Chimerism is when two embryos formed as a result of the development of two eggs fertilized separately by two spermatozoa, instead of forming twin siblings, merge at an early stage of development and are born as a single organism. Chimerism, which can occur in humans, other animals and plants, can give living things various characteristics.
Genetic chimerism is the biological term for the multiplication of cells from several zygotes to form a single organism. In animals, chimeras are formed by the fusion of fertilized eggs. In plants, mutations during cell division can lead to the formation of different types of structures from the same zygote. The name chimerism is derived from the mythological creature called chimera in Greek mythology, which is the result of the fusion of animal body parts into a single body. Even if this creature is a figment of the imagination, chimerism is a health problem in our time.
What are the Symptoms of Chimerism?
Chimerism is a very rare type of genetic defect that occurs when a person carries more than one DNA in their body. Usually, this difference does not cause symptoms, but the carrier's body may develop differences such as asymmetry and deformity or, in more rare cases, hermaphroditism.
This is different from mosaicism, in which the individual shows genetic differences due to mutations in the genetic material. Chimerism during and after pregnancy is biologically possible. Identical cases can also occur in fraternal twins. One twin can take cells from the other twin into its own body and make them part of a tissue or organ. However, when one twin dies in the womb, the other twin can take the cells of the deceased twin into its own body. This is called 'twin chimerism'.
What are the Types of Chimerism?
Chimerism is not a condition that can always be detected by external appearance, but it can be determined by examining the person and the family by experts in the field. We can list the types of chimerism as follows;
Tetra gametic chimerism: This is the most common type of chimerism. Such a structure consists of 4 gametes, hence the name tetra-gametic chimerism. This organism carries the cells and tissues of each of the twins. As a result, the chimeric individual born carries the DNA of the unborn twin.
Germline chimerism: Occurs when the germ cells of an organism are not genetically identical to themselves. Recently, it has been discovered that, due to placental fusion during development, marmosets may carry the germ cells of their twin siblings.
Chromosomal chimerism: This is when two different eggs fertilized separately by two different spermatozoa develop into two embryos that fuse early in development to form a single embryo.
Chimerism Test
Chimerism testing is a diagnostic test that is performed after an allo-stem cell transplantation procedure in which lymph nodes and blood cells from the donor are found in the recipient. This examination of blood and bone marrow samples provides important information about the risk of underlying disorders.