Two genomes in one
So a science report in The New York Times this past week tells us this whole individual genetic code business may be a little more complicated than we thought.
The article, which ran on Tuesday, suggests that a fair number of people have genetic signatures that are different from cell to cell. Instead of all being the same, and individual to that one person, the article says, “it’s quite common for an individual to have multiple genomes. Some people, for example, have groups of cells with mutations that are not found in the rest of the body. Some have genomes that came from other people.”1
When the science of genetics began, the article notes, “Scientists believed that they could look at the genome from cells taken in a cheek swab and be able to learn about the genomes of cells in the brain or the liver or anywhere else in the body.” But by the 1950s, “scientists began to get clues that this was not always true.”2
Sometimes, it appeared, one twin would share aspects of the genetic code of the other twin, as in the case of a woman whose blood samples showed two different blood types — one that was her own and the other that her twin brother had. Another woman had one genetic code in her blood cells and an entirely different genetic code in the cells that she passed on to some of her children. And women who have had children appear to retain some of the genetic code of their children — meaning that a woman who’s had sons may actually have some of their YDNA in her own cells.3
When the different genomes result from a fusion or exchange of cells from more than one fertilized egg, the phenomenon is called chimerism, and the person with the different genomes is called a chimera. “A chimera or chimaera is a single organism … that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction.”4
When it results in an individual who developed from a single fertilized egg, it’s called mosaicism: “In genetics, a mosaic or mosaicism denotes the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg.”5
It’s an absolutely fascinating bit of scientific information and the article makes it clear that it has implications for medical genetics — no more assuming that all cells everywhere come from the same source and will behave the same way — and for forensics — no more assuming that a DNA sample from blood will have the same genetic code as a DNA sample from tissue from elsewhere in the body.
So why do we care? Does this mean anything for us in the DNA testing we do as part of genetic genealogy? Is this sort of chimera or mosaic DNA something we need to be concerned with? Are these sorts of miscues going to show up in the scrapings we take from the insides of our cheeks or the saliva we spit into the tubes?
Not so we need to worry about it, according to Dr. David Mittelman, chief scientific officer of Gene by Gene, parent company to Family Tree DNA. He explained that “scientists have known for a long time that, in some tissue, if you take lots of cells and look at the DNA there are differences in the genomes from one cell to another” but that “while somatic mosaicism (as this phenomenon is known) is a fascinating aspect of human biology and is important for disorders such as cancer, it should not impact any of our genealogical tests.”
Dr. Mittelman said that the “variability in cells varies from tissue to tissue (for example microsatellite instability is greater in brain/liver tissue, but not so much in saliva and blood),” and that “in most cases, if there are changes, it is in a minority of cells, and so most will have the germline genotype (the one you got from mom/dad).”
The types of chimeras and mosaics reported in The New York Times article, he said, were “unlikely to happen at detectable levels in enough markers to make an impact in genealogical testing.”
The article suggests one possible exception — people who have had bone marrow transplants show a mix of genomes in cheek swab samples years after the transplants. But even there, the article says, “while the risk of confusion is real, it is manageable.”6
Bottom line: this is really interesting stuff, but it’s no reason to worry about whether your test with Family Tree DNA, AncestryDNA or 23andMe is going to be wonky in some way. Those results, with few exceptions, are just fine for genetic genealogy, helping you track your direct paternal line or direct maternal line or find cousins through autosomal DNA.
- Carl Zimmer, “DNA Double Take,” The New York Times online edition (http://www.nytimes.com : accessed 21 Sep 2013). ↩
- Ibid. ↩
- Ibid. ↩
- Wikipedia (http://www.wikipedia.com), “chimera (genetics),” rev. 21 Sep 2013. ↩
- Wikipedia (http://www.wikipedia.com), “mosaic (genetics),” rev. 15 Aug 2013. ↩
- Zimmer, “DNA Double Take.” ↩