Chimera (plant)
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A genetic chimerism or chimera ( ) is a single organism composed of cells with more than one distinct
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
. In animals, this means an individual derived from two or more
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...
s, which can include possessing
blood cell A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), ...
s of different
blood type A blood type (also known as a blood group) is a classification of blood, based on the presence and absence of antibodies and inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrate ...
s, subtle variations in form (
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
) and, if the zygotes were of differing sexes, then even the possession of both female and male
sex organs A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
. Animal chimeras are produced by the merger of two (or more)
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s. In plant chimeras, however, the distinct types of tissue may originate from the same
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...
, and the difference is often due to
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
during ordinary
cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
. Normally, genetic chimerism is not visible on casual inspection; however, it has been detected in the course of proving parentage. Another way that chimerism can occur in animals is by
organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an electronic keyboard instrument ** Hammond ...
transplantation, giving one individual tissues that developed from a different
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
. For example, transplantation of
bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...
often determines the recipient's ensuing
blood type A blood type (also known as a blood group) is a classification of blood, based on the presence and absence of antibodies and inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrate ...
.


Etymology

While German
dermatologist Dermatology is the branch of medicine dealing with the skin.''Random House Webster's Unabridged Dictionary.'' Random House, Inc. 2001. Page 537. . It is a speciality with both medical and surgical aspects. A dermatologist is a specialist medical ...
Alfred Blaschko Alfred Blaschko (4 March 1858 – 26 March 1922) was a German dermatologist who was a native of Freienwalde an der Oder. In 1881 he earned his medical doctorate at Berlin, and afterwards worked with Georg Wegner (1843–1917) in Stettin. Later ...
described
Blaschko's lines Blaschko's lines, also called the lines of Blaschko, are lines of normal cell development in the skin. These lines are only visible in those with a mosaic skin condition or in chimeras where different cell lines contain different genes. These li ...
in 1901, the genetic science took until the 1930s to approach a vocabulary for the phenomenon. The term ''genetic chimera'' has been used at least since the 1944 article of Belgovskii.


Animals

An animal chimera is a single
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
that is composed of two or more different populations of genetically distinct
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
that originated from different
zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...
s involved in
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
. If the different cells have emerged from the same zygote, the organism is called a
mosaic A mosaic is a pattern or image made of small regular or irregular pieces of colored stone, glass or ceramic, held in place by plaster/mortar, and covering a surface. Mosaics are often used as floor and wall decoration, and were particularly pop ...
. Innate chimeras are formed from at least four parent cells (two fertilised eggs or early embryos fused together). Each population of cells keeps its own character and the resulting organism is a mixture of tissues. Cases of human chimerism have been documented. This condition is either innate or it is synthetic, acquired for example through the infusion of
allogeneic Allotransplant (''allo-'' meaning "other" in Ancient Greek, Greek) is the Organ transplant, transplantation of cell (biology), cells, Biological tissue, tissues, or Organ (anatomy), organs to a recipient from a genetically non-identical donor of ...
blood cell A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), ...
s during transplantation or transfusion. In nonidentical twins, innate chimerism occurs by means of blood-vessel
anastomose An anastomosis (, plural anastomoses) is a connection or opening between two things (especially cavities or passages) that are normally diverging or branching, such as between blood vessels, leaf veins, or streams. Such a connection may be normal ...
s. The likelihood of offspring being a chimera is increased if it is created via
in vitro fertilisation In vitro fertilisation (IVF) is a process of fertilisation where an egg is combined with sperm in vitro ("in glass"). The process involves monitoring and stimulating an individual's ovulatory process, removing an ovum or ova (egg or eggs) ...
. 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 Intersex people are individuals born with any of several sex characteristics including chromosome patterns, gonads, or genitals that, according to the Office of the United Nations High Commissioner for Human Rights, "do not fit typical bina ...
differences may result if one set of cells is genetically female and another genetically male.


