Somatic mosaics are common in embryogenesis due to retrotransposition of long interspersed nuclear element-1 (LINE-1 or L1) and Alu transposable elements. In early development, DNA from undifferentiated cell types may be more susceptible to mobile element invasion due to long, unmethylated regions in the genome. Further, the accumulation of DNA copy errors and damage over a lifetime lead to greater occurrences of mosaic tissues in aging humans. As  longevity has increased dramatically over the last century, human genome may not have had time to adapt to cumulative effects of mutagenesis. Thus, cancer research has shown that somatic mutations are increasingly present throughout a lifetime and are responsible for most leukemia, lymphomas, and solid tumors. Trisomies, monosomies and related conditions
The most common form of mosaicism found through prenatal diagnosis involves trisomies. Although most forms of trisomy are due to problems in meiosis and affect all cells of the organism, some cases occur where the trisomy occurs in only a selection of the cells. This may be caused by a nondisjunction event in an early mitosis, resulting in a loss of a chromosome from some trisomic cells. Generally, this leads to a milder phenotype than in nonmosaic patients with the same disorder. In rare cases, intersex conditions can be caused by mosaicism where some cells in the body have XX and others XY chromosomes (46, XX/XY).
A phenotypic character called "twin spot" seen in Drosophila is a result of mitotic recombination. However, it also depends on the allelic status of the genes undergoing recombination. Twin spot occurs only if the heterozygous genes are linked in repulsion, i.e. the trans phase. The recombination needs to occur between the centromeres of the adjacent gene. This gives an appearance of yellow patches on the wild-type background in Drosophila. another example of mitotic recombination is the Blooms syndrome, which happens due to the mutation in the blm gene. The resulting BLM protein is defective. The defect in RecQ, a helicase, facilitates the defective unwinding of DNA during replication, thus is associated with the occurrence of this disease. Use in experimental biology
Genetic mosaics are a particularly powerful tool when used in the commonly studied fruit fly, where specially selected strains frequently lose an X or a Y chromosome in one of the first embryonic cell divisions. These mosaics can then be used to analyze such things as courtship behavior, and female sexual attraction.More recently, the use of a transgene incorporated into the Drosophila genome has made the system far more flexible. The flip recombinase (or FLP) is a gene from the commonly studied yeast Saccharomyces cerevisiae that recognizes "flip recombinase target" (FRT) sites, which are short sequences of DNA, and induces recombination between them. FRT sites have been inserted transgenically near the centromere of each chromosome arm of D. melanogaster.