The Incredible World of Chimera Genetics- Co-existence of genetically different cells in the same organism
Genetic Chimera is a single organism that comes to life from a composition of different zygotes, or the fertilized eggs. Even in humans, there are individuals with cells of two types, or even more, containing different genetic information. A feature that defines the Chimera well is that the individual cells in the body are not same but genetically distinct. In the Greek mythology, Chimera is a monster figure with an amalgamation of a lion’s head, goat’s body and dragon’s or serpent’s tail. Equidna – the half woman, half serpent went into a union with the giant Typhon, resulting in the birth of Chimera. It was later killed by the Corinthian hero Bellerophon, with the help from the winged horse, Pegasus.
The thought of two bodies living together as one, is haunting. Can only imagine how difficult it would be to continue a life that way. To make way for fresh and clear concept of Chimerism, we at Compilation11 thought to unravel the mysterious facts about Chimera organisms, the ones I am sure you must just have looked or heard some rumors about.
1 Chimeras Also Existed In Greek Mythology
Chimeras are animals consisting of more than one type of cells of different origin. That can at least be resorted to the fertile source of images that the Greek mythology hands down to us. The origin is a result of errors in the fertilization process and that of following cell division. These errors may occur in meiosis and gametogenesis i.e. the formation of sex cells in sexed species – in fertilization or in embryonic development.
2 Mechanism Of Being Chimera
The traditional explanation is that when a sperm is fused to the cell membrane of the ovum, there is a change in membrane properties. This change in membrane properties prevents any other sperm from attaching to the same egg.
This is what happens in most of the situations. An animal chimera is a result of involvement of different zygotes that are formed in a sexual reproduction. They are formed from at least four parent cells i.e. two fertilized eggs or embryos fused together. The condition may either be inherited or infusion of cells.
3 The Triploid Situation In Chimerism
An individual has only one type of maternal genes, but two types of paternal genes. As a result of the egg being fertilized by two spermatozoids from the same parent, a situation called triploid occurs. The human cells are normally diploid, containing 2n chromosomes: half of maternal origin and half of paternal origin.
The developments of molecular biology and techniques have made it possible to identify situations of chimerism. Triploid is relatively frequent in humans: it occurs in about 1% of all conceptions.
The triploid condition is not stable and tends to lose part of the chromosomes, becoming diploid cells. Only the missing chromosomes may be different in different cells, giving rise to different cell populations in the same individual. And there are a number of possible combinations.
4 Recent Cases And Studies
The case of an extraordinary Triploidy was recently described in the publication Human Genetics by Vivienne Souter and coworkers. The article described the existence of two twins, one of which is apparently a boy and the other a true hermaphrodite. These twins have the same genetic information from the mother, but only half from the father. This condition may have occurred either by an ovum that got fertilized by two spermatozoa – then the embryonic stem cells have developed at a subsequent stage, as usually occurs in the formation of identical twins – or because the ovule has split into two cells which were each fertilized by two spermatozoa.
Another investigator named Golubovsky predicted in 2003 that the situation of twins who are neither monozygotic nor dizygotic (false twins) results from Triploidy. His discovery confirmed that the theory has predictive power, and it allows predicting situations not yet known at the time of its formulation.
5 In Chimerism, Why Is One Of The Twins A Hermaphrodite And The Other Not?
This is an extremely rare situation. If one of the twins had not been hermaphrodite, this case of chimerism might have gone unnoticed.
The reason behind one of the twins being hermaphrodite is the variation in the ratio of XX cells and XY cells from tissue to tissue in both twins.
6 What Is Chimerism?
In the language of Zoology, Chimerism is the name given to an animal which has two or more populations of genetically distinct cells that originated in different zygotes. If the different cells emerged from the same zygote, it is called a mosaic. It is rare in humans, with only 40 in number.
Chimeras are formed from four (or two fertilized eggs) or three stem cells (one fertilized egg melts with an infertile egg or a fertilized egg fuses with an extra sperm). Each cell population maintains its own character and the animal is a mixture of the imparting parts.
7 Chimerism In Human Beings
The chimerism in humans occurs when two fertilized eggs fuse before the fourth day of gestation. This fusion mixes the genetic information and undergoes great mutations. If the fusion between the ovules occurs after the fourth day, it will produce Gemini Xiphagos (Siamese twins).
Within four days of gestation, if the fertilized ova are of the same sex, the individual will be born perfect. In case of the eggs being of different sexes, the individual born will be hermaphrodite. In cases where the ovules are of the same sex, the individual lives his entire life without realizing the unusual characteristics. More so because of fewer registered cases.
8 Genetic Information In A Chimera
In a general case, a person carries 50% of the genetic information of the mother and 50% of the genetic information of the father. There also are chimerism individuals who carry 100% of the genetic information of both and with different genetic information in their own body. In the above case, the woman possesses 100% of her father’s DNA in her hair and none of her mother’s. In a skin sample- 100% of the mother’s DNA and no trace of father’s DNA is found. The children are normal and carry 50% of the DNA of the biological father. Hence, chimerism is not hereditary but only a great fruit of chance that can happen to anyone.
You never know you can be a chimera; your children may be…You never know that!
9 Three situations That Occur In Chimerism
Genetic chimerism and its implications for the reliability of DNA tests (deoxyribonucleic acid) have not yet been discussed. At least three similar situations arise. They may be-
(1) Congenital genetic chimerism: This is when an individual holds two distinct genetic codes in his organism. They are distributed randomly in the various organs and tissues, and come from the fusion of two different embryos.
(2) Genetic mosaicism: When irregularity occurs in the initial process of cell division, a single fertilized ovum, after repeated subdivisions, generates a distinct genetic sequence and fuses it in the same individual.
(3) Genetic microchemistry: It occurs when there is exchange of cells by two individuals. They are common in normal gestation between the mother and the fetus; in the gestation of dizygotic twins; in vitro fertilization; or induced in the treatment of leukemia in patients who have undergone bone marrow transplantation.
10 Four Types Of Chimerism
Based on the ways the chimera comes to life, chimerism has been divided into following types-
(1) Tetragametic Chimerism- It is a condition of congenital chimerism which occurs through the fertilization of two separate ova by two sperm, followed by the aggregation at zygote stage. It possesses organs with different sets of chromosomes. This state has implications for organ or stem-cell transplantation.
(2) Microchimerism- It is the presence of small number of cells that are genetically indistinct from the host individual.
(3) Symbiotic chimerism- This type of chimerism occurs in Ceratoid anglerfish and is a natural part of their life. As soon as the male attains adulthood, he starts searching for female, using its strong olfactory receptors. The male is usually less than an inch in size, and bites into the female’s skin. He releases an enzyme that digests the skin of his mouth and her body. The two fuses down the blood vessel level and become a single hermaphroditic anglerfish.
(4) Germline Chimerism- It occurs when the germ cells of an organism are not genetically identical to its own.