The terms chimera and mosaic are both used to describe people with two sets of DNA in their cells. Chimerism is caused by the fusing of more than one zygote, and results in a person with more than one genetic identity. Mosaicism refers to DNA differences that arose from only one zygote.
Chimerism in Humans — When Embryos Fuse
When the embryos of dizygotic (fraternal) twins are fused, the result is a chimera, a person with at least two genetically distinct sets of cells. This happens very early in embryonic development, before stem cells have been assigned their jobs in the body. The embryo can develop as normal.
Although dizygotic twins share DNA, they are as different genetically as two typical siblings. Genetic testing can easily determine the DNA differences between dizygotic twins and other non-identical siblings.Therefore the differences between the two cell sets in a chimera is notable. Genetically, they are like two people in one body.
Cases of chimerism are rare, though they will almost certainly go undetected. Genetic testing is usually required to show chimerism. In some cases a chimera may have some notable physical attributes, such as differently colored eyes. Hermaphrodism can also be a result of chimerism.
Chimerism in Humans — a Real Life Example
A well-known case of chimerism is that of Karen Keegan who underwent genetic testing in 1998. Keegan required a kidney transplant, and was told that she couldn’t be the biological mother of two of her three sons. Further testing revealed that her sons were indeed her biological children, and that Keegan was a chimera.
Mosaicism in Humans
Everyone accumulates small changes to their DNA in their lifetime, most of which aren't harmful. These changes are called mutations, and can be caused by environmental factors such as medication and diet. Sometimes mutations affect some cells in the body but not others. This is a type of mosaicism.
X Chromosome Mosaicism
Another type of mosaicism can be found in females, or males with an extra X chromosome. A female inherits one of her X chromosomes from her mother, and one from her father. In each body cell, one of these X chromosomes is mostly inactive, ensuring she gets the right amount of X chromosome genes.
In some cells, the maternally-inherited X is active. In others, it is the paternally-inherited X that is active. This is because X inactivation is random and happens early in embryonic development.
Mosaicism and Genetic Disorders
Sometimes genetic disorders affect some cells and not others. For example, this can happen with the chromosomal disorder Down syndrome. People with Down syndrome have either a whole or partial extra chromosome 21. When only some of a person’s cells have the extra material, the disorder is mosaic Down syndrome.
Mosaic disorders sometimes manifest with milder features than disorders affecting all cells. This is because the unaffected cells are still able to produce proteins normally, and are therefore able to compensate.
For more information on chimeras and genetic testing, read Human Chimeras. Visit the Tech’s Ask a Geneticist site for answers to several reader questions on chimerism and mosaicism.
Sources
"Chimerism", published: 24th January 2010, Genetics Home Reference website, accessed: 31st January 2010, no author specified.
"How normal genes work", published by Scion Publishing Ltd 2009, Human Genetics, 2nd edition, p.29, authors: Gardner, Anne and Davies, Teresa.
"Mosaicism", published: 24th January 2010, Genetics Home Reference website, accessed: 31st January 2010, no author specified.
"The Twin Inside Me”, Five.TV website, accessed: 31st January 2010, no author specified.
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