--- title: Human Chimera: How a Man's Unborn Twin Fathered His Child description: An unusual case illustrates how promiscuous DNA can be. It turns out we have a lot of DNA that isn’t our own canonical: https://time.com/4091210/chimera-twins/ author: Alice Park article:opinion: false article:content_tier: free article:published_time: 2015-10-28T19:23:11.000Z article:modified_time: 2026-08-04T07:41:42.310Z article:section: Health og:title: How a Man's Unborn Twin Fathered His Child og:description: An unusual case illustrates how promiscuous DNA can be. It turns out we have a lot of DNA that isn’t our own og:url: https://time.com/4091210/chimera-twins/ og:site_name: TIME og:image: https://static.time.com/v3/assets/bltea6093859af6183b/bltde093bcdab65f2d0/6988d9b9bc6cfc69913e2a3c/time-logo.jpg?branch=production og:image:width: 1200 og:image:height: 675 og:image:alt: TIME og:type: article twitter:card: summary_large_image twitter:title: How a Man's Unborn Twin Fathered His Child twitter:description: An unusual case illustrates how promiscuous DNA can be. It turns out we have a lot of DNA that isn’t our own twitter:image: https://static.time.com/v3/assets/bltea6093859af6183b/bltde093bcdab65f2d0/6988d9b9bc6cfc69913e2a3c/time-logo.jpg?branch=production --- # How a Man's Unborn Twin Fathered His Child > Sponsored content. Supplied in partnership with TIME. 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Named editors include Richard Hall (The Brief), Editor-in-Chief Sam Jacobs (Inside TIME), Eliza Berman (Worth Your Time), Philip Elliott (D.C. Brief), Justin Worland (Future Proof) and Harry Booth (In the Loop). Health Matters is written by TIME's health desk. #### How do I sign up for a TIME newsletter? Go to time.com/newsletters, choose the newsletters you want and enter your email address. There is no cost and no subscription requirement. ## Video: The Peanut Question: Can Food Allergies Develop in the Womb? [Watch (HLS stream): The Peanut Question: Can Food Allergies Develop in the Womb?](https://cdn.jwplayer.com/manifests/TPek27Qi.m3u8) (2:33)  _Published 2010-11-08. A new study has reopened the debate about whether a pregnant mother's diet can lead to food allergies for her child_  by [Alice Park](https://time.com/author/alice-park/)  ## Alice Park Senior Correspondent Oct 28, 2015 7:23 PM UTC  by [Alice Park](https://time.com/author/alice-park/)  ## Alice Park Senior Correspondent Oct 28, 2015 7:23 PM UTC Paternity tests are well known for producing some unexpected surprises, but the case involving a Washington man presents a new head-scratcher. After undergoing fertility treatments and having a son, the man and his partner were surprised when blood work showed that the boy couldn’t be related to the father. After two paternity tests showed the father only shared 10% of his child’s DNA, the parents feared the fertility clinic had inseminated the mother with another man’s sperm. The couple hired a lawyer, who wrote in to Barry Starr’s Ask a Geneticist blog. Starr, director of outreach activities at Stanford University’s Department of Genetics, suggested that the couple get a 23andMe analysis done. The consumer-based genetic testing company provides more detailed relationship and ancestry type genetic data—using hundreds of thousands of markers on the genome—than the dozen or so markers that paternity tests cover. When the report arrived, Starr interpreted it for them and, he says, “it just leapt out at me: uncle. If it was a parent-child relationship, you would see 50% of the DNA related. If it is an uncle to a niece or nephew, it’s 25% related. This man and his son were 25% related.” It turned out that the DNA in the man’s sperm, which was 90% his DNA and 10% that of his twin’s, was from his unborn fraternal twin. Vanishing twin syndrome, which refers to the condition in which one twin dies and is “absorbed” by the other, or by the mother or the placenta, occurs in anywhere from 20% to 30% of pregnancies with multiple babies. Apparently, the father had absorbed some of his twin’s cells in the womb, effectively becoming a blend, or chimera, of himself and his brother. The man’s previous child’s DNA matched his, so, as Starr says, “He is like dad and uncle to his kids.” ### Tulsa Twins, 1947  Caption from LIFE. At a party, teen-agers of the Bounds twins' set munch doughnuts