“A chimera or chimaera is a single organism (usually an animal) that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic. Chimeras are formed from four parent cells (two fertilized eggs or early embryos fused together). Each population of cells keeps its own character and the resulting organism is a mixture of tissues. Chimeras are typically seen in animals; there are some reports on human chimerism. Plant chimeras are discussed separately.”
Aparently, this is not as rare as wuns beleeved:
“Twin blood group chimerism seems to be very rare in humans. The 30-40 previously reported cases usually were found by mere coincidence during routine blood grouping in hospitals or blood banks. Usually in these cases frank blood group mixtures of, for example, 50/50%, 25/75%, or 5/95% at most were seen. Smaller percentages are very difficult to notice during routine work-up. Using a sensitive fluorescence technique (sensitivity >0.01%) we detected blood group chimerism in 32/415 (8%) twin pairs and 12/57 (21%) triplet pairs, respectively, which is a higher incidence than reported previously.”
The stranj thing is that ther shud be problems with imune tolerans as the kase with organ transplants. If the imune system is not kimerik, shudn’t it atak the sels with diferent1 DNA?
“Immune tolerance or immunological tolerance is the process by which the immune system does not attack an antigen. It can be either ‘natural’ or ‘self tolerance’, in which the body does not mount an immune response to self antigens, or ‘induced tolerance’, in which tolerance to external antigens can be created by manipulating the immune system. It occurs in three forms: central tolerance, peripheral tolerance and acquired tolerance.”
Obviusli, sins kimeras exist, ther is som explanation as to wy the imune sistem does not atak the other sels. I found a kase report:
“We present here a rare case that involved the long-term coexistence of 2 mature, functional, and equilibrated immune systems in a single child after fetofetal transfusion between dizygotic twins. A dichorionic diamniotic pregnancy complicated by twin anemia-polycythemia sequence resulted in the demise of 1 twin. The detection of abnormal vessels on the dichorionic plate strongly suggested the existence of functional vascular anastomoses leading to blood chimerism in the survivor. Genetic, phenotypic, and immunologic analyses at 2 years revealed chimeric lymphoid and myeloid cells in the surviving twin, although no tissue mosaicism was detected, which indicates that early transfusion led to mutual immune tolerance. “
We report the case of a dizygotic twin with long-term mature and functional HLA-disparate immune cells. Blood exchange early in the fetal period through anastomosis of placental vessels likely contributed to the acquisition of tolerance. Whether chimeric twins are tolerant only with respect to hematopoiesis or to other tissues also remains a question.”
Other interesting quotes
“Chimerism occurs naturally in adult Ceratioid anglerfish and is in fact a natural and essential part of their life cycle. Once a male is born, it begins its search for a female. Using strong olfactory glands, the male searches until it locates a female anglerfish. The male, less than an inch in length, bites into her skin and releases an enzyme that digests the skin of his mouth and her body, fusing the pair down to the blood-vessel level. While this attachment has become necessary for the male’s survival, it will eventually consume him, as both anglerfish fuse into a single hermaphroditic individual. Sometimes in this odd ritual, more than one male will attach to a single female as a ‘parasite’. They will both be consumed into the body of the larger female angler. Once fused to a female, the males will reach sexual maturity, developing large testicles as their other organs atrophy. This process allows for sperm to be in constant supply when the female produces an egg, so that the chimeric fish is able to have a greater number of offspring.” (from the 1st Wiki link)
“The existence of chimerism is problematic for DNA testing, a fact with implications for family and criminal law. The Lydia Fairchild case, for example, was brought to court after DNA testing apparently showed that her children could not be hers. Fraud charges were filed against her and her custody of her children was challenged. The charge against her was dismissed when it became clear that Lydia was a chimera, with the matching DNA being found in her cervical tissue. Another case was that of Karen Keegan, who was also in danger of losing her children, after a DNA test for a kidney transplant seemed to show she wasn’t the mother of her children.” (ibid.)
ETA 6th jan.
I forgot to mention it, but conjoined twins do not hav problems with this as they ar always identikal twins.
1Diferent bekus from a diferent zygote, not just bekus of random mutations.