Mosaic (genetics)

Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication.

In the more common mosaics, different genotypes arise from a single fertilized egg cell, due to mitotic errors at first or later cleavages.

Somatic mutation leading to mosaicism is prevalent in the beginning and end stages of human life.

[10] Somatic mosaics are common in embryogenesis due to retrotransposition of long interspersed nuclear element-1 (LINE-1 or L1) and Alu transposable elements.

[10] In early development, DNA from undifferentiated cell types may be more susceptible to mobile element invasion due to long, unmethylated regions in the genome.

[10] 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.

[10] Thus, cancer research has shown that somatic mutations are increasingly present throughout a lifetime and are responsible for most leukemia, lymphomas, and solid tumors.

This may be caused by a nondisjunction event in an early mitosis, resulting in a loss of a chromosome from some trisomic cells.

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).

Revertant somatic mosaicism is a rare recombination event with a spontaneous correction of a mutant, pathogenic allele.

[19] Other endogenous factors can also lead to mosaicism, including mobile elements, DNA polymerase slippage, and unbalanced chromosome segregation.

The amount of tissue that is mosaic depends on where in the tree of cell division the exchange takes place.

[24][25] Genetic mosaics are a particularly powerful tool when used in the commonly studied fruit fly, where specially selected strains frequently lose an X[17] or a Y[18] chromosome in one of the first embryonic cell divisions.

After induction of FLP expression, cells that undergo recombination will have progeny homozygous for either the marker or the allele being studied.

Creating positively marked clones is possible using the so-called MARCM ("mosaic analysis with a repressible cell marker" system, developed by Liqun Luo, a professor at Stanford University, and his postdoctoral student Tzumin Lee, who now leads a group at Janelia Farm Research Campus.