Chimerism occurs when a woman is pregnant with twins and one embryo dies, and the other embryo absorbs the twin's cells. (Scientifically speaking, this type of chimerism is called tetragametic because the baby was derived from four gametes – one egg and one sperm for each embryo.)
What happens when one twin absorbs another? The vanishing twin's tissue gets absorbed by the surviving embryo(s) and the parent, too. This absorption process is completely harmless.
Experts aren't sure how many human chimeras exist in the world. But the condition is believed to be quite rare. It could be becoming more common with certain fertility treatments like in vitro fertilization, but this isn't proven. Only about 100 or so cases of chimerism have been recorded in modern medical literature.
Lydia Fairchild (born 1976) is an American woman who exhibits chimerism, having two distinct populations of DNA among the cells of her body.
Sometimes a DNA test can easily show that you are a chimera. A quick cheek swab, a strange result with three or four versions of a specific marker and BAM, you're a chimera. Sometimes you need to test your blood and your skin cells to find out. You get two different results from each and BAM, you're a chimera.
A chimera is essentially a single organism that's made up of cells from two or more "individuals"—that is, it contains two sets of DNA, with the code to make two separate organisms. These individuals often don't know they are a chimera.
A human chimera is made up of two different sets of DNA, from two different individuals. Experts aren't quite sure how common natural chimeras are in the human population, as only 100 cases have been documented so far. However, the prevalence of natural human chimeras is hypothesized to be as high as 10%.
Chimeras Aren't More Likely to Have Kids with Chimerism
Even when the ovaries or testes of a chimera are made up of cells from both “twins,” their sperm or egg cells will only get DNA from one “twin” or the other. This is due to a process called meiosis.
Blood group chimerism has not been directly linked to intersex traits or DSD in humans and is typically confined to peripheral blood cells, though it has been shown to extend to non-haematological tissues including buccal [13] and skin cells [14].
Most chimeras remain undetected, especially if both zygotes are of the same genetic sex. Many are discovered accidently, for example, during a routine blood group test. Even sex-discordant chimeras can have a normal male or female phenotype.
Chimerism is a rare congenital condition involving one person having two different sets of DNA. There are a few instances when it can occur: when a fetus absorbs a vanishing twin during pregnancy, when fraternal twins trade chromosomes with each other in utero, or when someone has a bone marrow transplant.
Does everybody have the same genome? The human genome is mostly the same in all people. But there are variations across the genome. This genetic variation accounts for about 0.001 percent of each person's DNA and contributes to differences in appearance and health.
In art the Chimera is usually represented as a lion with a goat's head in the middle of its back and with a tail that ends in a snake's head. This matches the description found in Hesiod's Theogony (7th century bc).
According to one study, about 36% of twin pregnancies experience vanishing twin syndrome. It also occurs in around half of multiple pregnancies, or pregnancies where a woman carries more than one baby. Some researchers think the number of women who experience this syndrome may be increasing.
Fetus in fetu is a parasitic twin that's completely enclosed inside the body of the healthy twin. This can go undiscovered at birth, only becoming obvious later. Fetus in fetu may initially be thought to be a tumor. It's a rare condition with fewer than 200 cases having been reported worldwide.
Vanishing twin syndrome is evaluated to happen in 36% of twin pregnancies and in half of the pregnancies that begins with at least three or more gestational sacs.
The person must have both ovarian and testicular tissue. This may be in the same gonad (an ovotestis), or the person might have 1 ovary and 1 testis. The person may have XX chromosomes, XY chromosomes, or both. The external genitals may be ambiguous or may appear to be female or male.
Chimeras can often breed, but the fertility and type of offspring depends on which cell line gave rise to the ovaries or testes; varying degrees of intersex differences may result if one set of cells is genetically female and another genetically male.
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.
Although this is quite rare it can happen and it's called superfetation. Two babies are conceived from separate acts in two different cycles. These babies can be from the same father or two different men. When heteropaternal superfecundation occurs, the babies are from different fathers.
Chimerism is a phenomenon in which an individual has cells with different genetic content from different zygotes. In dizygotic twins (DTs), chimerism is believed to occur through placental anastomoses that enable the bidirectional exchange of hematopoietic stem cells.
Causes of vanishing twin syndrome
Advanced maternal age (over 35) (This may simply be because multiple pregnancies are more likely in older women.) Chromosomal abnormalities in the vanished twin. Use of assisted reproductive techniques. Small placenta or other placental abnormalities.
In the roleplaying game Dungeons & Dragons, the chimera is an evil-aligned creature which looks like a lion with leathery wings on its back. To either side of its lion's head is the head of a goat and the head of a dragon.
The chimera had a fearsome weapon - she was able to breathe fire. This, combined with her lion's strength, goat's cunning and snake's venom, made her nearly invincible. The chimera lived in Lycia, where she terrorised the people and ravaged the land with her fire breath.
Federal US laws do not prohibit the creation of chimeras. Nonetheless, there are some restrictions on federal funding for research by the National Institute of Health (NIH).