Tetragametic chimerism

Tetragametic chimerism is a form of congenital chimerism. This condition occurs through the fertilization of two separate ova by two sperm, followed by aggregation of the two at the
blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...
or zygote stages. This results in the development of an organism with intermingled cell lines. Put another way, the chimera is formed from the merging of two nonidentical twins (a similar merging presumably occurs with identical twins, but as their genotypes are not significantly distinct, the resulting individual would not be considered a chimera). As such, they can be male, female, or have mixed intersex characteristics. As the organism develops, it can come to possess
organ Organ may refer to: Biology * Organ (biology), a part of an organism Musical instruments * Organ (music), a family of keyboard musical instruments characterized by sustained tone ** Electronic organ, an electronic keyboard instrument ** Hammond ...
s that have different sets of
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s. For example, the chimera may have a
liver The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
composed of cells with one set of chromosomes and have a
kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...
composed of cells with a second set of chromosomes. This has occurred in humans, and at one time was thought to be extremely rare although more recent evidence suggests that this is not the case. This is particularly true for the
marmoset The marmosets (), also known as zaris or sagoin, are 22 New World monkey species of the genera ''Callithrix'', ''Cebuella'', ''Callibella'', and ''Mico''. All four genera are part of the biological family Callitrichidae. The term "marmoset" is ...
. Recent research shows most marmosets are chimeras, sharing DNA with their
fraternal twin Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
s. 95% of marmoset fraternal twins trade blood through chorionic fusions, making them
hematopoietic Haematopoiesis (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. ...
chimeras. Most chimeras will go through life without realizing they are chimeras. The difference in phenotypes may be subtle (''e.g.'', having a
hitchhiker's thumb Hitchhiker's thumb, also known as ''distal hyperextensibility of the thumb'' is the condition of having a thumb that has a distal phalange that bends backwards in an angle of 90°, it is thought to be inherited in an autosomal recessive manner, h ...
and a straight thumb, eyes of slightly different colors, differential hair growth on opposite sides of the body, etc.) or completely undetectable. Chimeras may also show, under a certain spectrum of UV light, distinctive marks on the back resembling that of arrow points pointing downwards from the shoulders down to the lower back; this is one expression of pigment unevenness called
Blaschko's lines Blaschko's lines, also called the lines of Blaschko, are lines of normal cell development in the skin. These lines are only visible in those with a mosaic skin condition or in chimeras where different cell lines contain different genes. These li ...
. Affected persons may be identified by the finding of two populations of red cells or, if the zygotes are of opposite sex,
ambiguous genitalia Intersex people are individuals born with any of several sex characteristics including chromosome patterns, gonads, or genitals that, according to the Office of the United Nations High Commissioner for Human Rights, "do not fit typical bina ...
and intersex alone or in combination; such persons sometimes also have patchy skin, hair, or eye pigmentation (
heterochromia Heterochromia is a variation in coloration. The term is most often used to describe color differences of the iris, but can also be applied to color variation of hair or skin. Heterochromia is determined by the production, delivery, and concentra ...
). If the blastocysts are of opposite sex,
genitals A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
of both sexes may be formed: either
ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
and
testis A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...
, or combined
ovotestes An ovotestis is a gonad with both testicular and ovarian aspects. In humans, ovotestes are an infrequent anatomical variation associated with gonadal dysgenesis. The only mammals where ovotestes are not symptomatic of an intersex variation are mole ...
, in one rare form of intersex, a condition previously known as ''
true hermaphroditism True hermaphroditism, sometimes referred to as ovotesticular syndrome, is an intersex condition in which an individual is born with both ovarian and testicular tissue. Commonly, one or both gonad A gonad, sex gland, or reproductive gland i ...
''. Note that the frequency of this condition does ''not'' indicate the true prevalence of chimerism. Most chimeras composed of both male and female cells probably do not have an intersex condition, as might be expected if the two cell populations were evenly blended throughout the body. Often, most or all of the cells of a single cell type will be composed of a single cell line, i.e. the blood may be composed predominantly of one cell line, and the internal organs of the other cell line. Genitalia produce the hormones responsible for other sex characteristics. Natural chimeras are almost never detected unless they exhibit abnormalities such as male/female or hermaphrodite characteristics or uneven skin pigmentation. The most noticeable are some male
tortoiseshell cat Tortoiseshell is a cat animal coloration, coat coloring named for its similarity to tortoiseshell material. Like calico cat, calicoes, tortoiseshell cats are almost exclusively female. Male tortoiseshells are rare and are usually sterile.Atkins (2 ...
s and
calico cat A calico cat is a domestic cat of any breed with a tri-color coat. The calico cat is most commonly thought of as being typically 25% to 75% white with large orange and black patches (or sometimes cream and grey patches, which is called a mu ...
s (although most male tortoiseshells have an extra X chromosome responsible for the colouration) or animals with ambiguous sex organs. The existence of chimerism is problematic for
DNA testing Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...
, a fact with implications for family and criminal law. The
Lydia Fairchild Lydia Fairchild (born 1976) is an American woman who exhibits chimera (genetics), chimerism, having two distinct populations of DNA among the cells of her body. She was pregnant with her third child when she and the father of her children, Jamie ...
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 suspected (initially) of not being her children's biological mother, after DNA tests on her adult sons for a kidney transplant she needed, seemed to show she was not their mother. The tetragametic state has important implications for organ or
stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
transplantation. Chimeras typically have
immunologic tolerance Immune tolerance, or immunological tolerance, or immunotolerance, is a state of unresponsiveness of the immune system to substances or tissue that would otherwise have the capacity to elicit an immune response in a given organism. It is induced by ...
to both cell lines.


Microchimerism

Microchimerism is the presence of a small number of cells that are genetically distinct from those of the host individual. Most people are born with a few cells genetically identical to their mothers' and the proportion of these cells goes down in healthy individuals as they get older. People who retain higher numbers of cells genetically identical to their mother's have been observed to have higher rates of some autoimmune diseases, presumably because the immune system is responsible for destroying these cells and a common immune defect prevents it from doing so and also causes autoimmune problems. The higher rates of autoimmune diseases due to the presence of maternally-derived cells is why in a 2010 study of a 40-year-old man with scleroderma-like disease (an autoimmune rheumatic disease), the female cells detected in his blood stream via FISH (fluorescence in situ hybridization) were thought to be maternally-derived. However, his form of microchimerism was found to be due to a vanished twin, and it is unknown whether microchimerism from a vanished twin might predispose individuals to autoimmune diseases as well. Mothers often also have a few cells genetically identical to those of their children, and some people also have some cells genetically identical to those of their siblings (maternal siblings only, since these cells are passed to them because their mother retained them).


