mitchell Tsai

Last year, forensic scientists at the Washington State Patrol Crime Laboratory Division described how a saliva sample and a sperm sample from the same suspect in a sexual assault case didn’t match. - Mitchell Tsai

Bone marrow transplants can also confound forensic scientists. Researchers at Innsbruck Medical University in Austria took cheek swabs from 77 people who had received transplants up to nine years earlier. In 74 percent of the samples, they found a mix of genomes — both their own and those from the marrow donors. - Mitchell Tsai

Women can also gain genomes from their children. After a baby is born, it may leave some fetal cells behind in its mother’s body, where they can travel to different organs and be absorbed into those tissues. “It’s pretty likely that any woman who has been pregnant is a chimera,” Dr. Randolph said. - Mitchell Tsai

Now that scientists are beginning to appreciate how common chimerism and mosaicism are, they’re investigating the effects of these conditions on our health. “That’s still open really, because these are still early days,” Dr. Urban said. Recent studies on chimeras suggest that these extra genomes can even be beneficial. Chimeric cells from fetuses appear to seek out damaged tissue and help heal it, for example. - Mitchell Tsai