In genetics, a mosaic (or mosaicism) means the presence of two different genotypes in an individual which developed from a single fertilized egg. As a result, the individual has two or more genetically different cell lines derived from a single zygote.
Mosaicism may result from:
- Unusual events in cell division (mitosis).
- A gene mutation during development
- A chromosomal mutation during development
- X-inactivation: one X chromosome is randomly switched off in cells of a female mammal
- Viral infection that can range from small mutations to changing large pieces of DNA
Mosaicism had been understood since the 1880s to be generally related to genes. But a specific genetic mechanism that causes mosaics was uncovered by Curt Stern in 1936, when he demonstrated that recombination, which is a normal part of meiosis, can also take place in mitosis. When it does, it results in somatic (body) mosaics. These are organisms which contain two or more genetically distinct types of tissue.
Types of chimeras[change | change source]
A genetic chimera is an organism composed of two or more sets of genetically distinct cells.
Dispermic[change | change source]
When two fertilized eggs happen to fuse, this results in one organism with two genetically distinct cell populations. The resulting "dispermic chimera" has distinct DNA in different tissues, for example, a liver arising from one cell population and kidneys arising from the other. In animals, an easily observable examples of chimerism is a patchwork coloration of skin, hair, fur, and even the iris of the eyes.
Other causes of two-tone appearances[change | change source]
When eye colours vary between the two eyes, or within one or both eyes, the condition is called heterochromia iridis (= 'different coloured iris'). Apart from mosaicism, it can have non-genetic causes. For example, David Bowie had the appearance of different eye colours due to an injury that caused one pupil to be permanently dilated.
Somatic mosaic[change | change source]
Not all chimeras result from two zygotes fusing. Sometimes, mutations and chromosome rearrangements can result in the emergence of two or more genetically distinct cell lines within one embryo. For example, when chromosomes don't sort properly ("nondisjunction") that can gives rise to two different cell lines that become randomly distributed throughout the body, leading to a mosaic of cells within a tissue. Sometimes this is harmless, other times it can result in malformations and severe abnormalities or genetic diseases.
In plants, an example of somatic mosaicism can be two-tone coloration of leaves known as variegation. Sometimes viral infection of regular plants by something like the Tobacco mosaic virus can cause a variegated mosaic appearance but that does not result in an actual chimera.
Microchimerism[change | change source]
Modern medical and genomic sequencing technology are revealing that undetected mosaicism can be common. Perfectly healthy tissues can harbor small numbers of chimeric cells. Blood testing sometimes reveals that an individual has more than one blood type—as many as 8% of non-identical twins have chimeric blood. One common cause is maternal-fetal trafficking of cells during pregnancy. Small numbers of fetal cells have been observed in the tissues of mothers decades after they have given birth.
In cancer, individual cells may have significantly different genomes. They may have multiple and different gene mutations and/or the wrong number of chromosomes ("aneuploidy"). It has been known since the 1950s-60s that in tumor tissue there may actually be a mosaic of genetically different cells mixed in with the normal tissue cells.
X-inactivation[change | change source]
The most common kind of mosaicism in mammalian females is X-inactivation. Females have two X chromosomes (and males have only one). The two X chromosomes in a female are rarely identical. They have the same genes, but at some loci (positions) they may have different alleles (versions of the same gene).
In the early embryo, each cell independently and randomly inactivates one copy of the X chromosome. This inactivation lasts the lifetime of the cell, and all the descendants of the cell inactivate that same chromosome.
This phenomenon shows in the colouration of calico cats and tortoiseshell cats. These females are heterozygous for the X-linked colour genes: the genes for their coat colours are carried on the X chromosome. X-inactivation causes groups of cells to carry either one or the other X-chromosome in an active state.
It is an epigenetic change[change | change source]
Mosaicism refers to differences in the genotype of various cell populations in the same individual, but X-inactivation is an epigenetic change, a switching off of genes on one chromosome. It is not a change in the genotype. Descendent cells of the embryo carry the same X-inactivation as the original cells. This may give rise to mild symptoms in female 'carriers' of X-linked genetic disorders.
Related pages[change | change source]
Further reading[change | change source]
- "Every cell in your body has the same DNA. Except it doesn't". The New York Times. 21 May 2018. Archived from the original on 23 May 2018. Retrieved 23 May 2018.
- "From Many, One -- Diverse mammals, including humans, have been found to carry distinct genomes in their cells. What does such genetic chimerism mean for health and disease?". The Scientist. Archived from the original on 25 April 2017. Retrieved 23 May 2018.
References[change | change source]
- The term has also been used for organisms with cells derived from more than one zygote.
- Stern C. 1936. Somatic crossing-over and segregation in Drosophila melanogaster. Genetics 21, 625–730.
- Stern, Curt 1968. Genetic mosaics in animals and man. pp27–129, in Stern C. Genetic mosaics and other essays. Harvard University Press, Cambridge, Massachusetts.
- Medical Definition of Chimera|https://www.medicinenet.com/script/main/art.asp?articlekey=8905
- Stich HF & Steele HD (2004). "NA Content of Tumor Cells. III. Mosaic Composition of Sarcomas and Carcinomas in Man". Journal of the National Cancer Institute 28 (5): 1207-18. doi:10.1093/jnci/28.5.1207. PMID 13917181.
- Okamoto I et al (2004). "Epigenetic dynamics of imprinted X inactivation during early mouse development". Science 303 (5658): 644–9. doi:10.1126/science.1092727. PMID 14671313.
- Klug, William S. et al 2012. Concepts of genetics. 10th ed, Pearson, p187 & 196. ISBN 0-321-79578-4
- Curt Stern described the set-up as "functional mosaicism". Stern 1968 p103
- Puck J; Willard, HF (1998). "X Inactivation in females with X-linked disease". N. Engl. J. Med. 338 (5): 325–8. doi:10.1056/NEJM199801293380511. PMID 9445416. http://content.nejm.org/cgi/content/full/338/5/325.