Genetic testing assumes all cells in the body express a common genome. Yet different cells in the body may have distinct genomes, either due to chimera or mosaic effects.
The chimera process happens during gestation, where a twin receives some genomic information from its twin, or when a mother receives some genomic information from the father through the fetus. This is a common occurrence, with on the order of 50% of mothers evidencing some chimera effects.
Mosaic instances are due to cell mutations. When occurring during embryonic development, they can result in visible variations to phenotype and also produce disastrous effects, as in hemimegalencephaly. Later occurring changes are associated with disease, and have long been suspected of being a primary cause of cancers. Scientists are now looking for other such mosaic-linked diseases, including contributions to schizophrenia.
So how will genetic testing and forensics have to change to accommodate this common variability? Possibly some types of cells will be more resistant to such variability. It is suspected that hair cells are more likely definitive than cheek cells in producing genomic identity. Testing companies may need to revise their sample intake procedures.