Symbiotic chimerism in anglerfish

Chimerism occurs naturally in adult Ceratioid
anglerfish The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence ...
and is in fact a natural and essential part of their life cycle. Once the male achieves adulthood, it begins its search for a female. Using strong olfactory (or smell) receptors, 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 both 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 In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have separ ...
individual. Sometimes in this process, more than one male will attach to a single female as a symbiote. In this case, they will all be consumed into the body of the larger female angler. Once fused to a female, the males will reach sexual maturity, developing large
testicle A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...
s as their other organs
atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...
. 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.


Germline chimerism

Germline chimerism occurs when the germ cells (for example,
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
and
egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
cells) of an organism are not genetically identical to its own. It has been recently discovered that
marmoset The marmosets (), also known as zaris or sagoin, are 22 New World monkey species of the genera ''Callithrix'', ''Cebuella'', ''Callibella'', and ''Mico''. All four genera are part of the biological family Callitrichidae. The term "marmoset" is ...
s can carry the reproductive cells of their (fraternal) twin siblings due to placental fusion during development. (Marmosets almost always give birth to fraternal twins.)


Artificial chimerism

Artificial chimerism falls under the artificial category in which a chimera can exist. An individual that falls under this classification possesses two different sets of genetic pedigrees: one that was inherited genetically at the time of the formation of the human embryo and the other that was intentionally introduced through a medical procedure known as transplantation. Specific types of transplants that could induce this condition include bone marrow transplants and organ transplants, as the recipient's body essentially works to permanently incorporate the new blood stem cells into it. An example of artificial chimerism in animals are the quail-chick chimeras. By utilizing  transplantation and
ablation Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosion, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft materi ...
in the chick embryo stage, the neural tube and the neural crest cells of the chick were ablated, and replaced with the same parts from a quail. Once hatched, the quail feathers were visibly apparent around the wing area, whereas the rest of the chick's body was made of its own chicken cells.


Humans

Chimerism has been documented in humans in several instances. * The Dutch sprinter
Foekje Dillema Foekje Dillema (18 September 1926 – 5 December 2007) was a Dutch track and field athlete. Biography Dillema was named "athlete of the match" in 1949 after winning the 100 metres and 200 metres race during a tournament in London. She was an im ...
was expelled from the 1950 national team after she refused a mandatory sex test in July 1950; later investigations revealed a Y-chromosome in her body cells, and the analysis showed that she was probably a 46,XX/46,XY mosaic female. * In 1953, a human chimera was reported in the ''
British Medical Journal ''The BMJ'' is a weekly peer-reviewed medical trade journal, published by the trade union the British Medical Association (BMA). ''The BMJ'' has editorial freedom from the BMA. It is one of the world's oldest general medical journals. Origi ...
''. A woman was found to have blood containing two different blood types. Apparently this resulted from her twin brother's cells living in her body. A 1996 study found that such blood group chimerism is not rare. * Another report of a human chimera was published in 1998, where a male human had some partially developed female organs due to chimerism. He had been conceived by
in-vitro fertilization In vitro fertilisation (IVF) is a process of fertilisation where an egg is combined with sperm in vitro ("in glass"). The process involves monitoring and stimulating an individual's ovulatory process, removing an ovum or ova (egg or eggs) ...
. * In 2002,
Lydia Fairchild Lydia Fairchild (born 1976) is an American woman who exhibits chimera (genetics), chimerism, having two distinct populations of DNA among the cells of her body. She was pregnant with her third child when she and the father of her children, Jamie ...
was denied public assistance in
Washington state Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washington ...
when DNA evidence appeared to show that she was not the mother of her children. A lawyer for the prosecution heard of a human chimera in New England, Karen Keegan, and suggested the possibility to the defense, who were able to show that Fairchild, too, was a chimera with two sets of DNA, and that one of those sets could have been the mother of the children. * In 2002, an article in the ''New England Journal of Medicine'' described a woman in whom tetragametic chimerism was unexpectedly identified after she underwent preparations for kidney transplant. The transplant required the patient and her immediate family to undergo histocompatibility testing, the result of which suggested that she was not the biological mother of two of her three children. * In 2009, singer Taylor Muhl discovered that what was always thought to be a large birthmark on her torso was actually caused by chimerism. * In 2017, a human-pig chimera was reported to have been created; the chimera was also reported to have 0.001% human cells, with the balance being pig. * In 2021, a human-monkey chimera was created as a joint project between the
Salk Institute The Salk Institute for Biological Studies is a scientific research institute located in the La Jolla community of San Diego, California, U.S. The independent, non-profit institute was founded in 1960 by Jonas Salk, the developer of the polio vacci ...
in the US and
Kunming University Kunming University 昆明学院, founded in 1984, is Yunnan's only comprehensive professional university approved by the Ministry of Education of the People's Republic of China. Kunming University has 3 campuses with a total area of 499 mus. T ...
in China and published in the journal ''Cell''. This involved injecting human stem cells into monkey embryos. The embryos were only allowed to grow for a few days, but the study demonstrated that some of these embryos still had human stem cells surviving at the end of the experiments. Because humans are more closely related to monkeys than other animals, it means there is more chance of the chimeric embryos surviving for longer periods so that organs can develop. The project has opened up possibilities into organ transplantation as well as ethical concerns particularly concerning human brain development in primates.


