Chimera – Genetics

Chimera genetics refers to an organism containing cells with different genetic compositions. In genetics, human chimeras are individuals with cells from two or more genetically distinct sources. Identification of chimerism in humans involves genetic testing of multiple tissues or cells to detect variations in DNA profiles.

Chimerism can occur naturally through processes like twinning or fusion of embryos, and artificially through procedures like organ transplantation or gene therapy. Chimerism has applications in various fields like forensics, transplantation, and reproductive medicine. In this article, we will study the meaning of chimera, its formation, chimera in humans and its identification in detail.

In genetics, a chimera is an organism or tissue that contains at least two different sets of DNA from the fusion of different zygotes (fertilized eggs). The term comes from the Greek mythological figure chimera, a fire-breathing hybrid creature made up of parts from more than one animal. This can occur naturally or be induced artificially through techniques such as cell transplantation or genetic engineering.

Chimeras can exhibit a range of phenotypic characteristics, reflecting the contributions of the different cell populations within their bodies. These differences can manifest in various tissues, organs, or even whole body structures, leading to mosaicism. Chimeras are valuable research tools in genetics and developmental biology and also have applications in fields such as regenerative medicine and organ transplantation.

Also Read: Fertilizations And Implantation

Chimerism typically arises from the fusion of multiple embryos during early development or the exchange of cells between twin embryos in utero. Different types of chimerism have slightly different causes and symptoms.

Tetragametic Chimerism

Tetragametic chimerism is a rare condition where an individual possesses cells from at least four genetically distinct parents. It occurs during fertilization when two separate eggs are fertilized by two different sperm, resulting in the formation of fraternal twins. Subsequently, the embryos fuse in early development, leading to a single individual with cells from both embryos.

1. Tetragametic chimeras have two different sets of DNA, with one set originating from each twin. This can result in a mosaic of traits throughout the body.
2. Diagnosis occurs incidentally during genetic testing, such as when discrepancies are found between different tissue samples or during paternity testing.
3. Tetragametic chimerism can lead to variations in physical appearance, organ function, and genetic markers across different parts of the body.
4. While tetragametic chimerism is usually benign, it can complicate medical diagnoses and forensic investigations due to discrepancies in DNA profiles from different tissues.
5. The existence of tetragametic chimerism raises ethical questions regarding individual identity, reproductive rights, and legal parentage.

Microchimerism

Microchimerism occurs when a small number of cells carrying different genetic material from another individual persist within the body of a host organism. Microchimerism can occur naturally during pregnancy when fetal cells cross the placenta and enter the maternal circulation, leading to maternal-fetal microchimerism. It can also arise from blood transfusions, organ transplantation, or maternal cells entering the fetus during pregnancy. There are two main types of microchimerism:

• Maternal-Fetal Microchimerism: Fetal cells (from the fetus) persisting in the mother’s body after pregnancy.
• Fetal-Maternal Microchimerism: Maternal cells (from the mother) persisting in the fetus or newborn.

Microchimerism has been implicated in both beneficial and detrimental effects on health. It may contribute to immune tolerance during pregnancy and enhance maternal immune responses. However, it has also been associated with autoimmune diseases, transplant rejection, and certain cancers. Microchimerism is detected the through various techniques, including PCR-based methods to identify specific genetic markers unique to the microchimeric cells.

Also Read: Genome Editing – Definition, Techniques & Applications

Twin Chimerism

Twin chimerism occurs when twins exchange cells with each other while still in the womb, leading to a blend of their genetic material. This can result in an individual having cells with different DNA, potentially causing variations in traits or characteristics. Twin chimerism shows how interactions between siblings during development can influence an individual’s biological makeup. It highlights the complexity of human biology and the unique ways in which genetic diversity can manifest within individuals.

Artifical Chimerism

Artificial chimeras can be created through laboratory manipulation, such as:

1. Cell Transplantation: Genetically modified cells or tissues can be transplanted into an organism, integrating with the host’s cells and contributing to its genetic makeup.
2. Genetic Engineering: Techniques such as gene editing can be used to introduce specific genetic alterations into an organism’s cells, resulting in chimerism with both modified and unmodified cells.

Symbiotic chimerism in anglerfish refers to a unique mating strategy where a small male fuses permanently with a larger female. The male anglerfish fuses with the female, becoming a parasitic appendage known as a “sexual parasite.” The male loses its digestive organs, eyes, and other structures, as well as its DNA, depeninng entirely on the female for nutrition and reproductive functions.

Despite the loss of its individual identity, the male continues to provide sperm to fertilize the female’s eggs, ensuring the continuation of the species. Symbiotic chimerism allows the male to benefit from the female’s resources and protection, while the female gains a continuous source of sperm for reproduction.

An animal chimera is a single organism composed of cells from two or more genetically distinct individuals. Chimeras can occur naturally through the fusion of embryos or be created artificially through techniques like cell transplantation or genetic engineering.

• Chimeras exhibit a mosaic of traits, with different cell populations retaining their own genetic characteristics.
• At least four parent cells are involved in the formation of chimeras, often through the fusion of two fertilized eggs or early embryos.
• Animal chimeras are used in biomedical research to study development, disease mechanisms, and potential therapies.
• There are ethical and regulatory considerations surrounding the creation and use of animal chimeras, particularly regarding their welfare and potential implications for human health.
• Common examples include mice with humanized immune systems for disease research and pigs with human organs for transplantation.

