Mosaicism or genetic mosaicism is a condition in multicellular organisms in which a single organism possesses more than one genetic line as the result of genetic mutation. This means that various genetic lines resulted from a single fertilized egg. Genetic mosaics may often be confused with chimerism, in which two or more genotypes arise in one individual similarly to mosaicism. In chimerism, though, the two genotypes arise from the fusion of more than one fertilized zygote in the early stages of embryonic development, rather than from a mutation or chromosome loss. Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication. Anaphase lagging is the most common way by which mosaicism arises in the preimplantation embryo. Mosaicism can also result from a mutation in one cell during development, in which case the mutation will be passed on only to its daughter cells (and will be present only in certain adult cells). Somatic mosaicism is not generally inheritable as it does not generally affect germ cells. History
In 1929, Alfred Sturtevant studied mosaicism in Drosophila, a genus of fly. Muller in 1930 demonstrated that mosaicism in Drosophila is always associated with chromosomal rearrangements and Schultz in 1936 showed that in all cases studied these rearrangements were associated with heterochromatic inert regions, several hypotheses on the nature of such mosaicism were proposed. One hypothesis assumed that mosaicism appears as the result of a break and loss of chromosome segments.
This results in the cells of interest being marked brightly in a dark background. See also
Extrachromosomal array
Heterochromia
Parasitic twin
Vanishing twin
X0/XY mosaic
Human somatic variation

References
Further reading
Zimmer, Carl (21 May 2018). "Every Cell in Your Body Has the Same DNA. Except It Doesnt". The New York Times. Archived from the original on 23 May 2018. Retrieved 23 May 2018. "From Many, One -- Diverse mammals, including humans, have been found to carry distinct genomes in their cells. What does such genetic chimerism mean for health and disease?". The Scientist. Archived from the original on 25 April 2017. Retrieved 23 May 2018.