Chimerism

Have you heard about Chimerism? I've not heard about it until last night when I watched "I am my own twin" on Discovery Health Channel. It got me so interested to know more about this disorder. According to the show and some of my search on the internet, Chimerism is known as an extremely rare disorder that mixes the chromosomal population in a single organism. In these cases, chimerism may manifest as the presence of two sets of DNA, or organs that do not match the DNA of the rest of the organism. In some cases, hermaphroditic characteristics, in other words, having both male and female sex organs, can be signs of chimerism. Alternately, small patches of DNA can be present throughout the body. Chimerism tends to occur very early in the embryonic development. It is often the result of two non-identical twin embryos merging together instead of growing on their own.

Found an article in BBC News about this condition and it was dated back in 2003.

When two became one in the womb
Some babies could have started life as twins. Jane was a puzzle to doctors. She needed a kidney transplant, but her naturally conceived sons could not donate - because they were not biologically related. Tests eventually showed Jane was a chimera - her mother had been pregnant with non-identical twins who had fused together in the womb to make one person.
It meant some parts of her are derived from one twin, and the rest from the other.
Jane, 52, was seen by doctors at the Beth Israel Deaconess Medical Centre in Boston, Massachusetts. Medical staff had tested her three sons to see if they were a close enough tissue match to act as kidney donors. But New Scientist magazine reports genetic tests showed two of her sons could not be hers.

'Male and female organs'
Closer analysis found Jane was a tetragametic chimera - made up of two genetically distinct type of cells. The term chimera comes from the Greek word for a monster that was part lion, part goat and part serpent. She had one twin's cells in her blood, which showed up in the tissue-typing tests the scientists carried out. But in other tissues, including her ovaries, the other twin's genes dominated - explaining the initially mysterious test results.

A single embryo can be formed from two sets of egg and sperm. Jane's condition emerged by chance, and scientists say many other chimeras probably go through life unaware of their genetic make up (especially when two same sexes embryos fused together). Some do display physical signs, however, such as having different coloured eyes. Others report problems with the reproductive systems, and been found to have parts of both the male and female organs because they had cells from both sexes in their bodies. In the United States, around 30 cases of chimerism have been reported.

Swapping cells
In 1998, doctors at the University of Edinburgh highlighted a case in which two embryos, one male and one female, fused in development to form a single child. His condition was discovered when he was treated because his left testicle had not descended normally and surgeons discovered an ovary and a fallopian tube on the left side.

During pregnancy, some cells from the mother slip through into the foetus and vice versa.
Up to 90% of women are thought to carry their children's cells or DNA in their blood during pregnancy - and up to 50% for decades afterwards, a condition called microchimerism.
If a woman then has more children, the older sibling's cells could be passed back into the younger child during that pregnancy.

I am not sure at what degree chimerism is related to Vanishing twin syndrome. Based on what I know, researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and not been detected or recorded (even before a pregnant woman go for her first ultrasound). I believe in certain cases, instead of the two embryos vanished, they just fused together to become one, at this point nobody will ever know that the single baby born is actually a twin!