Chimerism and intersex

* The concept of a "human hermaphrodite" resulting from chimerism is largely a misconception. Most
intersex Intersex people are individuals born with any of several sex characteristics including chromosome patterns, gonads, or genitals that, according to the Office of the United Nations High Commissioner for Human Rights, "do not fit typical bina ...
individuals are not chimeras, and most human chimeras are not observed to have intersex traits. Theoretically, if a
gynandromorphic A gynandromorph is an organism that contains both male and female characteristics. The term comes from the Greek γυνή (''gynē'') 'female', ἀνήρ (''anēr'') 'male', and μορφή (''morphē'') 'form', and is used mainly in the field ...
human chimera were to have fully functioning male and female gonad tissue, popular speculation suggests such an individual could self-fertilize; this hypothesis is backed by the fact that
hermaphroditic In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have separ ...
animal species commonly reproduce in this way. However, no such case of functional self-fertilization has ever been documented in humans; many biologists have concluded that this form of "true hermaphroditism" does not occur in mammals, especially in humans. While humans are known to have sex characteristics that diverge from "true male" or "true female," these individuals fall under the social umbrella of intersex conditions and traits, and typically consider the term "hermaphrodite" to be a slur when applied to them.


Bone marrow recipients

* Several cases of unusual chimera phenomena have been reported in bone marrow recipients. ** In 2019, the blood and seminal fluid of a man in Reno, Nevada (who had undergone a
vasectomy Vasectomy, or vasoligation, is an elective surgical procedure for male sterilization or permanent contraception. During the procedure, the male vasa deferentia are cut and tied or sealed so as to prevent sperm from entering into the urethra and ...
), exhibited only the genetic content of his bone marrow donor. Swabs from his lips, cheek and tongue showed mixed DNA content. ** The DNA content of semen from an assault case in 2004 matched that of a man who had been in prison at the time of the assault, but who had been a bone marrow donor for his brother, who was later determined to have committed the crime. ** In 2008, a man was killed in a traffic accident that occurred in Seoul, South Korea. In order to identify him, his DNA was analyzed. Results revealed that the DNA of his blood, along with some of his organs, appeared to show that he was female. It was later determined that he had received a bone marrow transplant from his daughter.


Chimera identification

Chimerism is so rare that there have only been 100 confirmed cases in humans. However, this may be due to the fact that humans might not be aware that they have this condition to begin with. There are usually no signs or symptoms for chimerism other than a few physical symptoms such as hyper-pigmentation, hypo-pigmentation, or possessing two different colored eyes. However, these signs do not necessarily mean an individual is a chimera and should only be seen as possible symptoms. Again, forensic investigation or curiosity over a failed maternity/paternity DNA test usually leads to the accidental discovery of this condition. By simply undergoing a DNA test, which usually consists of either a swift cheek swab or a blood test, the discovery of the once unknown second genome is made, therefore identifying that individual as a chimera.


Research

The first known primate chimeras are the rhesus monkey twins, Roku and Hex, each having six genomes. They were created by mixing cells from
totipotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...
four-cell morulas; although the cells never fused, they worked together to form organs. It was discovered that one of these primates, Roku, was a sexual chimera; as four percent of Roku's blood cells contained two x chromosomes. A major milestone in chimera experimentation occurred in 1984 when a chimeric sheep–goat was produced by combining
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s from a
goat The goat or domestic goat (''Capra hircus'') is a domesticated species of goat-antelope typically kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the a ...
and a
sheep Sheep or domestic sheep (''Ovis aries'') are domesticated, ruminant mammals typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus ''Ovis'', in everyday usage it almost always refers to domesticated s ...
, and survived to adulthood. In August 2003, researchers at the
Shanghai Second Medical University The Shanghai Jiao Tong University School of Medicine (SJTUSM, ), formerly Shanghai Second Medical University, is a public medical school in Shanghai, China. Clinical medicine of SJTUSM is consistently ranked first among medical schools nationa ...
in China reported that they had successfully fused human skin cells and
rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit speci ...
ova to create the first human chimeric embryos. The embryos were allowed to develop for several days in a laboratory setting, and then destroyed to harvest the resulting
stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s. In 2007, scientists at the
University of Nevada The University of Nevada, Reno (Nevada, the University of Nevada, or UNR) is a Public university, public Land-grant university, land-grant research university in Reno, Nevada. It is the state's flagship public university and primary land grant ...
School of Medicine created a sheep whose blood contained 15% human cells and 85% sheep cells. On January 22, 2019 th
National Society of Genetic Counselors
released an article — Chimerism Explained: How One Person Can Unknowingly Have Two Sets of DNA, where they state “Tetragametic Chimerism, where a twin pregnancy evolves into one child, is currently believed to be one of the rarer forms. However, we know that 20 to 30% of singleton pregnancies were originally a twin or a multiple pregnancy. Due to this statistic, it is quite possible that tetragametic chimerism is more common than current data implies”.


