When Women Fail Maternity Tests: The Fascinating World of Natural Chimerism
Several years ago a woman named Lydia Fairchild applied for welfare. She was pregnant and had two children. They forced her to take a DNA test…which showed that she was not the mother of her own children. The children she carried and gave birth to. Fairchild was accused of welfare fraud and illegal surrogacy. Social services threatened to take the children. Her third child’s birth was witnessed, but again, tests showed she wasn’t the mother. She was accused of carrying somebody else’s baby.
There have been a handful of cases of failed maternity tests (and at least one failed paternity tests) that happened because the parent was a chimera. Specifically, a tetragametic chimera. In some cases, fraternal twins are conceived, but they merge together, kind of the reverse of the splitting process that creates identical twins. This results in one person with two sets of DNA. When the gonads are created from a different cell line from the skin, a swab test will show a mismatch between the child and the parent. This is very rare, but may be more common than we think. And yes, it’s possible for somebody to have two cell lines that carry different sex chromosomes. Another variant is twin chimerism, where one twin dies and the other absorbs a few cells, resulting in occasional parts of the body from the other cell line.
When Chimerism Haunted a Quarter Horse Breeder
There is, it turns out, no gene for brindle in horses. However, rarely, a brindle phenotype occurs. One such brindle horse is Dunbars Gold, a registered Quarter Horse stallion.
A breeder decided that a good way to test the possibility of a brindle gene in horses was to breed her brindle mare, “Sharp One,” to Dunbars Gold. The AQHA now requires DNA testing for registration.
Surprise, surprise, the foal’s DNA didn’t match…
…either parent’s on-file DNA. Oops. They tried a retest with the same result, and the AQHA told the breeder, Denise Charpilloz, that she was obviously sending in a sample from the wrong foal. Snag is, it was the only foal on the property.
Then somebody took a look at Dunbars Gold’s DNA sample on file, which had been taken using a hair follicle test typically used on horses. They noticed something very odd.
Dunbars Gold’s DNA sample had no Y chromosome. So, maybe that was the mystery. Maybe the DNA sample from Dunbars Gold had gotten switched with that of a mare. That didn’t explain, though, why “Deuce” also showed as not being a match to his dam. Or to the foal she had had the previous year.
You’ve probably already guessed. Both parents were stripy not because of some elusive equine brindle gene, but because they were chimeras. The brindle striping was, in fact, Blaschko’s lines, which are sometimes seen on human chimeras, especially those of darker-skinned ethnicity. The correct cell lines were identified and Deuce was duly registered. Dunbars Gold was a normally functioning male who had female genes in his skin and hair. Sharp One, meanwhile, had somehow managed to have her two ovaries develop from different cell lines, hence the “unrelated” foals.
This form of chimerism is extremely rare in horses, who seldom conceive twins. So it should be more common in dogs and cats, right?
Chimerism in Dogs and Cats
Venus the Chimera Cat is one of the queens of the internet. Her face is black on one side and ginger on the other. However, she’s probably not a chimera…she’s just a really striking calico. Calico or tortoiseshell in cats is caused by something called X-inactivation mosaicism. Because one of the genes for fur color in cats somehow ended up on the X chromosome, female cats that are heterozygous for this gene, which determines whether a cat is red-based or black-based, have different X chromosomes activated in different cells, resulting in those oh-so-pretty patterns. This is also why male tortoiseshells are extremely rare and when they do show up tend to have fertility problems — all male torties either have an extra X chromosome or are XX/XY chimeras.
In fact, despite what you would think of as a higher likelihood of chimerism in litter bearing animals, nope, it’s not really any more common. Or at least if it is it’s not always visible. If you have chocolate and yellow lab puppies fuse in the womb you may get a really pretty chocolate/yellow pattern. If they’re two yellow lab puppies you might not ever know.
Do Chimeras have Health Problems?
Most of the time, no. You or I could be a twin or even tetragametic chimera without even knowing it. There are a couple of things that can happen with human chimeras, though.
- Blaschko’s Lines, which are differently-pigmented areas of the skin. They generally don’t come with any issues, though. Other than looking odd.
- A slight increased risk of auto immune disease in twin chimeras. If your thyroid comes from a different cell line, your immune system is more likely to attack it, for example.
- Fertility issues in XX/XY chimeras. A minority of XX/XY chimeras may have issues. We talked about the mare who’s ovaries had different DNA. This kind of thing can result in an ovary on one side of the body and a testes on the other…in practice, this generally means a testes on one side and something that wanted to be an ovary but didn’t quite make it on the other, which is sometimes initially mistaken for cryptorchidism (having a testical fail to descend). Such individuals often have low fertility. Some chimeras may also have ambiguous genitalia. However, the vast majority of intersex people aren’t chimeras.
It’s likely the vast majority of human chimeras don’t have any clue about it. It generally doesn’t affect their lives. Until their offspring test as not related to them…
So, this is the fascinating world of tetragametic and twin chimeras. Because nature is seriously weird sometimes.