Evident in Down’s syndrome in which there is a mixture of cells containing 46 and 47 chromosomes. Other examples of mosaicism include Edward’s syndrome and Turner’s syndrome. It can arise as a consequence of spontaneous DNA mutations, chromosomal abnormalities, and epigenetic changes in chromosomal DNA. The term itself denotes the presence of two or more populations of cells (that may or may not include the germ line cells) in one individual developing from a single fertilized egg. There are in essence two types of mosaicism. In somatic mosaicism, a particular percentage of cells are affected by post-zygotic mutation (cancer being one the most prominent forms of such mosaicism). Somatic mosaicism was first reported by Curt Stern (1902-1981) who demonstrated that when genetic recombination occurs in mitosis it gives rise to this type of mosaicism. Gonadal or germ line mosaicism refers to the presence of a mutation in all of the germ line, but not in the rest of the body, and thus that the mutation occurred in sperm or ova prior to fertilization. The most common of mosaicism revealed by mean of prenatal diagnosis (e,.g., aminocentisis) include those arising from chromosomal mosaicism (e.g., trisomy 21 in Down’s syndrome). In such cases, trisomy 21 mosaicism can occur in in the germ line, in somatic tissues, or both. There is a lack of information about on the incidence of mosaicism in Down’s syndrome individuals due to persisting technical problems in its identification, and in part resolved by in situ FISH analysis. Nevertheless, it is estimated that about 2-4% of individuals with Down’s syndrome have mosaicism for cells with three copies of chromosome 21 (i.e., a combination of normal and trisomy cells within individual tissues).
See Amniocentesis, Chromosome, Copying errors, DNA (deoxyribonucleic acid), Down’s syndrome, Edward’s syndrome, Fluorescent in situ hybridization (FISH), Genetic recombination, Mitosis, Mutation (biology), Trisomy 21, Turner’s syndrome