Sponges

Chimerism has been found in some species of marine sponges. Four distinct genotypes have been found in a single individual, and there is potential for even greater genetic heterogeneity. Each genotype functions independently in terms of reproduction, but the different intra-organism genotypes behave as a single large individual in terms of ecological responses like growth.


Mice

Chimeric mice are important animals in biological research, as they allow for the investigation of a variety of biological questions in an animal that has two distinct genetic pools within it. These include insights into problems such as the tissue specific requirements of a gene, cell lineage, and cell potential. The general methods for creating chimeric mice can be summarized either by injection or aggregation of embryonic cells from different origins. The first chimeric mouse was made by
Beatrice Mintz Beatrice Mintz (January 24, 1921 – January 3, 2022) was an American embryologist who contributed to the understanding of genetic modification, cellular differentiation, and cancer, particularly melanoma.Martha J. Bailey, ''American women in ...
in the 1960s through the aggregation of eight-cell-stage embryos. Injection on the other hand was pioneered by Richard Gardner and Ralph Brinster who injected cells into blastocysts to create chimeric mice with germ lines fully derived from injected
embryonic stem cells Embryonic stem cells (ESCs) are pluripotent stem cells derived from the inner cell mass of a blastocyst, an early-stage pre- implantation embryo. Human embryos reach the blastocyst stage 4–5 days post fertilization, at which time they co ...
(ES cells). Chimeras can be derived from mouse embryos that have not yet implanted in the uterus as well as from implanted embryos. ES cells from the inner cell mass of an implanted blastocyst can contribute to all cell lineages of a mouse including the germ line. ES cells are a useful tool in chimeras because genes can be mutated in them through the use of
homologous recombination Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids (usually DNA as in cellular organisms but may ...
, thus allowing
gene targeting Gene targeting (also, replacement strategy based on homologous recombination) is a genetic technique that uses homologous recombination to modify an endogenous gene. The method can be used to delete a gene, remove exons, add a gene and modify ind ...
. Since this discovery occurred in 1988, ES cells have become a key tool in the generation of specific chimeric mice.


Underlying biology

The ability to make mouse chimeras comes from an understanding of early mouse development. Between the stages of fertilization of the egg and the implantation of a blastocyst into the uterus, different parts of the mouse embryo retain the ability to give rise to a variety of cell lineages. Once the embryo has reached the blastocyst stage, it is composed of several parts, mainly the
trophectoderm The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the ...
, the
inner cell mass The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of ...
, and the
primitive endoderm In amniote embryology, the hypoblast, is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst, or from the blastodisc in reptiles and birds. The hypoblast gives rise to the yolk sac, which in turn gives rise to ...
. Each of these parts of the blastocyst gives rise to different parts of the embryo; the inner cell mass gives rise to the embryo proper, while the trophectoderm and primitive endoderm give rise to extra embryonic structures that support growth of the embryo. Two- to eight-cell-stage embryos are competent for making chimeras, since at these stages of development, the cells in the embryos are not yet committed to give rise to any particular cell lineage, and could give rise to the inner cell mass or the trophectoderm. In the case where two diploid eight-cell-stage embryos are used to make a chimera, chimerism can be later found in the
epiblast In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is t ...
, primitive
endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...
, and trophectoderm of the mouse
blastocyst The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called the t ...
. It is possible to dissect the embryo at other stages so as to accordingly give rise to one lineage of cells from an embryo selectively and not the other. For example, subsets of blastomeres can be used to give rise to chimera with specified cell lineage from one embryo. The Inner Cell Mass of a diploid blastocyst, for example, can be used to make a chimera with another blastocyst of eight-cell diploid embryo; the cells taken from the inner cell mass will give rise to the primitive endoderm and to the epiblast in the chimera mouse. From this knowledge, ES cell contributions to chimeras have been developed. ES cells can be used in combination with eight-cell-and two-cell-stage embryos to make chimeras and exclusively give rise to the embryo proper. Embryos that are to be used in chimeras can be further genetically altered in order to specifically contribute to only one part of chimera. An example is the chimera built off of ES cells and tetraploid embryos, which are artificially made by electrofusion of two two-cell diploid embryos. The tetraploid embryo will exclusively give rise to the trophectoderm and primitive endoderm in the chimera.


Methods of production

There are a variety of combinations that can give rise to a successful chimera mouse and according to the goal of the experiment an appropriate cell and embryo combination can be picked; they are generally but not limited to diploid embryo and ES cells, diploid embryo and diploid embryo, ES cell and tetraploid embryo, diploid embryo and tetraploid embryo, ES cells and ES cells. The combination of embryonic stem cell and diploid embryo is a common technique used for the making of chimeric mice, since gene targeting can be done in the embryonic stem cell. These kinds of chimeras can be made through either aggregation of stem cells and the diploid embryo or injection of the stem cells into the diploid embryo. If embryonic stem cells are to be used for gene targeting to make a chimera, the following procedure is common: a construct for homologous recombination for the gene targeted will be introduced into cultured mouse embryonic stem cells from the donor mouse, by way of electroporation; cells positive for the recombination event will have antibiotic resistance, provided by the insertion cassette used in the gene targeting; and be able to be positively selected for. ES cells with the correct targeted gene are then injected into a diploid host mouse blastocyst. Then, these injected blastocysts are implanted into a pseudo pregnant female surrogate mouse, which will bring the embryos to term and give birth to a mouse whose germline is derived from the donor mouse's ES cells. This same procedure can be achieved through aggregation of ES cells and diploid embryos, diploid embryos are cultured in aggregation plates in wells where single embryos can fit, to these wells ES cells are added the aggregates are cultured until a single embryo is formed and has progressed to the blastocyst stage, and can then be transferred to the surrogate mouse.


