Surely all babies are special, but a DNA test recently revealed that one pair of twin toddlers in New Jersey is especially noteworthy. While the mother of the twins was applying for public assistance, she had claimed one particular man was their father. In response, the man took a DNA paternity test, and found he was the father of only one of them, the New York Times reports; the other was apparently fathered by someone else. In case you're wondering, a judge ordered the father to pay child support for only one twin.
How often does this happen? Cases like this in the legal literature are practically unprecedented, the Times reported, but biologically, they may not be that rare.
In the early 1990s, a team of biologists estimated that, among paternity cases, one in 13,000 involve twins conceived from the sperm of different fathers. (The New York Times reported this statistic too.) But such cases could actually be much more common than those numbers make it sound. It's estimated that at least one pair of fraternal twins in every 12 are fertilized from different sexual encounters that occurred within the mother's single fertile period. How often those two instances involve different male partners depends, then, on the woman's circumstances. The prevalence of so-called "heteropaternal" twins may be underestimated because the vast majority of twins never undergo paternity testing, as a team of biologists pointed out in a paper published in 1997.
Scientists estimate that at least one pair of fraternal twins in every 12 are fertilized from different sexual encounters.
There could be another twist to a case like this. The twins could actually both be the offspring of the same man, despite what the DNA tests might imply.
Theoretically, this could happen if the father has chimerism, a condition where different cells in a person's body each contain one of two different genetic make-ups, as if they belonged to two different people. If a man with chimerism produces sperm with the make-up of two different people, then perhaps one sperm of each type could fertilize two separate eggs. Scientists have documented a number of people with chimerism, including women whose chimerism made it seem, in genetic tests, that they weren't their own children's mother. (They were.) That said, we weren't able to find any cases of a man producing sperm of two different types.
The New Jersey paternity test could simply be wrong. LabCorp, the company that performed the test, says it's 99.99 percent accurate, which likely refers to the chances that the DNA markers that LabCorp tests for are shared by two unrelated people. But human mistakes can also be a big source of error in DNA testing, and the chances of that aren't well-quantified, as Discover magazine reported in 2006.
So which of these scenarios is the most likely explanation for the woman with the differently fathered twins? There aren't ready numbers for the prevalence of chimerism in the general population, nor even how often DNA tests fail. It's not unreasonable to take the New Jersey judge's conclusion as true. The mother said in court she had had sex with another man, plus, as we mentioned, it's not uncommon for two eggs to get fertilized from sperm from two different sessions of sex.
The New York Times called the New Jersey case "a tangled web of love and biology." We don't want to make any unsubstantiated claims about love, but for biology, at least, that claim seems to be true.