mosaicism
Mosaicism is when a single individual is built from two or more genetically different cell populations — like a mosaic artwork made of tiles that are not all the same colour, even though they form one picture. Some cells carry one genome and others carry a variant of it, side by side in the same body.
It usually arises from an error after fertilization, once the embryo has started dividing. If a mistake such as nondisjunction or a new mutation happens in one cell early on, all of that cell's descendants inherit the change while the rest of the body does not. The earlier the error, the larger the affected fraction tends to be.
Mosaicism explains why some chromosomal conditions are milder than expected: if only a portion of cells carry an extra chromosome, the imbalance is partial. It also matters for testing, because a blood sample might miss a variant present mainly in other tissues. Germline (gonadal) mosaicism, where only some egg or sperm cells carry a variant, is a subtle reason a healthy-testing parent can have more than one affected child.
A person with mosaic Down syndrome might have 47,XX,+21 in 70% of cells and a normal 46,XX in the rest, often producing a milder presentation than full trisomy 21.
A blend of cell lines can soften the effect of a chromosomal change.
Mosaicism arises after fertilization, so the variant is present in only some cells of one person; a germline mutation passed in the gamete affects every cell of the child.