|MadSci Network: Development|
Chimera twins are very rare, resulting from the fusion of two separate embryos early in development. There are a number of examples of clinical cases of somatic mosaicism that were apparent from techniques other than DNA analysis. The OMIM entry for the XG blood group describes a chimera twin apparent because red blood cells had different ABO types: http://www.ncbi.nlm.nih.gov:80/entrez/dispomim.cgi?id=314700 The citation is: Ducos, J.; Morty, Y.; Sanger, R.; Race, R. R. : Xg and X chromosome inactivation. Lancet II: 219-220, 1971. PubMed ID : 4104885 There is also a report of a possible chimera twin in which one genotype was affected by Duchenne muscular dystrophy: http://www.ncbi.nlm.nih.gov:80/entrez/dispomim.cgi?id=310200 There is a dramatic first-person account of the difficulty of being a chimera twin in which the two genotypes were male and female. Because the fusion of twins occurred relatively late in development, after sexual differentiation had begun, the affected individual was born with an intersexual phenotype: http://www.medhelp.org/www/ais/stories/jane2.htm In general, a chimera twin could be suspected if the two genotypes differ with respect to some trait that is evident through medical tests or observation. This trait must be cell autonomous, meaning that the phenotype of an individual cell is wholly determined by its genotype and is not influenced by other genotypes present in that individual. Ordinarily, we would not expect chimera twins made up of cells of the opposite sex to be intersexual, because sexual differentiation is driven by the production (or lack of production) of androgens by cells in the presumptive gonad. Half of all chimera twins are made up of genotypes of the opposite sex, so these would be very easy to spot if sex determination were cell autonomous. As another example, consider a chimera twin in which one genotype is albino. This individual would be suspected of somatic mosaicism because of their appearance. There are other mechanisms of somatic mosaicism besides chimera twins. To take a very common example, all females are somatic mosaics for X-linked traits because of X-inactivation. In the case of women heterozygous for color blindness, they have mosaic retinas with some cells that are color blind, which can be detected by very sensitive tests for color vision. In some individuals, there will be a large clone of color-blind cells in the center of the visual field and these individuals will be effectively color blind in one eye. To fully answer your question, I am sure that there are undetected cases of chimera twins. If the two genotypes are not different with respect to some obvious phenotypic difference that is cell autonomous, somatic mosaicism will not be apparent, although it might be discovered accidentally during medical tests of some kind. Yours, Paul Szauter Mouse Genome Informatics
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