What is Mosaicism?
Every cell in the human body is ultimately derived from one initial cell: the fertilized egg, which is also called the zygote. After fertilization, the zygote then proceeds to divide. As new cells form, the chromosomes duplicate themselves so that the resulting cells have the same number of chromosomes as the original cell. However, errors sometimes happen, and one cell ends up with a different number of chromosomes. From then on, all cells originating from that cell will have the different chromosomal number, unless another error takes place. (All like cells originating from a single type of cell is called a cell line—for example, the skin cell line, the blood cell line, the brain cell line, etc.) When a person has more than one type of chromosomal makeup, this is called mosaicism, like the mosaic style of art in which a picture is made up of different colors of tiles. In Down syndrome, mosaicism means that some cells of the body have trisomy 21, and some have the typical number of ch
Some children and adults with chromosome 15q duplications are said to have ‘mosaicism’, meaning that their extra 15th chromosome is present in some, but not all, of their body cells. A child with a mosaic form of idic(15) is conceived with the usual pattern of 46 chromosomes; but somewhere along the line, during those early cell divisions, one cell accidentally divides incorrectly and produces an extra, inverted 15th chromosome (idic(15)). All the subsequent cells that come from that one have an idic(15) pattern, although cells produced prior to the idic(15) error continue to produce cells with the usual 46 chromosome pattern. Chromosome studies on a person with mosaic idic(15) show some cells with ‘normal’ chromosomes (from the original cell divisions) and some with idic(15). Mosaicism occurs by chance in this and many other chromosomal disorders.
The term “mosaicism” is used to describe the presence of more than one type of cell in a person. For example, a person may have some of the cells in their body with 46 chromosomes, while other cells in their body have 47 chromosomes. An example of mosaicism is mosaic Down syndrome. About 95 percent of people with Down syndrome have trisomy 21, where there is an extra #21 chromosome in every cell of their body. Three to 4 percent of people with Down syndrome have translocation Down syndrome, where all or part of the extra #21 chromosome is attached to another chromosome. The remaining 1 to 2 percent of individuals with Down syndrome are mosaic, where there are at least two types of cells, some with the usual number of chromosomes (46 total), and others with an extra #21 chromosome (47 total). Rarely, a person can have more than two types of cell lines. Mosaicism is usually described as a percentage. For example, when a baby is born with Down syndrome, the physician will take a blood sam
Every cell in the human body comes from one initial cell: the fertilized egg, which is also called the zygote. After fertilization, the zygote then proceeds to divide. As new cells form, the chromosomes duplicate themselves so that the resulting cells have the same number of chromosomes as the original cell. However, mistakes sometimes happen and one cell ends up with a different number of chromosomes. From then on, all cells originating from that cell will have the different chromosomal number, unless another mistake happens. (All like cells originating from a single type of cell is called a cell line; for example, the skin cell line, the blood cell line, the brain cell line, etc.) When a person has more than one type of chromosomal makeup, that is called mosaicism, like the mosaic style of art in which a picture is made up of different colors of tiles. In Down syndrome, mosaicism means that some cells of the body have trisomy 21, and some have the typical number of chromosomes. How i
