As hard as it may be to believe, every human starts out as a single cell (around one two-hundredth of an inch in size) formed when the right sperm cell meets the ovum in the mother. The sperm and egg each carry a half-set of chromosomes, merging to create a single genome that is a combination of each parent. Since all other cells from the body are duplicates and clones of this one (with the exception of other egg or sperm cells), it is common sense that these cells would share a genome, but it appears that common sense is wrong.
As we get better at reading and examining human DNA, it is becoming more clear that each of our cells has its own more personal genome, different from the rest of the body. This phenomenon of multiple distinct geneotypes that come from a single cell is called “mosaicism”. We often associate genetic changes and mutations with cancer or disease, but this isn’t always the case. Due to regular wear and tear, as well as specific re-shuffling of the DNA during replication, our cells produce and change their genomes constantly, sometimes causing malfunctions or cancer, but they have been known to spontaneously correct diseased genes, as well.
Mosaicism is key in the immune system, where it helps fight new or different viruses and other diseases, and also linked to neuron formation and normal brain activity. Unfortunately, this means that our current genetic screenings of blood, skin, or other easily-accessed DNA may not be painting the whole picture.
Genome Mosaicism—One Human, Multiple Genomes (Science)
A similar phenominon is seen in females, where one of the two x-chromosomes is inactivated (as only one dose of the genes is required) randomly across big areas of the body. This is best known as being the reason for calico cats‘ colouration, but can also lead to some bizarre genetic consequences, such as women who are color blind in one eye, but not the other.
Mosaicism can be contrasted with chimerism, where the organism is made from more than one fertilized cell, strangely enough this is quite common in humans: Most people are born with a few cells from their mother, and mothers often carry a few blood cells belonging to their children.