I'd like to tell you a curious story. Jane was a 52 year old woman in need of a kidney transplant. Thankfully she had three loving sons who were all very happy to give her one of theirs. So Jane's doctors performed tests to find out which of the three boys would be the best match, but the results surprised everyone. In the words of Jeremy Kyle, the DNA test showed that Jane was not the mother of two of the boys... Hang on, said Jane, child birth is not something you easily forget. They're definitely mine. And she was right. It turns out Jane was a chimera.
Chimerism is the existence of two genetically different cell lines in one organism. This can arise for a number of reasons- it can be iatrogenic, like when someone has an organ transplant, or it can be naturally occurring. In Jane's case, it began in her mum's womb, with two eggs that had been fertilised by different sperm creating two embryos. Ordinarily, they would develop into two non-identical twins. However in Jane's case the two balls of cells fused early in development creating one person with both cell lines.
Thus when doctors did the first tissue typing tests on Jane, just by chance they had only sampled the 'yellow' cell line which was responsible for one of her sons. When they went back again they found the 'pink' cell line which had given rise to the other two boys.
This particular type of human chimerism is thought to be pretty rare- there are only 30 case reports in the literature. (Though remarkably both House and CSI's Gil Grissom have encountered cases.) What happens far more frequently is fetal microchimerism- which occurs in pregnant women when cells cross the placenta from baby to mum. This is awesome because we used to think the placenta was this barrier which prevented any cells crossing over. Now we've found both cells and free floating DNA cross the placenta, and that the cells can hang around for decades after the baby was born. Why? As is often the case in medicine we're not sure but one theory is that the fetal cells might have healing properties for mum. In pregnant mice who've had a heart attack, fetal cells can travel to the mum's heart where the develop into new heart muscle to repair the damage.
Whilst we're still in the early stages of understanding why this happens, we already have a practical application. In the United States today, a pregnant woman can have a blood test which isn't looking for abnormalities in her DNA but in that of her fetus. The DNA test isn't conclusive enough to be used to diagnose genetic conditions, but it is a good screening test for certain trisomies including Down's syndrome.
Now, we started with a curious tale, so lets close with a curious fact, and one that's appropriate for Mother's Day: This exchange of cells across the placenta is a two way process. So you may well have some of your mum's cells rushing through your veins right now. In my case they're probably the ones that tell me to put on sensible shoes and put that boy down...
(FYI: This is a story I originally posted on my own blog)