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Mosaicism of Genes and Chromosomes
In genetics, the terms mosaic and mosaicism are used to describe cell populations which have two different genotypes. The genotype of a cell or organism is the collective term for all of its genetic material. In an organism with mosaicism, therefore, some cells in the body are genetically different from the rest, even though the cells are in the same organism. Mosaicism is a fairly common phenomenon in plants, and can also occur in animals and humans.
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Causes of Mosaicism
In almost all cases, an individual develops from a single fertilized egg which contains 46 chromosomes. Occasionally, this process occurs with a fertilized egg with 47 chromosomes. This occurs as a result of trisomy, in which there is an extra copy of a chromosome in a cell.
When trisomy occurs, the result is usually that all cells in the individual have the extra chromosome. The most common type of trisomy is trisomy of chromosome 21, which causes Down syndrome.
In some cases, only some of the cells have the extra chromosome. There are two possible reasons why this might occur. The explanation which is believed to be most likely is that the sperm or egg itself had an extra copy of the chromosome, and the extra copy was not passed on to all cells after fertilization.
The second possible explanation is that trisomy mosaicism occurs as a result of errors during early cell division just after fertilization of an egg. As the fertilized egg develops into a fetus, cells that initially acquired the extra chromosome give rise to new and larger populations of cells with the extra chromosome. All cells produced from the initial abnormal cell have the trisomy, but cells produced from the normal cells have the usual 46 chromosomes.
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Examples of Mosaic Chromosome Syndromes
Two of the most common genetic syndromes known to sometimes appear as mosaic versions are Down syndrome and Klinefelter syndrome. In each case, a child born with the syndrome has some or all of the characteristics of the more common disorder, but in a milder form.
For example, children born with mosaic Down syndrome have a higher IQ than children born with “complete" Down syndrome. However, even though in general children with this mosaic chromosome syndrome have milder symptoms, there is no way to predict at birth what degree of symptoms a child with the mosaic form will have as he or she grows up.
The same is true of mosaic Klinefelter syndrome, a syndrome which develops in male children who have an extra copy of the X chromosome. In “complete" Klinefelter syndrome, children might have learning difficulties and physical abnormalities, as well as speech problems and delayed or absent sexual development. In the mosaic version, these symptoms are typically less severe because not all of the cells in the body have retained the extra X chromosome.