Chimera & Mosaicism

Chimera according to the genome glossary is an organism which possesses cells or tissues with a different genotype. This can be caused due to mutated cells of the host organism or cells from a different organism or species. (http://en.wikipedia.org/wiki/Chimera_(genetics))

Mosaicism - the condition in which an organism has two or more cell populations that differ in genetic makeup condition - a mode of being or form of existence of a person or thing; "the human condition" (encyclopediahttp://en.wikipedia.org/wiki/Mosaic_(genetics))

"In medicine (genetics), a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one individual, who has developed from a single fertilized egg. Mosaicism may result from a mutation during development, which is propagated to only a subset of the adult cells. Although the two can have some common symptoms, mosaicism is distinctly different from chimerism. In chimerism, the two or more genotypes arise from more than one zygote, while in mosaics; these genotypes arise from only a single cell. Sometimes intersex conditions can be caused by mosaicism - where some cells in the body have XX and others XY chromosomes." (http://www.medgen.ubc.ca/wrobinson/mosaic/mosaicism.htm)

In Greek mythology, the original "Chimera" (pronounced as kih-MEE-ra, and the adjective form "chimerical" pronounced as kih-MER-ih-cal) was a fire-breathing female monster with the head of a lion, the body of a goat and the hindquarters of a dragon. The terrifying Chimera may have merely been a product of a substandard family environment -- her father was the giant Typhon, her mother the half-serpent Echidna. Her siblings were the Cerebrus (the three-headed hound who eventually found work guarding the gates of Hell), Hydra (a nine-headed aquatic monster) and Orthrus (the runt of the litter, a prosaic two-headed dog). As per the mythology, Chimera pounced around making everyone wretched until one day a man named Bellerophon, riding on the winged horse Pegasus slays her.( http://www.word-detective.com/091801.html)

Thought to be a myth, it was found that cases of Chimera actually existed in humans when certain criminal cases erupted. One such case was when a woman has been raped by a man and the women claimed a particular man was the culprit. However, authorities had found it difficult to convict the suspect. It so happened that the DNA taken women did not match the DNA of the man. After extensive investigation the women's statements revealed it to be same person being responsible for the crime. When another series of DNA tests were taken from the suspect, this time from his hair it was found that the DNA sample matched. Those people who have two different types of DNA in their body are called chimeras after the mythological creature with a head of lion, body of a goat and a tail of a serpent. These people can also be called mosaics. It has found that these people have two different types of DNA in different parts of their body. Though very rare, this could have been due to various reasons. (http://www.word-detective.com/091801.html)

The most common reason that chimeras are formed if developing fraternal twin embryos join together to become one embryo. This is different from identical twins that are formed when a single embryo splits into two. This occurrence takes place in the very early stages of the embryo development , so when it develops the baby that is born is healthy but with two types of DNAs. Another reason for chimeras to form is when developing fraternal twin share the same blood supply. This happens when the twins with different DNA share a placenta and cells from their blood mix together. This results in the twins being chimeras. Chimeras are also formed through mistakes or mutations taking place in the division of the cells in the development embryo. This mainly takes place when the cells split into two so that they are able to be formed into more of themselves for the growth of the embryo. It also results in the doubling up of the DNA by the cells, which is divided between the two new halves. During this process, if something goes wrong, then some new cells result in having different DNA. If this happens in the early stages of the development, then the tissues which are developed from the cells will have different DNA. (http://content.nejm.org/)

It is difficult to say exactly how many chimeras are in the world. You are only ably to know if a person is a chimera by testing his or her DNA. This causes many problems with regards to solving of crime cases and also results in complication with organ donations. (http://www.nature.com/nsu/020429/020429-13.html)

Human chimeraism came into being with advent of blood typing and it was found that some people had more than two types of blood group. Most of the cases of chimeraism are the blood chimeraism. Twin embryos often share a blood supply in the placenta, allowing blood stem cells to pass from one embryo and settle in the bone marrow of the other, seeding a lasting source of blood. As a result, almost 8% of non-identical twin pairs have chimeric blood. Apart from those who have chimera due to different blood types, there are even larger numbers of people due to carrying smaller numbers of different blood cells that may have passed through the mother and the fetus across the placenta or from a blood transfusion. (http://content.nejm.org/)

The rarest cases of true chimeraism are when many tissues of the body are affected and are caused when non-identical twin embryos join shortly after fertilization. According to clinical geneticist David Bonthron, who led the team at the University Of Edinburgh, UK that reported on the British hermaphrodite boy "If you've got two embryos there's the chance of two becoming one" In 1995, Bonthron described a boy who was partly parthenogenetic which meant the cells from his blood and some other tissues contained none of his father's chromosomes. However, he had two identical sets of one half of his mother's chromosomes. "Although it is not unknown for an egg to start developing without being fertilized, fully parthenogenetic human embryos cannot develop to term." Bonthron, who is currently working at the University of Leeds, UK, thinks that the partially parthenogenetic boy got his odd genetic constitution due to an egg that suddenly split into two cells, only on was fertilized. The second cell then copied its maternal chromosomes, allowing the resulting chimera to become a viable embryo, resulting in the formation of hermaphrodites or any related problem in the person's sexual organs and their chromosomal sex is identified by their blood test.

It has been also been said that with the increase in in-vitro fertilization (IVF), there has an increase in the cases of chimeras.

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