and sip Cokes whenever they are not dancing with serious faces to sentimental music. At right, Barbara dances with Jimmy Dick. Nina Leen—The LIFE Picture Collection/Getty images  Caption from LIFE. Contrasts in teen-age clothes are shown by Betty Bounds (right), wearing a dainty 1947 outfit, while Barbara poses in sloppy get-up. Nina Leen—The LIFE Picture Collection/Getty images  Caption from LIFE. Chores are receiving new respect, for 1947 teen-agers think of marriage much more seriously than their wartime equivalents did. Note the frilliness of Betty's shorts. Nina Leen—The LIFE Picture Collection/Getty images  Identical twins Barbara and Betty Bounds eat al fresco with their parents in Tulsa, Okla. Nina Leen—The LIFE Picture Collection/Getty images  Barbara and Betty Bounds pose in matching outfits, 1947. Nina Leen—The LIFE Picture Collection/Getty images  Caption from LIFE. Although teen-age girls are more romantic and less boisterous than they used to be, they still like to put on some old clothes, whizz around with boys and even get a little grease on their hands. Nina Leen—The LIFE Picture Collection/Getty images  Betty Bounds talks to classmates at school. Nina Leen—The LIFE Picture Collection/Getty images  Barbara and Betty Bounds look at a globe with a friend. Nina Leen—The LIFE Picture Collection/Getty images  Caption from LIFE. Always handy, combs are often carried in sock. No girl is ever without one. Nina Leen—The LIFE Picture Collection/Getty images  The Bounds sisters take a ride with friends. Nina Leen—The LIFE Picture Collection/Getty images  Barbara and Betty Bounds sunbathe side by side. Nina Leen—The LIFE Picture Collection/Getty images  Identical twins Barbara and Betty Bounds ride a horse together. Nina Leen—The LIFE Picture Collection/Getty images  Nancy Churchwell, a friend, tastes a mixture for fudge cake as Barbara Bounds, 17, watches skeptically. Nina Leen—The LIFE Picture Collection/Getty images  The Bounds sisters get ready for ballet class. Nina Leen—The LIFE Picture Collection/Getty images  The Bounds go for a ride with friends. Nina Leen—The LIFE Picture Collection/Getty images Such cases are exceedingly rare, but they do occur. Previous cases of chimeras revealed people with cells that contained DNA different from their own. Mothers retain some cells of their children, and a recent Danish study found that women who gave birth to boys retained cells with Y chromosomes. These can migrate throughout the body and have been found in the lungs, thyroid, muscle, blood, heart and even the brain of the mother. But what makes this man’s case unusual is that some of the cells he obtained from his twin were apparently germline cells, which develop into eggs or sperm. Some cells outside of a developing fetus—in this case, the man’s fraternal twin (not identical, since their DNAs are different)—manage to get incorporated during the chaotic series of cell division and proliferation that occurs when an embryo starts to grow. It’s known that germ cells that go on to make egg and sperm are also quite mobile and do migrate: in this man’s case, apparently from his unborn twin to him. It’s not that an entirely new genome can be subsumed into another one, which would result in an excess of genetic material that is lethal. But some cells with differing DNA can be folded into a developing fetus and not be rejected because the fetus’ immune system isn’t developed enough to see it as foreign. Because the incorporation happened in utero, presumably early in development, the immune system was trained to see both the man’s DNA and his twin brother’s as self. Dieter Egli, a stem cell scientist at Columbia University, notes that chimerism is what occurs with organ transplants, and, on a more sophisticated level, with the new stem cell-based treatments that researchers are exploring now. Egli is quite familiar with the unusual results of mixing DNA and the exquisite balance that’s required for proper human development; he is among the few who have used stem cells to clone human cells. The purpose isn’t to make mini-me’s, but to create a method for reliably producing cells from patients that can then be used to treat various diseases, like Alzheimer’s and spinal cord injury. “This is certainly very unusual and interesting because it’s a germline chimerism,” he