Also Read: Genetic Linkage

Genetic chimerism in humans refers to a rare condition where an individual has cells with different genetic makeups, resulting from the fusion of two or more embryos during early development. This can occur naturally:

• When fraternal twin embryos fuse in the womb, leading to an individual with cells from both embryos. Chimeras may exhibit a mosaic of traits, with different parts of their bodies having distinct genetic compositions.
• From processes such as the persistence of fetal cells in the mother’s body after pregnancy, known as maternal-fetal microchimerism.

In addition to macrochimerism involving entire tissues or organs, microchimerism involves the presence of a small number of foreign cells in an individual’s body, often from a different individual. While most cases of chimerism are asymptomatic, rare cases may present with medical conditions such as autoimmune diseases or disorders related to blood or organ transplants.

Diagnosis mostly happens incidentally during genetic testing, such as when discrepancies are found between different tissue samples or during paternity testing. While chimerism is usually benign, it can complicate medical diagnoses and forensic investigations due to variations in DNA profiles from different tissues.

Chimerism raises ethical questions regarding individual identity, reproductive rights, and legal parentage, especially in cases where it affects medical or legal decisions. Chimerism may also have psychosocial implications for individuals and families, raising questions about identity, family relationships, and personal history.

Germline chimeras are organisms with genetically different cells in their reproductive cells (germline), resulting from genetic engineering techniques. Germline chimeras are created through methods such as gene editing, where modified cells are introduced into early-stage embryos or germ cells. Genetically modified cells incorporated into the germline can be passed on to offspring, resulting in heritable genetic modifications. Germline chimeras are valuable tools for studying gene function, disease mechanisms, and various therapies in animal models.

However, the creation of germline chimeras raises ethical concerns regarding safety, unintended consequences, and implications for future generations. Germline chimeras hold promise for advancing fields such as regenerative medicine, transgenesis, and disease modeling, but careful consideration of ethical and societal implications is essential.

Also Read: Genetic Linkage

Chimera identification is a complex process that requires careful analysis and interpretation of genetic data to distinguish between true chimeras and other genetic anomalies. Chimera identification can be done by:

• Genetic Testing: Chimera identification involves genetic testing of various tissues to detect differences in DNA profiles.
• Discrepancies in DNA: Discrepancies between DNA profiles from different tissues may indicate the presence of chimerism.
• Techniques: Various techniques, such as polymerase chain reaction (PCR), DNA sequencing, and microarray analysis, may be used to detect and analyze genetic differences.
• Tissue Sampling: Samples may be collected from different parts of the body, including blood, skin, hair follicles, and buccal swabs, for comparison.
• Incidental Discovery: Chimerism is sometimes identified incidentally during medical tests, such as paternity testing or prenatal screening.
• Confirmation: Confirmation of chimera status may require additional testing and analysis to rule out other possible explanations for genetic discrepancies.
• Forensic Applications: Chimera identification has forensic applications, particularly in cases where DNA evidence from different tissues may not match due to the presence of multiple cell populations.

Advantages of Chimera are:

1. Chimeric organisms help study human diseases and test therapies.
2. Chimeras offer a potential solution for organ shortages by growing humanized organs in animals.
3. They help in understanding stem cell behavior and tissue regeneration processes.
4. Chimeras facilitate gene function studies and testing of gene editing techniques.
5. Natural chimeras provide insights into evolutionary processes and genetic diversity.

Disadvantages of chimerism are:

1. Manipulating genetic composition raises ethical dilemmas regarding the treatment of animals and the creation of human-animal hybrids.
2. Chimeric research requires strict oversight and regulation to ensure ethical standards, safety, and animal welfare.
3. Transplanted organs from chimeric animals may still face rejection by the recipient’s immune system, despite efforts to humanize them.
4. Understanding the interactions between different genetic compositions within chimeric organisms can be challenging and may lead to unforeseen consequences.
5. There may be public resistance or ethical objections to the creation and use of chimeric organisms, affecting research funding and societal acceptance.

Chimera genetics includes various phenomena, from natural occurrences like symbiotic chimerism in anglerfish to artificial formation like germline chimeras in genetic engineering. Genetic chimerism in humans, including tetragametic and microchimerism, shows the complexity of genetic variation and its implications for health and identity. Identification of chimeras involves genetic testing of various tissues, diagnosing discrepancies in DNA profiles.

Also Read:

• Non-communicable Disease 
• Gene Mapping
• Difference Between Gene and DNA
• Recombinant DNA Technology
• Difference between Transformants and Recombinants

What is Chimerism in Genetics?

Chimerism in genetics refers to an organism containing cells with different genetic compositions, often resulting from the fusion of multiple embryos or genetic manipulation.

Define Heterochromia Iridum?

Heterochromia iridum is a condition characterized by variations in eye color within a single individual.

Is it True that a Mule is a Chimaera?

Yes, mules can be considered chimeras because they are hybrids resulting from the fusion of genetically distinct horse and donkey embryos.

How Common are Human Chimeras?

The prevalence of human chimeras is not precisely known, but it’s considered rare in the general population.

Are Chimeras Only Female?

No, chimeras can occur in both males and females.

What are the Disadvantages of Chimerism?

One disadvantage of chimerism is potential complications in medical diagnoses and forensic investigations due to discrepancies in DNA profiles from different tissues.