Plants


Structure

The distinction between sectorial, mericlinal and periclinal plant chimeras are widely used.


Graft chimeras

These are produced by grafting genetically different parents, different
cultivar A cultivar is a type of cultivated plant that people have selected for desired traits and when propagated retain those traits. Methods used to propagate cultivars include: division, root and stem cuttings, offsets, grafting, tissue culture, ...
s or different species (which may belong to different genera). The tissues may be partially fused together following
grafting Grafting or graftage is a horticultural technique whereby tissues of plants are joined so as to continue their growth together. The upper part of the combined plant is called the scion () while the lower part is called the rootstock. The succ ...
to form a single growing organism that preserves both types of tissue in a single shoot. Just as the constituent species are likely to differ in a wide range of features, so the behavior of their
periclinal In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the l ...
chimeras is like to be highly variable. The first such known chimera was probably the
Bizzaria The Bizzaria of Florence (''Citrus medica'' + ''C. aurantium''), which is probably the first graft chimera obtained, is a graft between the Florentine citron and sour orange. It produces branches of regular Florentine citron including such le ...
, which is a fusion of the
Florentine citron The Florentine citron – citron hybrid of Florence ( it, cedrato di Firenze) – is a very fragrant citrus fruit, which is named after its most known origin of cultivation. Its scientific name is ''Citrus × limonimedica'' 'Florentina' Lush. ...
and the
sour orange Bitter orange, Seville orange, bigarade orange, or marmalade orange is the citrus tree ''Citrus'' × ''aurantium'' and its fruit. It is native to Southeast Asia and has been spread by humans to many parts of the world. It is probably a cross bet ...
. Well-known examples of a graft-chimera are ''Laburnocytisus'' 'Adamii', caused by a fusion of a ''
Laburnum ''Laburnum'', sometimes called golden chain or golden rain, is a genus of two species of small trees in the subfamily Faboideae of the pea family Fabaceae. The species are '' Laburnum anagyroides''—common laburnum and '' Laburnum alpinum''— ...
'' and a
broom A broom (also known in some forms as a broomstick) is a cleaning tool consisting of usually stiff fibers (often made of materials such as plastic, hair, or corn husks) attached to, and roughly parallel to, a cylindrical handle, the broomstick. I ...
, and "Family" trees, where multiple varieties of apple or pear are grafted onto the same tree. Many fruit trees are cultivated by grafting the body of a sapling onto a
rootstock A rootstock is part of a plant, often an underground part, from which new above-ground growth can be produced. It could also be described as a stem with a well developed root system, to which a bud from another plant is grafted. It can refer to a ...
.


Chromosomal chimeras

These are chimeras in which the layers differ in their
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
constitution. Occasionally, chimeras arise from loss or gain of individual chromosomes or chromosome fragments owing to misdivision. More commonly cytochimeras have simple multiple of the normal chromosome complement in the changed layer. There are various effects on cell size and growth characteristics.


Nuclear gene-differential chimeras

These chimeras arise by spontaneous or induced mutation of a nuclear gene to a dominant or recessive allele. As a rule, one character is affected at a time in the leaf, flower, fruit, or other parts.


Plastid gene-differential chimeras

These chimeras arise by spontaneous or induced mutation of a plastid gene, followed by the sorting-out of two kinds of plastid during vegetative growth. Alternatively, after selfing or
nucleic acid thermodynamics Nucleic acid thermodynamics is the study of how temperature affects the nucleic acid structure of double-stranded DNA (dsDNA). The melting temperature (''Tm'') is defined as the temperature at which half of the DNA strands are in the random coil o ...
, plastids may sort-out from a mixed egg or mixed zygote respectively. This type of chimera is recognized at the time of origin by the sorting-out pattern in the leaves. After sorting-out is complete, periclinal chimeras are distinguished from similar looking nuclear gene-differential chimeras by their
non-mendelian inheritance Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each paren ...
. The majority of variegated-leaf chimeras are of this kind. All plastid gene- and some nuclear gene-differential chimeras affect the color of the plasmids within the leaves, and these are grouped together as
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
chimeras, or preferably as variegated leaf chimeras. For most variegation, the mutation involved is the loss of the
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s in the mutated tissue, so that part of the plant tissue has no green pigment and no
photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...
ability. This mutated tissue is unable to survive on its own, but it is kept alive by its partnership with normal photosynthetic tissue. Sometimes chimeras are also found with layers differing in respect of both their nuclear and their plastid genes.


Origins

There are multiple reasons to explain the occurrence of plant chimera during plant recovery stage: (1) The process of shoot
organogenesis Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal orga ...
starts form the multicellular origin. (2) The endogenous tolerance leads to the ineffectiveness of the weak selective agents. (3) A self-protection mechanism (cross protection). Transformed cells serve as guards to protect the untransformed ones. (4) The observable characteristic of transgenic cells may be a transient expression of the marker gene. Or it may due to the presence of agrobacterium cells.


