Well, let me throw a bit of a curve ball at you. Because this stuff can get complicated pretty fast. The first twist is mosaicism.
Most genetic syndromes affect all the cells in the body.
Which makes sense. But it is possible for a person to have a genetic syndrome that only impacts a percentage of their cells. A genetics test can provide an estimate on the range of affected cells, and which cells are likely impacted. Mosaicism is important because it means that the impacts of the syndrome are ammeliorated because it is not present in every cell. But it is currently impossible to tell just how impacted a child with tetrasomy 18p mosiac will be.