says. “What is interesting is to explore in what way this affects the person’s feeling of identity. These are questions that are more important as we start to use cell therapies and cell transplantations as well.” It’s likely that more than just the man’s semen is a mix of his and his brother’s DNA. Starr says that the man’s skin shows different a striping pattern because his skin cells are a chimera of the two as well—his twin’s skin tone was slightly darker and therefore shows up more distinctly against his lighter skin. His other tissues and organs are also likely a mix of the two DNAs, making matching for his blood type or other organs a challenge. Starr says that he and his colleagues plan to describe the case in more detail in a journal article; so far they have presented it at the International Symposium on Human Identification and at the American Society of Human Genetics. ### See the Funniest and Weirdest Baby Photos  Shot for TIME, 2015 Evan Kafka for TIME  Shot for TIME, 2015 Photo-illustration by Evan Kafka for TIME  Shot for TIME, 2015 Photo-illustration by Evan Kafka for TIME  Shot for TIME, 2015 Photo-illustration by Evan Kafka for TIME  Personal work, 2009 Evan Kafka—Getty Images  Shot for Huggies, 2011 Evan Kafka  Personal work, 2009 Evan Kafka  Personal work, 2009 Evan Kafka—Getty Images  Personal work, 2009 Photo-illustration by Evan Kafka—Getty Images  Personal work, 2011 Evan Kafka—Getty Images  Personal work, 2009 Evan Kafka—Getty Images  Personal work, 2009 Evan Kafka—Getty Images  ## Transcript >> About 6 percent of American children have food allergies, with 1 percent of them allergic to peanuts. And those numbers only continue to climb. What causes the body to mistakenly react so violently to such common foods? And when did the cases of mistaken identity start? Do they go as far back as the womb? And can what an expectant mom eat while she's pregnant affect whether or not her child gets a food allergy? That's a strong possibility, says Dr. Scott Sicherer, an allergy researcher at Mount Sinai School of Medicine. He recently studied more than 500 infants, aged 3 months to 15 months, and asked their moms how often they ate peanuts while they were pregnant. >> The more peanut that the mother had reported eating during pregnancy, the more likely it was that we found that their child, when they were entered into the study, had a strong positive test to peanut. >> The results are only the latest in the ongoing discussion about pregnancy and food allergies. Between 1998 and 2000, the American Academy of Pediatrics suggested that moms who came from families with histories of food allergies avoid certain highly- allergenic foods such as milk, eggs, soy, and peanut. But the lack of scientific evidence for this advice forced the Academy to rescind its recommendation in 2008. The results still don't make it any easier for pregnant moms to decide what to eat while they're expecting. And that's true even for researchers like Sarah Taylor- Black, a fellow in Sicherer's lab, who sees the issue both as a scientist and as a mother- to- be. >> There's conflicting data, and so that there is no hard and fast recommendation these days in that you can should just eat what you want, which is what I've been doing. And I have plenty of peanut butter and other allergenic foods during my pregnancy. So, we'll see how it works out. >> For now, until additional research clarifies exactly how food allergies start and which people are more vulnerable to developing then, it doesn't make sense to ask moms to restrict what they eat. >> I think this study basically-- fortunately or unfortunately tells us that we have more to answer on this, and thhat a parent should not feel guilty that they caused or didn't do something right and it led to a child's allergy. I think families should still stick with what they feel comfortable doing. ```json [{"@context":"https://schema.org","@type":"NewsArticle","@id":"https://time.com/4091210/chimera-twins/","mainEntityOfPage":{"@type":"WebPage","@id":"https://time.com/4091210/chimera-twins/"},"headline":"How a Man's Unborn Twin Fathered His Child","datePublished":"2015-10-28T19:23:11.000Z","dateModified":"2026-08-04T07:41:42.310Z","description":"An unusual case illustrates how promiscuous DNA can be. 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