Detection

Untransformed cells should be easy to detect and remove to avoid chimeras. This is because it is important to maintain the stable ability of the transgenic plants across different generations. Reporter genes such as
GUS Gus is a masculine name, often a diminutive for Angus, August, Augustine, or Augustus, and other names (e.g. Aengus, Argus, Fergus, Ghassan, Gustav, Gustave, Gustafson, Gustavo, Gussie). It can also be used as the adaptation into English of ...
and
Green Fluorescent Protein The green fluorescent protein (GFP) is a protein that exhibits bright green fluorescence when exposed to light in the blue to ultraviolet range. The label ''GFP'' traditionally refers to the protein first isolated from the jellyfish ''Aequorea ...
(GFP) are utilized in combination with plant selective markers (herbicide, antibody etc.) However, GUS expression depends on the plant development stage and GFP may be influenced by the green tissue autofluorescence.
Quantitative PCR A real-time polymerase chain reaction (real-time PCR, or qPCR) is a laboratory technique of molecular biology based on the polymerase chain reaction (PCR). It monitors the amplification of a targeted DNA molecule during the PCR (i.e., in real ...
could be an alternative method for chimera detection.


Viruses

In 2012, the first example of a naturally-occurring RNA-DNA hybrid virus was unexpectedly discovered during a
metagenomic Metagenomics is the study of genetic material recovered directly from environmental or clinical samples by a method called sequencing. The broad field may also be referred to as environmental genomics, ecogenomics, community genomics or microb ...
study of the acidic extreme environment of Boiling Springs Lake that is in
Lassen Volcanic National Park Lassen Volcanic National Park is an American national park in northeastern California. The dominant feature of the park is Lassen Peak, the largest lava dome, plug dome volcano in the world and the southernmost volcano in the Cascade Range. L ...
, California.Thompson, Helen (20 April 2012)
"Hot spring yields hybrid genome: Researchers discover natural chimaeric DNA-RNA virus".
''Nature''. Retrieved 27 March 2020.
The virus was named BSL-RDHV (Boiling Spring Lake RNA DNA Hybrid Virus).Devor, Caitlin (12 July 2012
"Scientists discover hybrid virus".
''Journal of Young Investigators''. Retrieved 31 March 2020.
Its
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
is related to a DNA
circovirus ''Circovirus'' is a genus of viruses, in the family ''Circoviridae''. Birds (such as pigeons and ducks) and pigs serve as natural hosts, though dogs have been shown to be infected as well. It is a single stranded DNA virus (ssDNA). There are 49 ...
, which usually infect birds and pigs, and a RNA
tombusvirus ''Tombusvirus'' is a genus of viruses, in the family ''Tombusviridae''. Plants serve as natural hosts. There are 17 species in this genus. Symptoms associated with this genus include mosaic. The name of the genus comes from ''Tomato bushy stunt ...
, which infect plants. The study surprised scientists, because DNA and RNA viruses vary and the way the chimera came together was not understood.BioMed Central Limited (18 April 2012)
"Could a newly discovered viral genome change what we thought we knew about virus evolution?".
''ScienceDaily''. Retrieved March 31, 2020.
Other viral chimeras have also been found, and the group is known as the CHIV viruses ("chimeric viruses").


Ethics and legislation


Ethics

The US and Western Europe have strict codes of ethics and regulations in place that expressly forbid certain subsets of experimentation using human cells, though there is a vast difference in the regulatory framework. Through the creation of human chimeras comes the question: where does society now draw the line of humanity? This question poses serious legal and moral issues, along with creating controversy. Chimpanzees, for example, are not offered any legal standing, and are put down if they pose a threat to humans. If a chimpanzee is genetically altered to be more similar to a human, it may blur the ethical line between animal and human. Legal debate would be the next step in the process to determine whether certain chimeras should be granted legal rights. Along with issues regarding the rights of chimeras, individuals have expressed concern about whether or not creating human chimeras diminishes the "dignity" of being human.


Legislation


The Human Chimera Prohibition Act

On 11 July 2005, a bill known as The Human Chimera Prohibition Act, was introduced into the
United States Congress The United States Congress is the legislature of the federal government of the United States. It is bicameral, composed of a lower body, the House of Representatives, and an upper body, the Senate. It meets in the U.S. Capitol in Washing ...
by Senator
Samuel Brownback Samuel Dale Brownback (born September 12, 1956) is an American attorney, politician, diplomat, and member of the Republican Party (United States), Republican Party who served as the United States Ambassador-at-Large for International Religious Fr ...
; however, it died in Congress sometime in the next year. The bill was introduced based on the findings that science has progressed to the point where the human and nonhuman species can be merged to create new forms of life. Because of this, serious ethical issues arise as this blurs the line between humans and other animals, and according to the bill with this blurring of the lines comes a show of disrespect for human dignity. The final claim brought up in The Human Chimera Prohibition Act was that there is an increasing amount of zoonotic diseases. With that being said, the creation of human-animal chimeras can allow these diseases to reach humans. On 22 August 2016, another bill, The Human-Animal Chimera Prohibition Act of 2016, was introduced to the
United States House of Representatives The United States House of Representatives, often referred to as the House of Representatives, the U.S. House, or simply the House, is the Lower house, lower chamber of the United States Congress, with the United States Senate, Senate being ...
by
Christopher H. Smith Christopher Henry Smith (born March 4, 1953) is an American politician serving his 21st term as the United States House of Representatives, U.S. representative for . Though it has taken various forms, his district has always been situated in Centr ...
. It identifies a chimera as: * a human embryo into which a nonhuman cell or cells (or the component parts thereof) have been introduced to render the embryo's membership in the species Homo sapiens uncertain; * a chimera human/animal embryo produced by fertilizing a human egg with nonhuman sperm; * a chimera human/animal embryo produced by fertilizing a nonhuman egg with human sperm; * an embryo produced by introducing a nonhuman nucleus into a human egg; * an embryo produced by introducing a human nucleus into a nonhuman egg; * an embryo containing at least haploid sets of chromosomes from both a human and a nonhuman life form; * a nonhuman life form engineered such that human gametes develop within the body of a nonhuman life form; or * a nonhuman life form engineered such that it contains a human brain or a brain derived wholly or predominantly from human neural tissues. The bill prohibits the attempts to create a human-animal chimera, the transfer or attempt to transfer a human embryo into a nonhuman womb, the transfer or attempt to transfer a nonhuman embryo into a human womb, and the transport or receive of any purpose of an animal chimera. Penalties for violations of this bill include fines and/or imprisonment of up to 10 years. The bill was referred to the Subcommittee on Crime, Terrorism, Homeland Security, and Investigations on October 11, 2016, but died there.


See also

* 46,XX/46,XY * Chimera (molecular biology) *
Conjoined twins Conjoined twins – sometimes popularly referred to as Siamese twins – are twins joined ''in utero''. A very rare phenomenon, the occurrence is estimated to range from 1 in 49,000 births to 1 in 189,000 births, with a somewhat higher incidence ...
*
Genetic chimerism in fiction Human Chimera (genetics), genetic chimerism, which can not only cause a wide range of illnesses but also lead to the same person having more than one profile in genetic fingerprinting, has served as a plot device in many works of fiction. Most known ...
*
Half-sider budgerigar A half-sider budgerigar is an unusual budgerigar variant that is one color on one side of its body and a different color on the other side, divided bilaterally down the center. This is not a simple genetic mutation, as can be observed in other bu ...
*
Retron A retron is a distinct DNA sequence found in the genome of many bacteria species that codes for reverse transcriptase and a unique single-stranded DNA/RNA hybrid called multicopy single-stranded DNA (msDNA). Retron msr RNA is the non-coding RNA ...
*
Vanishing twin A vanishing twin, also known as twin resorption, is a fetus in a multigestation pregnancy that dies ''in utero ''and is then partially or completely reabsorbed. In some instances, the dead twin is compressed into a flattened, parchment-like sta ...
*
X-inactivation X-inactivation (also called Lyonization, after English geneticist Mary Lyon) is a process by which one of the copies of the X chromosome is inactivated in therian female mammals. The inactive X chromosome is silenced by being packaged into ...
(lyonization)


References


Further reading

* * Appel, Jacob M. "The Monster's Law", ''
Genewatch The Council for Responsible Genetics (CRG) was a nonprofit NGO with a focus on biotechnology. History The Council for Responsible Genetics was founded in 1983 in Cambridge, Massachusetts. An early voice concerned about the social and ethica ...
'', Volume 19, Number 2, March–April 2007. * Nelson, J. Lee (Scientific American, February 2008)
Your Cells Are My Cells
* Weiss, Rick (August 14, 2003)

. ''
The Washington Post ''The Washington Post'' (also known as the ''Post'' and, informally, ''WaPo'') is an American daily newspaper published in Washington, D.C. It is the most widely circulated newspaper within the Washington metropolitan area and has a large nati ...
''. * Weiss, Rick (February 13, 2005). ttps://www.washingtonpost.com/wp-dyn/articles/A19781-2005Feb12.html ''U.S. Denies Patent for a too-human hybrid'' ''
The Washington Post ''The Washington Post'' (also known as the ''Post'' and, informally, ''WaPo'') is an American daily newspaper published in Washington, D.C. It is the most widely circulated newspaper within the Washington metropolitan area and has a large nati ...
''. * * *


External links


"Chimerism Explained"

Chimerism and cellular mosaicism
Genetic Home Reference, U.S. National Library of Medicine, National Institute of Health.

* ttp://aggie-horticulture.tamu.edu/tisscult/chimeras/chimera.html Plant Chimeras in Tissue Culture* Ainsworth, Claire (November 15, 2003)
"The Stranger Within"
''
New Scientist ''New Scientist'' is a magazine covering all aspects of science and technology. Based in London, it publishes weekly English-language editions in the United Kingdom, the United States and Australia. An editorially separate organisation publishe ...
'' . (Reprinted her


Embryogenesis of chimeras, twins and anterior midline asymmetries

Natural human chimeras: A review
{{DEFAULTSORT:Chimera (Genetics) Reproduction Intersex and medicine Genetic anomalies Twin